A Pilot is the airspace behind the piston or diaphragm in a valve, which is vented to open the valve. For maximum performance, its volume should be small to provide fast response as well as being large enough to minimize compression of the pilot volume. Its is usually exhausted by means of a built-in solenoid or an external pilot valve.

Considerations on deciding on the proper pilot volume includes the size of the pilot valve, the ratio of the piston being piloted, and the valve style, a barrel sealer or chamber sealer.

A barrel sealer piston valve with a large OD and a small seat diameter can provide a piston that acts much like a hydraulic valve where the speed of motion is directly related to the rate the pilot is vented. A small pilot volume and a large fast high flow pilot valve provides good performance with this style piston.

A chamber sealer piston requires a small pilot volume and high flow fast pilot to open them quickly. Sprinkler valve or other high flow fast pilots are recommended.

On a low leakage barrel sealer piston with a seat near the diameter of the piston in a barrel sealing configuration, high performance can be achieved with a smaller pilot valve such as a blowgun and a larger pilot volume to reduce compression of the pilot air volume when the piston opens. The narrow ratio pistons remain closed until the pilot pressure is very low due to the small area exposed to the chamber pressure while closed. This reduces the initial amount of air pressure in the pilot when the valve cracks open. When the valve pops open, with a larger pilot volume, the pressure rise in the pilot is low, so the piston pops open and remains open without requiring a large flow pilot. Low piston friction and low EQ bleed flow is required to get this high performance. Floating o ring seals are recommended for narrow ratio pistons. Too large of a pilot volume in this style of pilot arrangement does increase the time from when the trigger is pulled to when the pilot pressure is low enough to unseat the main piston. This time increases the amount of time the chamber has to bleed down through the piston EQ port into the pilot. A low flow EQ port is recommended as well as a moderate pilot volume to reduce pilot pressure rise as the piston opens.

See the section on piston valves for more information on piston valve configurations.