Abstract:
A hydraulic system servicing apparatus includes an old fluid holding vessel and a new fluid supply vessel with a selector valve which draws fluid from the holding vessel during flushing operations and draws fluid from the supply vessel during fluid replacement and bleeding operations. A pump moves the fluid through the selector valve and supplies it to a flow reversing valve which has a master cylinder line and a plurality of bleeder valve lines for connection to the hydraulic system. A shutoff valve is provided in each bleeder valve lines for directing fluid through the different branches of the hydraulic system. To flush the system, the flow reversing valve is cycled to circulate the fluid from the old fluid holding vessel in opposite directions and return it to the old fluid holding vessel. To replace the fluid and bleed the system, the fluid selector valve directs new fluid through the flow reversing valve into the system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to the servicing of hydraulic systems and more particularly to an apparatus for flushing contaminants from hydraulic systems by recirculating the used fluid and subsequently replacing it with new fluid and bleeding air from the system. 
     2. Description of the Prior Art 
     Hydraulic fluid is used to transmit the pressure exerted on a motor vehicle&#39;s brake pedal to the slave cylinders provided at the wheels of the vehicle. The most commonly used hydraulic brake fluids consist of glycol-based liquids identified as DOT3 and DOT4 on the basis of the boiling points resulting from their particular composition. 
     Copending U.S. Ser. No. 10/030,455, hereby incorporated by reference, discloses a novel approach for estimating the condition of brake fluid based on the discovery that moisture content is closely correlated to the copper content in the fluid. The invention consists of a reactive test strip adapted to measure and indicate the concentration of copper ions in brake fluid in terms of a readily visible color change. The strip is immersed in the fluid and the resulting color acquired by reacting with the fluid is compared to a copper concentration-versus-color chart or to a standard color representative of the maximum concentration determined empirically to correspond to a fluid condition considered appropriate for normal operation. 
     Another important aspect of brake system maintenance is the corrosive nature of some of their constituents, which, upon contamination of the brake fluid, progressively contribute to damage of the metallic tubing and other parts of the brake system. In conventional brake fluids, amines are added to inhibit corrosion and prevent damage to metal parts that operate in contact with the fluid. As the brake fluid ages, its anticorrosive properties are measured in terms of reserve alkalinity; that is, the amount of amines remaining in the fluid to buffer the acidity resulting from a breakdown of the fluid constituents. Over time, thermal oxidation and volatization produce a significant reduction of the amine content and the concurrent decrease in anticorrosive properties. Tests have shown that the reserve alkalinity of DOT3 and DOT4 fluids is reduced to about 20 percent of its original value after 18 to 20 months of normal operation. Therefore, brake fluids need to be checked and periodically replaced in order to prevent the development of potentially unsafe conditions in the brake system. Accordingly, industry maintenance recommendations are typically based on service time and milage of the vehicle. 
     Following the work described in U.S. Ser. No. 10/030,455, it has been found that copper content is predictably related also to time and milage of vehicle operation. Thus, this correlation has similarly been used to determine the need to replace the fluid based on “virtual age”; that is, the wear and tear on the brake fluid, rather than the actual service time and/or milage. If a color change in the test strip indicates a higher-than-desirable copper concentration, the brake fluid is considered in need of replacement without further tests. This invention is described in copending U.S. Ser. No. 10/132,978. 
     The prior art methods for replacing deteriorated and contaminated brake fluid with new fluid are varied, however, the most common method used is a two-person operation which takes about 45 minutes to an hour to complete. Usually, the master cylinder is drained of the old fluid, cleaned and then refilled with new clear fluid. Then one person attaches a drain line to the bleeder valve of one of the wheel cylinders and opens the bleeder valve and the other person pumps the brake pedal to move the new fluid through the system to expel the old fluid out through the bleeder valve and through the drain line into a suitable container. When the clear new brake fluid emerges from the open bleeder valve, draining of the old fluid from that branch of the system is terminated and the valve is closed. This operation is repeated for each wheel cylinder with new brake fluid being added to the master cylinder as needed during the draining operation. When the brake system is drained and refilled in this manner, it must subsequently be bled to remove the air introduced during the draining and refilling operation. This prior art method of replacing old brake fluid with new fluid has several drawbacks; it is time consuming, requires the services of two people, and uses an excessive amount of brake fluid in that the new fluid which emerges from the bleeder valves during the replacement and bleeding operations cannot be reused due to it having been exposed to system contamination. 
     Some of these drawbacks have been overcome by a closed system fluid replacement apparatus and method disclosed in U.S. Pat. Nos. 6,206,055 and 6,302,167 to Peter C. Hollub. The Hollub apparatus includes a vacuum wand which extracts the old fluid and contaminants from the master cylinder. A fill pump is used for supplying new fluid under pressure from a closed container to the master cylinder and at the same time a vacuum pump is connected to extract the old fluid from all of the bleeder valves simultaneously and directing it to a closed waste fluid tank. Also disclosed is the periodic shutting off and restarting of the fill and vacuum pumps to produce a fluid surging to flush contaminants from the system. The Hollub apparatus does away with the need for using two-man and reduces the time required to complete the replacement operation. However, the Hollub apparatus is not especially efficient in flushing the old brake fluid and contaminants from the brake system. When the old brake fluid is extracted from all the brake lines simultaneously, the fluid will take the path of least resistance and the longer lines will often have insufficient flow to clean out the sludge and corrosion by-products. Also, supplying the new brake fluid to the system while extracting the old brake fluid provides a single flow of fluid through the system, that is, as the old fluid moves out, the new fluid moves in. It has been found that a single flow of fluid through the system oftentimes leaves some sludge and corrosion by-products behind. 
     Therefore a need exists for a new and improved apparatus for flushing, replacing the fluid and bleeding hydraulic systems which overcomes the shortcomings of the prior art. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a new and improved servicing apparatus for flushing, replacing the fluid and bleeding hydraulic systems is disclosed. Hydraulic systems of the type serviceable by the apparatus of the present invention normally include a master cylinder which supplies fluid under pressure to a plurality of slave cylinders each of which has a bleed valve for bleeding air from the system. Most modern brake systems are provided with an Antilock Brake System (ABS) in the form of a computer controlled module which also has at least one bleeder valve. The servicing apparatus has a plurality of bleeder valve lines each of which is for connection to a different one of the bleeder valves and an independently operable shutoff valve is provided in each of these bleeder valve lines. The servicing machine also includes a master cylinder line which is connected to the top end of the master cylinder by means of a suitable adapter. A new fluid supply source and an old fluid holding vessel are provided in the servicing apparatus and both are coupled to a different one of a pair of inlet ports of a flow selector valve. The flow selector valve has an outlet port that is connected to the inlet of a pump and the flow selector valve is operable to direct fluid to the pump from either the new fluid supply source or the old fluid holding vessel. The pump is preferably of the type which produces a pulsating output and supplies fluid under pressure to a two-position flow reversing valve. To initiate a servicing operation, the flow selector valve is positioned to direct fluid from the new fluid supply source vessel through to the two-position flow reversing valve which in turn directs the fluid under pressure through the master cylinder line to the top of the master cylinder. This causes the fluid to flow through the hydraulic system and out through the bleeder valves of the slave cylinders and into the servicing apparatus to fill the old fluid holding vessel. After initiation, the flow selector valve is operated to a first position wherein fluid from the old fluid holding vessel is directed through the hydraulic system and out through the bleeder valves as described above. Selective operation of the shutoff valves provided in the bleeder valve lines will allow fluid to flow out of all of the bleeder valves simultaneously or one at a time in any desired sequence. The fluid from the bleeder valves is directed through the flow reversing valve into a manifold which returns it to the old fluid holding tank. When the flow reversing valve is actuated to its second position, the fluid supplied by the pump will be directed under pressure through the hydraulic system in a direction that is opposite to the flow direction provided in the flow reversing valve&#39;s first position. In this manner, switching the flow reversing valve back and forth between its first and second positions will move the old brake fluid in one direction through the hydraulic system and then in the reverse direction to flush the system. 
     The servicing apparatus may also include a vibration mechanism, a filter and a pressure port with a detachable nozzle. The vibration device is attachable to the master cylinder, and is movable to other components of the hydraulic system, to loosen contaminants in the system. The filter is used to remove particulate contaminants from the old fluid that is being returned to the old fluid holding vessel for re-circulation through the hydraulic system during the flushing operation. The nozzle is a hand operated device for connection to the pressure port for initial cleaning of the master cylinder and for bench bleeding the various components of the hydraulic system when the flow reversing valve is in the first position. 
     When flushing the hydraulic system has been completed, the flow selector valve is operated to shut off the flow from the old fluid holding vessel and supply fluid from the new fluid supply source to the inlet port of the pump. With the flow reversing valve in either its first or second position, and the shutoff valves in the bleeder valve lines are opened in any desired sequence, the new fluid will be supplied to the hydraulic system to replace the old fluid and simultaneously bleed the air from the system. 
     The servicing apparatus can be configured in various ways for detecting the completion of the fluid replacement and bleeding operations. In a first embodiment an optical sensor is provided for determining the clarity of the fluid being returned to the holding vessel from the hydraulic system and providing an indication when clean fluid is detected. To aid in such a determination, a fluid compatible dye may be injected into the replacement fluid at the beginning of the fluid replacement and bleeding operations. In a second embodiment, an ion-selective electrode is used to provide a suitable indication when the ions of a trace element, such as copper, are no longer detected in the fluid emerging from the hydraulic system. In still another embodiment, the new fluid supply is provided in pre-packaged containers that having an amount of fluid that is equal to or slightly greater than the fluid capacity of the hydraulic system being serviced. 
     In another embodiment, the apparatus is provided with three-way valves instead of shutoff valves to provide a cross-flushing capability of systems having interconnected components. 
     In the preferred embodiment, the pump and all of the valves of the servicing apparatus are electrically operated and are controlled by an electronic control unit. The electronic control unit is programable to change the operational sequence, timing and other functions of the servicing machine to suit the particular hydraulic system to be serviced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a first embodiment of the servicing apparatus of the present invention which is shown connected to a typical hydraulic system. 
     FIG. 2 is a fragmentary perspective view showing a hand held nozzle being connected to a pressure port of the servicing apparatus for bench bleeding a typical master cylinder. 
     FIG. 3 is a schematic diagram showing a fragmentary portion the servicing apparatus of the present invention which has been modified to include a trace element or dye injection system. 
     FIG. 4 is a fragmentary schematic diagram showing a second embodiment the servicing machine of the present invention as having a first type of pre-packaged new fluid supply container. 
     FIG. 5 is a fragmentary schematic diagram showing a third embodiment of the servicing machine of the present invention as having a second type of pre-packaged new fluid supply container. 
     FIG. 6 is a perspective view showing a vibrator demountably attached to a hydraulic system master cylinder. 
     FIGS. 7,  8 , and  9  are schematic diagrams showing different types of hydraulic system that are serviceable by a second embodiment of the servicing apparatus of the present invention. 
     FIG. 10 is a schematic diagram showing a fragmentary portion of the second embodiment of the servicing apparatus of the present invention for use in servicing hydraulic systems of the types shown in FIGS. 7,  8 , and  9 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring more particularly to the drawings, FIG. 1 shows a hydraulic system which is indicated generally by the reference numeral  10  and the hydraulic servicing apparatus of the present invention which is indicated generally by the reference numeral  12 . The hydraulic system  10  is a schematically illustrated typical automotive braking system which includes: a master cylinder  14 ; an antilock brake system (ABS) modulator  16 , an ABS control computer  18 , and four slave cylinders  20 ,  22 ,  24  and  26 . The ABS modulator  16  is provided with a bleeder valve  28  and each of the slave cylinders  20 ,  22 ,  24 , and  26  has a bleed valve  30 ,  32 ,  34  and  36  respectively. It will be understood that not all automotive systems are the same, in some systems the ABS modulator is not used, and a proportioning valve (not shown) is used. In drum brake systems, the slave cylinders are in the form of wheel cylinders and in disc brake systems they are in the form of calipers. 
     The hydraulic servicing apparatus  12  is provided with five bleeder valve lines  38 ,  40 ,  42 ,  44  and  46  for demountable connection to the bleeder valves  28 ,  30 ,  32 ,  34  and  36  respectively. Five independently operable shutoff valves  48 ,  50 ,  52 ,  54  and  56  are provided with each being mounted in a different one of the bleeder valve lines  38 ,  40 ,  42 ,  44  and  46 . The shutoff valves  48 ,  50 ,  52 ,  54  and  56  are preferably in the form of solenoid operated two-way poppet valves each of which has a first port  58  connected to its respective one of the bleeder valve lines  38 ,  40 ,  42 ,  44  and  46  and a second port  60  which is connected to a manifold  62 . The manifold  62  connects the second port  60  of each of the shutoff valves  48 ,  50 ,  52 ,  54  and  56  to a first fluid flow port  64  of a flow reversing valve  66 . 
     The flow reversing valve  66  is in the preferred form of a solenoid operated two-position spool valve with its first position being shown in solid lines and its second position being shown in phantom lines. In addition to the first fluid flow port  64 , the flow reversing valve  66  has a second fluid flow port  68 , a first fluid return port  70 , a second fluid return port  72  and a fluid inlet port  74 . In the first position of the flow reversing valve  66 , the first fluid flow port  64  is in communication with the second return fluid port  72 , the second fluid flow port  68  is in communication with the fluid inlet port  74  and the first fluid return port  70  is blocked. In the second position of the flow reversing valve  66 , the first fluid flow port  64  is coupled to the fluid inlet port  74 , the second fluid flow port  68  is in communication with the first fluid return port  70  and the second fluid return port  72  is blocked. The second fluid flow port  68  of the flow reversing valve  66  is coupled by a master cylinder line  76  to the top of the master cylinder  14 , and the master cylinder line may be provided with a pressure take-off port  77  for reasons which will hereinafter be described. The usual top closure (not shown) of the master cylinder  14  is removed while the flushing, fluid replacement and bleeding operations are being preformed and replaced by an adapter  78  that locates the distal end of the master cylinder line  76  in the reservoir of the master cylinder. The first and second fluid return ports  70  and  72  respectively of the flow reversing valve  66  are connected to a fluid return manifold  80  which, in the preferred embodiment, passes returning fluid through a filter  82  to an old fluid holding vessel  84 . The filter  82  is for removing particulate contaminants which, if re-circulated through the hydraulic system, as will hereinafter be described in detail, could clog the internal filters (not shown) of the ABS modulator  16 . 
     The old fluid holding vessel  84  is provided with a first outlet port  86  which is coupled by a suitable line  88  to a first inlet port  90  of a fluid selector valve  92 . The fluid selector valve  92  has a second inlet port  94  and an outlet port  96 , with the second inlet port  94  being coupled by a suitable line  98  to a new fluid supply source which in a first embodiment is in the form of a tank  100 . The fluid selector valve  92  is in the preferred form of a solenoid operated spool valve having a first position which is shown in solid lines and a second position which is shown in phantom lines. In the first position of the fluid selector valve  92 , the first inlet port  90  is in communication with the outlet port  96  and in the second position, the second inlet port  94  is in communication with the outlet port  96 . The outlet port  96  of the fluid selector valve  92  is coupled by a line  101  to the inlet of a pump  102  which is driven by a suitable electric motor  104 . When operating, the pump  102  will draw fluid through the fluid selector valve  92  from either the old fluid holding vessel  84  or the new fluid supply vessel  100 , depending on the position of the fluid selector valve  92 , and the fluid will be supplied under pressure to the fluid inlet port  74  of the flow reversing valve  66 . The pump  102  is preferably of the type having a pulsating output, such as a piston pump or a gear-on-gear pump, to produce a surging fluid flow through the hydraulic system  10  to enhance the flushing and bleeding operations of the servicing apparatus  12 . The desired pulsating fluid flow can also be accomplished by cyclically interrupting power to the pump  102  or by cycling of the shutoff valves  48 ,  50 ,  52 ,  54 , and  56  between their open and closed positions. 
     Operation 
     To begin the servicing operation, the bleed valve lines  38 ,  40 ,  42 ,  44 , and  46  are connected to the bleeder valves  28 ,  32 ,  34 ,  36  and  38  respectively, and the bleeder valves are all opened. With the flow reversing valve  66  in its first position and the fluid selecting valve  92  in its second position, the pump  102  is operated to supply fluid from the new fluid supply tank  100  to the top of the master cylinder  14 . That fluid will flow through the hydraulic system  10  and into the servicing apparatus  12  to fill the apparatus with fluid and when full, the flushing. operation of the hydraulic system  10  can begin. 
     The flushing operation is started by actuating the fluid selector valve  92  to its first position and starting the pump  102 . It does not matter if the flow reversing valve  66  is in the first or second position at the beginning of the flushing operation, however, since it was put into the first position by the operation described above, this description will be started with the flow reversing valve  66  in the first position. The pump  102  will draw fluid from the old fluid holding vessel  84  through the fluid selector valve  92  and supply that fluid under pressure to the inlet port  74  of the flow reversing valve  66 . The fluid will move through the flow reversing valve  66  and out through the second fluid flow port  68  thereof into the master cylinder line  76 . The fluid supplied to the master cylinder line  76  will be directed into the master cylinder  14  and will move through the hydraulic system  10  in a manner determined by the bleeder line shutoff valves  48 ,  50 ,  52 ,  54  and  56 . In FIG. 1 the shutoff valve  56  is shown in the open position and all the others are in their normally closed positions. In this state, the fluid will flow only through the slave cylinder  20  to flush that cylinder and its associated lines. The fluid flowing through the open shutoff valve  56  will flow into the fluid return manifold  80  and from there through the filter  82  into the old fluid holding tank  84 . After the fluid is moved through the hydraulic system  10  in this direction for a suitable length of time, the flow reversing valve  66  is switched to its second position which reverses the flow of fluid through the system. More specifically, the reversed fluid flow will move from the first fluid flow port  64  of the flow reversing valve  66 , through the open shutoff valve  56 , through the slave cylinder  20 , through the ABS modulator  16  into the bottom of the master cylinder  14 . The fluid entering the bottom of the master cylinder  14  moves out of the master cylinder through the master cylinder line  76  and is returned to the flow reversing valve  66 . The fluid will move through the flow reversing valve  66  by entering through the second fluid flow port  68  and exiting through the first fluid return port  70  thereof. The fluid emerging from the first fluid return port  70  enters the fluid return manifold  80  and passes through the filter  82  into the old fluid holding tank  84 . By cycling the flow reversing valve  66  back and forth between its first and second positions, fluid flow through the slave cylinder  20  and its associated lines is reversed to agitate the fluid and thus enhance the removal of sludge and corrosion by-products from that branch of the system. Such a flushing operation is repeated for each of the other slave cylinders and their associated lines and for the ABS modulator  16  by sequentially opening the shutoff valves  48 ,  50 ,  52 , and  54 . Opening the other shutoff valves  48 ,  50 ,  52 , and  54  one at a time with the other shutoff valves in their normally closed positions, the slave cylinders  22 ,  24  and  26  as well as the ABS modulator  16  and their associated lines can be individually flushed in the manner described above. 
     When the flushing operation is complete, the old fluid holding tank  84  is emptied by opening a shutoff valve  106  which has an inlet port  108  and an outlet port  110  and is preferably a solenoid operated poppet valve. The inlet port  108  of the shutoff valve  106  is coupled to a second outlet port  112  of the old fluid holding tank  84  and the outlet port  110  of the shutoff valve  106  is coupled to the inlet of a waste fluid disposal tank  114 . The outlet of the waste fluid disposal tank  114  is preferably connected to a pump  116  which is operated by an electric motor  118  to empty the fluid from the old fluid holding tank  84 . Alternatively, the fluid can be allowed to flow into the waste fluid disposal tank under the influence of gravity. The waste fluid disposal tank  114  is provided with suitable fittings  120  at its inlet and outlet so that it  114  can be removed for fluid disposal purposes. 
     Replacing the old fluid in the hydraulic system  10  with new fluid is accomplished in a first embodiment by switching the fluid selector valve  92  to its second position to place it in communication with the new fluid supply vessel  100 . The pump  102  is operated to draw the fluid out of the new fluid supply vessel  100  and move it through the flow reversing valve  66  into the hydraulic system  10 . The fluid can move through the flow reversing valve  66  in either position thereof, however, it is preferred that the flow reversing valve be in the second position. The reason for this is that any air which may be in the hydraulic system  10  will naturally tend to move upwardly and by introducing the fluid into the system at its lowest points, the fluid flow will enhance the upward movement of the air and thereby simultaneously bleed the system. 
     The servicing apparatus  12  can be configured in various ways for detecting the completion of the fluid replacement and bleeding operation. Detection can be accomplished by placing a fluid condition sensor  122  in the master cylinder line  76  and, in a first embodiment, that sensor is an optical sensor which provides a suitable indication upon detection of clean new fluid in the line  76 . The indication provided by the optical sensor can be an audio or visual signal or it can produce an electrical signal which terminates the operation of the apparatus  12 . An optical sensor suitable for this purpose is identified as Model No. TSL230, available from Texas Advanced Optoelectronic Solutions, Plano, Tex. This method can be enhanced by injecting an additive such as hydraulic fluid compatible dye into the system in a manner to be hereinafter described. In a second embodiment, the fluid condition sensor  122  is in the form of an ion-selective electrode which provides an indication of the type described above when the metallic ions inherently present in the brake fluid being replaced are no longer detected in the fluid emerging from the hydraulic system  10 . An ion-selective electrode suitable for this purpose is Model No. CU3005, available from Weiss Research, Houston, Tex. 
     Reference is now made to FIG. 3 which shows an additive injection sub-system  124  by which the fluid compatible dye, is introduced into the hydraulic system  10 . The injection sub-system  124  includes a solution container  126  having an outlet port  128  which is coupled to a shutoff valve  130  in the preferred form of a normally closed solenoid controlled poppet valve. The shutoff valve  130  has an outlet port  132  that is coupled to the inlet port  134  of a metering valve  136  in the preferred form of an adjustable orifice. The outlet of the metering valve  136  is connected to the line  101  that leads from the fluid selector valve  92  to the pump  102 . When the shutoff valve  130  is actuated to its open position, the pump  102  will draw liquid from the solution container  126  and deliver it to the hydraulic system  10  in the manner hereinbefore described. Instead of using the adjustable orifice  136  to control the amount of dye injected into the system, controlling the time that the shutoff valve  130  is open will accomplish the same objective. 
     In a second embodiment, the old fluid from the hydraulic system  10  is replaced by supplying a known quantity of new fluid to the hydraulic system and when all of the known quantity of new fluid has been introduced, the replacement operation is terminated. The new fluid supply tank  100  of the hereinbefore described embodiment is replaced by either one of two pre-packaged containers which hold an amount of new fluid that is equal to, or slightly larger, than the capacity of the hydraulic system being serviced. 
     The first of the pre-packaged containers is seen in FIG. 4 to include a collapsible bag  138  having an outlet fitting  140  with the bag being placed in a housing  142 . The housing  142  has a removable lid  146  with an inlet port  144  formed therein and a suitable air hose  148  is connected to the inlet port. The housing  142  is provided with an outlet opening  150  through which the outlet fitting  140  of the collapsible bag  138  protrudes. A residual pressure valve  152  is mounted on the distal end of the outlet fitting  140  of the bag  138  to prevent fluid leakage. Flow through the outlet fitting  140  will occur when the differential pressure across the valve  152  exceeds the residual pressure setting thereof. A line  154  connects the discharge end of the residual pressure valve  152  to the second inlet port  94  of the fluid selector valve  92 . When the fluid selector valve is in its second position, as shown in solid lines in FIG. 4, and the pump  102  is operated, a negative pressure is applied on the discharge end of the residual pressure valve  152 . A positive pressure is applied to the inner end of the residual pressure valve  152  when the collapsible bag  138  is compressed upon the introduction of air under pressure into the housing  142 . The combination of the negative and positive pressures on across the residual pressure valve  152  will provide the necessary differential pressure and the fluid within the collapsible bag  138  will be supplied to the hydraulic system  10 . 
     FIG. 5 shows the second type of pre-packaged container as being in the form a cartridge  156  having an outlet fitting  158  which extends through an opening provided in a cartridge retaining housing  160 . A residual pressure valve  162  of the type hereinbefore described is mounted on the distal end of the outlet fitting  158  to prevent fluid flow from the cartridge  156  until a predetermined differential pressure is applied across the residual pressure valve  162 . A piston  164  is axially movable in the cartridge  156  to push the new fluid out through the outlet fitting  158  and through the fluid selector valve  92  into the hydraulic system  10  as described above. A suitable ram  166 , which can be pneumatic, hydraulic or electric, is employed to move the piston  164  axially in the cartridge  156 . 
     The master cylinder line  76  is provided with the pressure take-off port  77  as hereinbefore mentioned and a hand held nozzle  168  is demountably connected to that port. The nozzle is shown in position to accomplish bench bleeding of the master cylinder  14 . To accomplish such an operation, all the bleeder line shutoff valves  48 ,  49 ,  50 ,  52  and  54  are de-energized to their normally closed positions to prevent fluid flow through the hydraulic system  10 . Then the pump  102  is operated to supply fluid under pressure to the pressure take-off port  77  and to the nozzle  168 . The nozzle is positioned to deliver the pressurized fluid to one of the outlet ports of the master cylinder  14  with the other outlet port thereof being closed with a suitable plug  170 . Fluid flowing into the master cylinder in this manner will force the air out of the master cylinder to complete the bench bleeding operation. In brake systems that are in need of servicing, the master cylinders contain large amounts of sludge and contamination by-products and for that reason it is a common practice to drain the master cylinder and clean it prior to starting the servicing operation. The hand held nozzle  168  can also be used to clean out the master cylinder by connecting it to the pressure take off port  77  and reversing the rotation of the pump  102  to supply a negative pressure to the nozzle. 
     Referring now to FIG. 6 wherein a suitable vibrator  172  is shown as an electrically operated unit that is demountably attached to the master cylinder  14 , and is movable to other system components, to induce vibrations for loosening stubborn contaminants and putting them in suspension for flushing out of the system. A vibrator suitable for this purpose is commercially available from Makita, La Mirada, Calif., Model No. VR251DWDE. 
     It has been found that a low, or no pressure area exists in the master cylinders of some hydraulic systems and no fluid will flow in that area during flushing operations. When the hydraulic system is put in service after having been serviced as described above, fluid from that low, or no pressure area, will be circulated and contaminate an otherwise clean system. This is overcome by the simple step of pushing the actuator rod  174 , as indicated by the arrow  176  in FIG. 6, to move the master cylinder piston (not shown) forward about one inch and holding it there during the flushing operation. This provides a fluid flow path through the otherwise stagnant area of the master cylinder to flush the contaminants therefrom. 
     The hydraulic servicing apparatus  12  is preferably operated by an electronic control unit  178  which performs the switching operations and controls the timing of the servicing machine  12 . Power to the control unit  178  can be obtained from any suitable source such as a self-contained battery (not shown), connecting to the vehicle&#39;s power supply or by an external power supply such as a standard a 120 VAC source. The computer is programable so that the switching, timing and other functions of the apparatus can be customized to service various types of hydraulic systems. 
     The hydraulic brake system  10  shown in FIG. 1 is of the type wherein the slave cylinders  20 ,  22 ,  24 , and  26  are connected through the ABS modulator  16  to the master cylinder  14  by separate brake lines. This is not always the case and reference is now made to FIGS. 7,  8 , and  9  which show different brake systems  10   a ,  10   b , and  10   c , respectively, each having a different brake line configuration. 
     The hydraulic brake system  10   a  of FIG. 7 shows the larger compartment of a master cylinder  180  as having a single brake line  182  connected to a line  184  which extends between the two front slave cylinders  186  and  188 . Similarly, a single line  190  extends from the smaller compartment of the master cylinder  180  and is connected to a cross line  192  which extends between the two rear slave cylinders  194  and  196 . Thus, the two front slave cylinders  186  and  188  are in fluid communication with each other and the two rear slave cylinders  194  and  196  are in fluid communication with each other. 
     The hydraulic brake system  10   b  of FIG. 8 shows the larger compartment of the master cylinder  180  as having two brake lines  198  and  200  extending therefrom with the line  198  being connected to the front slave cylinder  186  and the line  200  being connecter to the other front slave cylinder  188 . As in the brake system  10   a  of FIG. 6, the system  10   b  of FIG. 8 has a single line  202  extending from the smaller compartment of the master cylinder  180  and is connected to a cross line  204  which extends between the two rear slave cylinders  194  and  196 . In this brake line configuration, the two front slave cylinders  186  and  18 B are in fluid communication with each other by virtue of their both being connected to the same compartment of the master cylinder  180 , and the rear slave cylinders  194  and  196  are in fluid communication with each other by virtue of the cross brake line  204 . 
     The hydraulic brake system  10   c  of FIG. 9 shows a first brake line  206  extending from the master cylinder  180  to the front slave cylinder  186  and a diagonal brake line  208  extending from the front slave cylinder  186  to the rear slave cylinder  196 . Thus, the front cylinder  186  is in fluid communication with the rear slave cylinder  196 . A second brake line  210  extends from the master cylinder  180  to the other front slave cylinder  188  and a diagonal brake line  212  extends from the front slave cylinder  188  to the other rear slave cylinder  194 . Therefore, the front and rear slave cylinders  188  and  194 , respectively, are in fluid communication with each other. 
     In hydraulic brake systems of the type wherein pairs of the slave cylinders are in fluid communication with each other, such as the systems  10   a ,  10   b , and  10   c , discussed above, a brake flushing technique known as cross-flushing can be advantageously employed. Cross-flushing is accomplished by injecting fluid into one of the two cylinders that are in fluid communication with each other so it flows through both cylinders and the associated brake line to flush that portion of the system. The advantage of cross-flushing is that the fluid flow restrictions inherent in master cylinders and ABS modulators are eliminated resulting in a significant increase in the volume and velocity of the flushing fluid to quickly and effectively purge contaminants from that portion of the hydraulic system. This same technique can be use in bleeding operations for efficiently removing air from the hydraulic system. 
     Reference is now made to FIG. 10 wherein a fragmentary portion of a second embodiment of the hydraulic servicing apparatus of the present invention is indicated generally by the reference numeral  14   a , with this embodiment being configured to accomplish the above describe cross-flushing technique on the hydraulic system  10   a . The solenoid operated two-way poppet valves  50 ,  52 ,  54  and  56  of the previously describe system  14  have been replaced by solenoid operated three-way poppet valves, three of which are shown and indicated by the reference numerals  214 ,  216  and  218 . It will be understood that each of the three-way poppet valves is connected to a different one of the slave cylinders and the following detailed description of the connection and operation of the poppet valves  214  and  216  also applies to the other poppet valves. The three-way poppet valves  214  and  216  each have a bleeder valve line  220  and  222 , respectively, extending from their first ports  224  to the bleeder valves  226  of the slave cylinders  194  and  196  of the hydraulic system  10   a . Second ports  228  of the valves  214  and  216  are connected to the manifold  62  which is in fluid communication with the first fluid port  64  of the flow reversing valve  66 . The third port  230  of each three-way poppet valve is connected to a collection manifold  232  which is in fluid communication with a first port  234  of a shutoff valve  236 , and the second port  238  of the shutoff valve  236  is connected by a fluid return line to the fluid return manifold  80 . The shutoff valve  236  is preferably a solenoid operated two-way poppet valve and its function will herein after be described. 
     A Cross-flushing operation on the two slave cylinders  194 , and  196  is accomplished with the flow reversing valve  66  in its second position (shown in solid lines in FIG. 10) so that fluid supplied by the pump  108  is directed through the flow reversing valve  66  and the manifold  62  to the second ports  228  of the three-way poppet valves  214 ,  216 , and  218  and the fourth poppet valve(not shown). The poppet valve  214  is shown in a first position wherein the first and second ports  224  and  228  are in fluid communication with each other and the third port  230  is blocked. The other three way poppet valves  216  and  218  are in the second position wherein the first and third ports  224  and  230  are in fluid communication with each other and the second ports  228  are blocked. Since the second ports  228  of all except the poppet valve  214  are blocked, the fluid supplied through the manifold  62  will flow only into the second port  228  of the valve  214  and out through its first port  224 . Fluid emerging from the port  224  of the valve  214  is directed by the bleeder valve line  220  into and through the slave cylinder  194  and through the cross-line  192  into the slave cylinder  196 . Since the master cylinder  180  has a relatively high resistance to fluid flow, fluid in the cross-line  192  will take the path of least resistance and will flow and directly and sequentially through the slave cylinders  194  and  196 . The fluid will exit the slave cylinder  196  through the bleeder valve line  222  and flow into the first port  224  of the three-way poppet valve  216 . With the poppet valve  216  in the second position, fluid will flow out through its third port  230  into the manifold  232  and through the shutoff valve  236  into the fluid return manifold  80  into the holding vessel  84 . 
     The shutoff valve  236  is open position during cross-flushing operations to provide a flow path for fluid returning from the hydraulic system being serviced. When the shutoff valve  236  is actuated to its closed position, the third port  230  of the three-way poppet valves  214 ,  216 ,  218  and the fourth valve (not shown), will be blocked and those valves will then function as two way poppet valves. Therefore, when the shutoff valve  236  is closed, the apparatus  14   a  will function in the manner hereinbefore described with reference to the apparatus  14 . 
     While the principles of the invention have now been made clear in illustrated embodiments, many modifications will be obvious to those skilled in the art which do not depart from those principles. The appended claims are therefore intended to cover such modifications within the limits only of the true spirit and scope of the invention.