Check valves require little attention over long periods of time. Leakage may be caused by the presence of a tiny particle of foreign matter between the checking device (ball, cone, or poppet) and its seat. To remove the foreign matter, it is necessary to remove the valve from the aircraft and completely disassemble the valve. If no scratches are found on the valve seat or the checking device, wash all parts in clean hydraulic fluid of the same type as that used in the system.

While the valve is disassembled, inspect the housing and the checking device for evidence of corrosion. Replace the valve if there is corrosion or excessive roughness. A slightly rough surface can be smoothed by buffing. A cone-type check valve may have a tendency to lean to one side, in which case the movable part may dig into the soft aluminum body of the housing and stick there.

When installing a check valve, remember that the arrow marked on the housing must point in the direction of the flow of the fluid through the valve. Before removing a check valve from a line, it is good practice to mark the adjacent structure, indicating the direction in which the arrow points. Also, observe the following precaution during installation of check valves: Grip the wrench flats of the check valve at the end to which the connecting tubing is being installed. Do not grip the opposite end. This will prevent the possibility of distorting the valve body, causing the valve to leak.

Sequence valves are used to control a sequence of operations; they ensure that actuating units operate at the proper time and in the proper sequence. Sequence valves may be mechanically operated or pressure-operated valves. An example of the use of a sequence valve is in a landing gear actuating system. In a landing gear actuating system, the landing gear doors must open before the landing gear starts to extend. Conversely, the landing gear must be retracted before the doors close. A sequence valve installed in each landing gear actuating line performs this function.

Sequence valves may be installed in one or both cylinder lines of an actuating system, depending upon the type of action desired. A direct line will go to the first unit to be operated, and a branch line goes from the sequence valve to the second unit.