Pre-filled power hydraulic system

A motor vehicle has a manually operated change speed gearbox driven by an internal combustion engine through a clutch which is automatically disengageable and re-engageable by operation of a power hydraulic clutch control. The hydraulic control comprises a fluid reservoir connected by a conduit to a control valve connected to the reservoir by a conduit which includes a pump and a one-way valve and a pressure accumulator and a piston and cylinder or fluid pressure operated actuator connected by piping to the control valve. The clutch is operated by the actuator causing disengagement of the clutch when the control valve is operated to relieve fluid pressure in the actuator. The hydraulic control is assembled and filled with hydraulic fluid at some place which can be distant from the vehicle manufacturer's production line. The filled control is transported to where the vehicle is being made and the control is installed in the vehicle; possibly after the filled control has been stored first.

This invention relates to a power hydraulic system. 
The power hydraulic system is of the type (hereinafter called "the type 
referred to") comprising a reservoir of hydraulic fluid, a control valve, 
conduit means connecting the reservoir with the control valve whereby 
hydraulic fluid may be supplied to the valve and returned therefrom to the 
reservoir, and said conduit means including therein a pump and an 
accumulator for the storing therein of hydraulic fluid under pressure 
delivered by the pump from the reservoir and for the supply of hydraulic 
fluid from said accumulator to the control valve. 
In a power hydraulic system of the type referred to the control valve has 
service porting which can be connected by piping to an actuator operated 
by hydraulic pressure applied to said actuator, and the control valve has 
at least first and second states such that the valve in said first state 
connects the actuator to the supply of hydraulic fluid under pressure and 
in the second state the valve connects the actuator to the reservoir 
whereby hydraulic fluid can be returned to the reservoir from the 
actuator. 
A semi-automatic transmission for a motor vehicle is known for driving at 
least one ground running wheel of the vehicle, using rotary output from an 
engine of the vehicle, the semi-automatic transmision being of the kind 
(hereinafter called "the kind referred to") comprising a change-speed 
gearbox having a rotary output shaft for supplying rotary motion to said 
ground running wheel and a rotary input shaft for receiving input torque 
from the engine and a plurality of gear ratios between said shafts each 
gear ratio being engageable and disengageable in response to operation of 
gear ratio selector means in response to manual action of a driver of the 
vehicle, clutch means engageable for transmitting torque to said input 
shaft from the engine and disengageable to interrupt the transmission of 
said torque to the input shaft, and clutch control means responsive to a 
first signal indicative of a wish by the driver to disengage the currently 
engaged said gear ratio and responsive to a second signal indicative that 
another said gear ratio has been engaged, and said clutch control means 
responding to said first signal to cause the clutch means to disengage and 
responding to said second signal to cause the clutch means to re-engage. 
In a known semi-automatic transmission of the kind referred to the clutch 
control means comprises a power hydraulic system of the type referred to 
in which the control valve is connected by said piping to the actuator so 
that on occurrence of said first signal the control valve goes to said 
first condition and the actuator receives hydraulic fluid under pressure 
so that the actuator operates to cause disengagement of the clutch means, 
and on occurrence of said second signal the control valve goes to said 
second condition permitting return of hydraulic fluid to the reservoir 
from the actuator in which latter the hydraulic pressure drops to cause a 
re-engagement of the clutch means. 
The actuator may be a piston and cylinder unit. 
When building a vehicle comprising a semi-automatic transmission of the 
kind referred to the hydraulic clutch control means comprising the 
combination of said power hydraulic system of the type referred to and the 
piping and actuator is mounted in the vehicle on the production line, for 
example by installing the power hydraulic system and the actuator, and 
connecting to them the piping. Then the combination is filled with 
hydraulic fluid. This may be done by connecting a vacuum source to the 
combination to remove the air and then connecting the evacuated 
combination to a supply of hydraulic fluid to fill the combination with 
the fluid. This manner of providing the said combination has 
disadvantages. Firstly it requires reliable simultaneous supplies to the 
production line of the various component parts of the combination to be 
connected together on the line. Secondly the connecting together of those 
component parts on the line can be time consuming and increase the time it 
takes to manufacture the vehicle on the production line. Thirdly the 
filling of the combination with hydraulic fluid on the production line 
also extends the production time on the line. Fourthly should the 
assembled and filled combination be defective, the reason for this can be 
the subject of dispute between the supplier of the constituent parts of 
the hydraulic clutch control means and the vehicle manufacturer. 
According to a first aspect of the invention a power hydraulic system of 
the type referred to and intended to be mounted in a motor vehicle is 
characterised in that the said system is assembled and filled with 
hydraulic fluid which is retained within the system during subsequent 
transportation and/or storage prior to the filled system being mounted in 
the vehicle. 
According to a second aspect of the invention a power hydraulic system 
formed according to the first aspect is provided in combination with an 
hydraulic actuator and piping for conveying hydraulic fluid between the 
actuator and service porting of the control valve, and said piping and 
actuator are filled with hydraulic fluid which is retained therein during 
subsequent transportation and/or storage prior to the filled piping and 
actuator being mounted in the vehicle. 
According to a third aspect of the invention the combination according to 
the second aspect is connected together and filled with hydraulic fluid 
which is retained in the combination during subsequent transportation 
and/or storage prior to the filled piping and actuator being mounted on 
the vehicle. 
According to a fourth aspect of the invention there is provided a motor 
vehicle having a semi-automatic transmission of the kind referred to 
wherein the hydraulic clutch control means comprises the combination 
provided according to the second or third aspects of the invention. 
The combination according to the third aspect of the invention avoids or 
considerably mitigates the aforesaid disadvantages at least some of which 
can be avoided or at least mitigated by arrangement in accordance with the 
first or second aspect .

In the drawings the same reference numerals indicate like or comparable 
parts. 
FIG. 1 shows a motor vehicle 2 having rear road wheels 4 driven via a 
propellor shaft 6 connected with an output shaft 8 of a change-speed 
gearbox 10 in which a plurality of gear ratios are each respectively 
engaged, or disengaged in response to manual operation, by the vehicle's 
driver, of a gear ratio selector device 12, in this example in the form of 
a gear lever, from which signals, in this example in the form of movement, 
are transmitted via a connection or linkage 14 to gear ratio change means 
(known per se) in the gearbox. On the other hand any other form of 
manually operable gear selector device may be used from which gear ratio 
disengage and engage signals can be transmitted to gear ratio change means 
in the gearbox. With reference also to FIG. 2, driving torque is input to 
the gearbox 10 through an input shaft 16 driven via a clutch 18, in a 
housing 20, by an internal combustion engine 22. As an alternative the 
transmission 10, 18 may drive front road wheels 24 or the front and rear 
road wheels 24 and 4. Turning to FIG. 2, the clutch 18 in this example is 
a push-type diaphragm spring clutch known per se having a cover 26 secured 
to a fly-wheel or other counter pressure plate 28 rotated by the engine 
22. The clutch includes a pressure plate 30 acted on by diaphragm spring 
32 and a driven plate 34 with an internally splined central hub engaging 
splines on the gearbox input shaft 16. The clutch also has release bearing 
35 acted on by a release fork or lever 36 pivoted at 38 and also at 40 to 
a thrust rod 42 of an hydraulic actuator 44 (see FIGS. 3 and 4) in an 
hydraulic control 46. Operation of the hydraulic control 46 is under 
control of an electronic control 48 comprising computer means connected by 
signal paths 50 and 52 to the hydraulic control. One path 50 instructs 
operation of the hydraulic control 46 to cause dis-engagements and 
re-engagements of the clutch 18, and the other path supplies electonic 
control 48 with information on the extent of clutch engagement and 
disengagement. Electronic control 48 is also connected by two signal paths 
54 and 56 to the gearbox 10 and the gear lever 12 respectively. The 
information in the signal on line 54 indicates when a gear ratio is 
engaged and when no gear ratio is engaged. Information in the signal on 
line 56 indicates when the driver desires or does not desire to change 
gear. Gear lever 12 comprises a shaft 58 arranged to pivot universally at 
60. A tube 62 surmounted by a hand knob 64 is pivotably mounted at 66 on 
the shaft 58 so that the tube can wobble or rock slightly relatively to 
the shaft. Such wobbles caused by the driver grasping the knob 64 causes 
switch or transducer means 68 to initiate a signal on line 56 indicating 
the driver's wish to change gear. In response the controls 48 and 46 
function causing clutch 18 to disengage automatically. Further manual 
movement of the knob 64 causes the shaft 58 to move to cause operation of 
the gear ratio change means to cause disengagement of the hitherto engaged 
gear ratio and the engagement of another gear ratio. That engagement 
produces a signal on line 54 causing the controls 48 and 46 to function so 
that the clutch 18 is re-engaged automatically. 
With reference to FIGS. 3 and 4, the hydraulic control 46 comprises a 
control valve 70 having an inlet port 72 connected by a conduit 74 to an 
hydraulic fluid reservoir 76 of plastics which is connected by another 
conduit 78 to another port 80 of the control valve. That valve also has a 
service port 82 connected by piping 84, which may be flexible, to the 
actuator 44 formed by a cylinder 86 in which slides a piston 88 acted on 
by spring 90 and acting on the thrust rod 42 sliding in a sealing boot 92. 
The actuator 44 also includes a position sensor (in the form of a 
transducer) to provide the signal on line 52 (FIG. 2) and comprising an 
electrical inductance coil 94 containing a sliding ferro-magnetic core 96 
on the piston. 
Conduit 78 opens out to the interior of the reservoir 76 through a filter 
98. Other filter means (not shown) may be provided between the reservoir 
interior and the inlet to conduit 74. The reservoir has a filler cap 100. 
Conduit 74 includes a pump 102 driven by an electric motor 104 integral 
therewith and an accumulator 106 known per se comprising a sturdy vessel 
internally divided by a tough, flexible, resilient membrane 108 into the 
chamber 110 for hydraulic fluid and chamber 112 containing gas under high 
pressure. The conduit 74 also includes a one-way valve 114 comprising a 
ball 116 urged by spring 118. The conduit also includes at least one 
pressure transducer 120 signalling via line 52 (FIG. 2), which may be a 
data bus, the fluid pressure prevailing in the chamber 110 so that the 
electronic control 48 (FIG. 2) operates the motor 104 (when the vehicle 
ignition system is switched on) to drive the pump 102 to raise hydraulic 
pressure in chamber 110 when that pressure is lower than a predetermined 
minimum but stops the motor to halt pumping when the pressure in chamber 
110 attains a pre-determined maximum. On the other hand there may be two 
pressure transducers 120 and 122 arranged to observe or detect attainment 
of said minimum and maximum pressure values respectively. 
The control valve 70 can be a sliding spool valve actuated by an integral 
electric motor 124 in the form of a solenoid actuator when the solenoid 
124 is energised, in response to a signal on line 50 (FIG. 2), the control 
valve 70 adopts a first condition closing off the port 80 and connecting 
the inlet port 72 with the service port 82 so that hydraulic fluid under 
pressure from the accumulator 106 is applied to the actuator 44 to 
disengage the clutch 18 (FIG. 2). When the solenoid 124 is de-energised, 
either in response to signals from the control 48 or if the ignition is 
switched off, the control valve 70 adopts a second condition closing off 
port 72 and connecting the service port 82 with the outlet port 80 whereby 
hydraulic fluid from the actuator 44 can return to the reservoir 76 via 
the conduit 78 as the clutch 18 (FIG. 2) re-engages under the effect of 
the resiling diaphragm spring 32 which acts to push the piston 88 to the 
left in FIG. 4. 
Initially the power hydraulic control 46 described and shown in FIGS. 3 and 
4 is formed as a complete module in which the control valve 70 is 
permanently connected to the reservoir 76 and to the actuator 44 by the 
conduit 74 and 78 and the piping 84 at an assembly station in a workshop. 
The assembled control 46 can then be transferred to a filling station in 
the workshop where the filler cap 100 can be removed from the filler neck 
126 which is then connected to a hose selectively connectable to either a 
source of vacuum or to a supply of hydraulic fluid. With the solenoid 124 
de-energised the port 82 is connected to the port 80 and firstly vacuum is 
applied to evacuate the combination comprising the reservoir 76, pump 102, 
chamber 110, cylinder 86 and conduits and piping 74, 78 and 84. Then the 
supply of hydraulic fluid is turned on and the combination is filled with 
fluid to the extent desired. During filling the control valve 70 can 
continue to connect the ports 80 and 82, or the solenoid 124 may be 
energised to connect ports 72 and 82. Immediately after filling, the 
capacity of the chamber 110 may be substantially its normal minimum 
capacity. 
Alternatively the hydraulic control 46 can be provided with sealable bleed 
ports or nipples which are initially opened to allow hydraulic fluid 
introduced through the neck 126 to fill the control. Whereafter the bleed 
ports are closed. 
During filling of the control 46 with hydraulic fluid, the piston 88 may be 
held in any desired position in the cylinder 86. For example, if the 
piston 88 is held in FIG. 4 at its furthest travel to the right the 
cylinder 86 has maximum capacity. Then after filling, the piston 88 can be 
moved to the left in FIG. 4 to displace excess fluid from the reservoir 76 
through neck 126. 
After filling, the cap 100 is replaced The filled control 46 may be taken 
to a testing station in the workshop where the pump 102 can be operated 
and the control valve 70 actuated to simulate clutch engagement and 
dis-engagement, and the operation of transducers 120 and 122 and 94, 96 
checked and the whole examined for leaks. Thereafter the filled control 46 
can be stored ready for supply to a vehicle manufacturer or transported to 
the vehicle manufacturer's premises ready for installation in the 
pre-filled state in a vehicle. 
In a modification the piping 84 can be divided into two portions 84a and 
84b each provided with a respective coupling part 128a and 128b 
connectable together to form the coupling 128 known per se. Each coupling 
part 128a, 128b includes a coupling valve 128c, 128d, respectively 
automatically closeable to seal off the piping portions 84a and 84b when 
the coupling parts are disconnected. When the coupling parts 128a and 128b 
are connected this act applies pressure to the valves therein opening them 
to communicate pipe portion 84a with pipe portion 84b. 
When a said coupling part 128a is not connected to a said coupling part 
128b the hydraulic control 46 is divided into two hydraulic control parts 
46a and 46b. The hydraulic control part 46a comprises the reservoir 76, 
the conduits 74 and 78, the control valve 70, the piping portion 84a, and 
the coupling part 128a. The hydraulic control part 46b comprises the 
coupling part 128b, the piping portions 84a, and the hydraulic actuator 
44. 
With the hydraulic control parts 46a and 46b disconnected one from the 
other when their coupling parts 128a and 128b are separated, it is 
possible to fill the control part 46a with hydraulic fluid via the filler 
neck 126. It is also possible to fill the actuator 44 and the piping 
portion 84b of the control part 46b with hydraulic fluid through the 
coupling part 128b after applying valve opening pressure to open its valve 
so that vacuum can first be applied to evacuate the interior of the 
control part 46b via the coupling part 128b and then the evacuated control 
part 46b is filled with hydraulic fluid; thereafter the valve opening 
pressure is released to allow the coupling part 128b to seal off the 
piping portion 84b. 
A filled hydrdaulic control part 46a and a filled hydraulic control part 
46b can be connected together at any time to form a pre-filled complete 
hydraulic control 46. For example the coupling part 128a of a filled 
control part 46a can be connected to a coupling part 128b of a filled 
control part 46b, for example at the vehicle manufacturer's premises, for 
example as the hydraulic control 46 is being installed in a vehicle. 
One or more control parts 46a can be filled, stored or transported 
seperately from any control part 46b. Thus any filled control part 46a can 
eventually be connected to any filled control part 46b. This can be the 
case even if a control part 46a and a control part 46b are connected 
together by their coupling 128 to form a control 46 which is then 
pre-filled with hydraulic fluid and thereafter the coupling parts 128a and 
128b disconnected so that, if desired, the filled control part 46a can be 
subsequently connected to a different filled control part 46b and vice - 
versa. 
In the modification in FIG. 7, the complete hydraulic control part 46 
comprises two hydraulic control parts 46c and 46d connected together by a 
fluid-tight coupling 228 formed by two connectable coupling parts 228a and 
228b each mounted on an end of a respective piping part 84a or 84b. 
Initially the control parts 46c and 46d are separate due to the coupling 
parts 228a and 228b not being connected together. To seal off the ends of 
the piping parts 84a and 84b at the coupling parts 228a and 228b each 
coupling part is formed with an intact but breakable fluid-tight seal, for 
example a rupturable membrane. The control part 46c can be filled with 
hydraulic fluid in like manner to the control part 46a as described above. 
In the control part 46d, the actuator 44 is provided with an openable and 
closable valve or nipple 87 which can be connected to a hose selectively 
connectable to either a source of vacuum or to a supply of hydraulic 
fluid. With the nipple 87 open the hydraulic control part 46d is evacuated 
and then filled with hydraulic fluid, and then the nipple is closed and 
disconnected from the hose. 
A filled hydraulic control part 46c can be stored and/or transported 
separately from a filled hydraulic control part 46d. The coupling parts 
228a and 228b are so arranged that when they are connected together, the 
act of connection causes breakage of the seals in the coupling parts thus 
allowing communication between the piping parts 84a and 84b. This 
connection can be accomplished at any location, for example the vehicle 
manufacturer's premises, between any filled control part 46c and any 
filled control part 46d. The connection of the coupling parts 228a and 
228b can be permanent in the sense that they are at least difficult to 
inadvertently disconnect. 
A problem of leakage of hydraulic fluid might arise during handling, 
storing and transporting the pre-filled hydraulic control 46 on its own 
(or the pre-filled control part 46a or 46c) if the reservoir 76 is not 
effectively sealed. And yet an air vent in the reservoir or in the cap is 
desirable to allow escape of gas from the reservoir to prevent a build up 
of gas pressure therein. Such gas can reach the reservoir 76 for example 
via the conduit 78 from the control valve 70, after seeping passed the 
membrane 108 from the accumulator chamber 112. One way to avoid leakage of 
hydraulic fluid through the vent is to provide the latter with a removable 
fluid tight seal or cover which is manually removed at the time of 
installing the hydraulic control 46 in a vehicle. 
Unfortunately it is possible to forget to remove a manually removable seal. 
To avoid this an automatically displaceable seal 130 is provided. The seal 
130 comprises a generally upright spindle 132 having two spaced pistons 
134 and 136 fast therewith and a handle 138 at one end. As shown in FIG. 
3, the piston 136 can make a fluid tight seal with a cylindrical wall 140 
of a passage 142 through an upper wall 144 of the reservoir 76. 
Simultaneously the lower piston 134 makes a fluid tight seal with a 
cylindrical wall 145 of a passage 146 through an internal partition 148 in 
the reservoir 76. The partition 148 defines a chamber 150 which, except 
for the passage 146, is in fluid tight isolation from the remainder of the 
reservoir interior. An outlet passage 152 leads from the reservoir 76 to 
the conduit 74 (FIG. 4). Initially the seal 130 is displaced downwards so 
that the handle 138 catches in a non-gastight manner, on the top of the 
wall 140. In this position the piston 134 is at A so that the passages 142 
and 146 are open, the pistons 136 and 134 having been displaced therefrom. 
The hydraulic control 46 (or the control part 46a or 46c) is then 
pre-filled as described above. After filling the control 46 (or the 
control part 46a or 46b) with hydraulic fluid, the sealing device 130 is 
pulled up manually to seal off the passages 142 and 146 as shown in FIG. 
3. Now the interior of the reservoir 76 is completely sealed off from the 
external atmosphere. An abutment 153 may be provided on the spindle 132 to 
abut against the underside of wall 140 to prevent excessive lifting of the 
device 130. To prevent the sealing device 130 being inadvertently 
displaced, the handle 138 and upper end of the spindle 132 are then 
enclosed by a cup-shaped cover 154 which may be welded or otherwise 
adhered or connected in a fluidtight manner to the top of the reservoir. 
In its side wall the cover 154 has an air vent 156. When the pre-filled 
hydraulic control 46 is eventually installed in a motor vehicle and the 
vehicle ignition is then switched on, the electronic control 48 starts the 
pump 102 operating. This creates suction in the chamber 150 and the 
resultant pressure differential causes the sealing device 130 to be 
automatically displaced downwardly to open the passages 142 and 146. 
In FIG. 3 it will be seen that piston 134 has a through passage 158 between 
its end faces. When the device 130 is providing its sealing function and 
the pre-filled hydraulic control 46 (or the control part 46a or 46c) is in 
store or being transported, an increase in temperature can cause thermal 
expansion of the fluid in conduit 74 (FIG. 4). This increase in fluid 
volume can escape from the chamber 150 through the passage 158 into the 
remainder of the reservoir 76. 
In the embodiment in FIG. 5, the filler cap 100 snap-fits on the neck 126 
and retains in a gastight manner a very supple flexible diaphragm 170 of 
an impermeable, elastic material, for example a rubber, which in this 
example is in the form of bag having a bellows or concertina shape. The 
top wall 144 of the reservoir 76 has a hole 172 therethrough. A 
cylindrical neck 174 united in a gastight manner with the wall 144, 
externally of the reservoir 76 surrounds the hole 172. A circular, 
flexible diaphragm 176 of elastic material, for example a rubber, is 
clamped in a gastight manner against the end of neck 174 by a cap 178 
snap-fitted onto the neck. This cap has a hole 180 therethrough initially 
covered in a fluidtight manner by a manually removable seal arrangemenr 
182, for example foil adhered to cap 178. Because the cap 100 is being 
used in conjunction with the bellows 170, the cap 100 in FIG. 5 has an 
air-vent 184 therethrough. It will also be seen from FIGS. 5 and 6 that 
the diaphragm 176 has a rectilinear slit 186 therethrough, the slit having 
effectively no width so that its opposite edges are always in a gastight 
sealing contact except when the pressure differential across the diaphragm 
176 exceeds a pre-determined value sufficient to so distort the diaphragm 
that the edges of the slit 186 separate. Effectively the diaphragm 176 is 
a two-way valve. 
The hydraulic control 46 in FIG. 5 may be pre-filled with hydraulic fluid 
using the neck 100 or 174 to give access to the reservoir interior and 
then replacing the cap 100 and bellows 170 or the cap 178 and diaphragm 
176 accordingly, after filling. Alternatively a filling passage 188 may be 
provided, into which a fluid tight plug 190 is fitted after filling the 
control 46 with hydraulic fluid. 
Once the pre-filled hydraulic control 46 is installed in a vehicle, the 
seal arrangement 182 is removed to uncover the hole 180. 
A purpose of the arrangement of the diaphragms 170 and 176 in FIG. 5 is to 
try and restrict ingress of new moisture laden air into the reservoir 76, 
because water deteriorates the performance of hydraulic fluid. Under 
normal running conditions the pump 102 raises the hydraulic fluid pressure 
in the accumulator 106 from the predetermined minimum to the predetermined 
maximum and the volume of hydraulic fluid in the reservoir 76 decreases; 
whereas whilst the hydraulic pressure in the accumulator is falling 
towards the pre-determined minimum because the control valve 70 has been 
operated and fluid has been returned via conduit 78 to the reservoir, the 
volume of hydraulic fluid in the reservoir increases. The bag shaped 
diaphragm 170 embraces or defines sufficient internal volume that its 
volume can change to adequately compensate for the increase and decrease 
in the volume of fluid in the reservoir 76 during normal running. 
Furthermore the diaphragm 170 is so supple whereas the diaphragm 176 is 
sufficiently stiff that during the aforesaid normal running the change in 
the volume of fluid in the reservoir is accomodated by the diaphragm 170, 
and the diaphragm 176 is not distorted by any pressure gradient 
thereacross to an extent which would open the slit 186. Also whilst the 
seal 182 is still in position and the pre-filled control 46 is being 
stored or transported prior to use in a vehicle, changes in the ratio of 
the volume of air to the volume of fluid in the reservoir (due to thermal 
expansion or contraction of the fluid) can be accommodated by changes in 
the volume bounded by the diaphragm 170. pressure. 
When the vehicle provided with the hydraulic control in FIG. 5 stands 
stationary for several hours, with the ignition turned off, hydraulic 
fluid from the accumulator 106 is progressively substantially completely 
discharged therefrom due to expansion of the gas chamber 112 (see FIG. 4). 
This discharged fluid makes its way to the reservoir 76 and considerably 
increases the volume of fluid in the reservoir. This so increases the air 
pressure in the reservoir 76 that the pressure gradient across the 
diaphragm 176 distorts it and opens the slit 186 to relieve the gas 
pressure Should the vehicle ignition now be switched on, the pump 102 acts 
to considerably decrease the volume of fluid in the reservoir 76 whilst 
the fluid pressure in the accumulator 106 is being raised to the 
predetermined maximum value. This decrease in the volume of fluid 
decreases the gas pressure in the reservoir 76 so that the pressure 
gradient across diaphragm 176 can be sufficient to open the slit 186 to 
raise the gas pressure in the reservoir 76. 
If desired the hydraulic control 46 in FIG. 3 may be provided with a 
two-way valve as described with reference to FIG. 5 and signified in FIG. 
3 by the cap 178. In addition or as an alternative, the reservoir 76 in 
FIG. 3 may have a diaphragm 170 as disclosed in FIG. 5.