It is a common agricultural practice to use large mobile spraying equipment to rapidly and efficiently spray fields with various products such as herbicides, pesticides and fertilizers.  Such spraying equipment often includes a mobile unit such as a tractor equipped with a large tank for carrying the chemicals desired to be sprayed.  The chemicals are typically pumped from the tank through flow lines to a plurality of spray nozzles that are typically mounted on an elongated boom structure pulled behind the mobile vehicle.  Flow through the spray nozzles is typically controlled by mechanical spring valves that stop flow to the spray nozzles when the pressure in the flow lines falls below a predetermined level. 
A problem with such mechanical spring valves is that they typically do not have the capability of instantaneously stopping and starting a uniform discharge through the spray nozzles.  This lack of control is particularly problematic at the ends of fields where spraying must be temporarily stopped and restarted as the spraying equipment turns to make subsequent passes across the field.  To overcome the lack of control provided by conventional spring valves, attempts have been made to utilize pneumatically actuated valves for controlling flow through the spray nozzles.  Two different types of pneumatic valves have been utilized: normally open; and normally closed.  A normally open valve uses a heavy spring to hold the valve open, and relies on air pressure to push the valve closed.  By contrast, a normally closed valve uses a heavy spring to hold the valve closed, and relies on air pressure to open the valve. 
A variety of problems exist with respect to the above identified valves.  For example, one problem relates to such valves' dependence upon a source of compressed air such as an air compressor.  If the air compressor fails, a normally open valve will have no way of preventing the undesirable leakage of chemicals from the spraying equipment.  By contrast, if the air compressor fails in a system utilizing normally closed valves, the spraying equipment is completely inoperative. 
In addition to the above problems, known valve arrangements typically have a sealed piston that slides within the valve.  Typically, displacement areas within the valve are vented to the outside atmosphere to eliminate the compression of gasses within the valve as the piston moves.  In dusty environments such as those encountered by agricultural sprayers, a valve that vents to atmosphere rapidly becomes contaminated with dirt.  Such dirt both damages the piston seals and increases friction within the valve.  Also, the pistons are commonly lubricated with grease that rapidly entraps field dust and further intensifies the problem of contamination induced friction and wear.