When the system pressurizes, the check valve closes, and the regulator is cut out; therefore, a leaking check valve does not effect the cutout time of the regulator, but it does affect the cut-in time.

The purpose of the check valve is to trap fluid under pressure in the system during the regulator cutout operation; however, it cannot do this if there is leakage around the seat. Even a slight leak around the valve seat causes the regulator to cut in faster than it should, but a bad leak causes the regulator to cycle rapidly (chatter). This rapid cycling, as indicated on the system pressure gauge, is usually caused only by a leaking valve. Thus, a leaking check valve gives normal regulator cutout and faster than normal cut-in operation.

The regulator bypass valve may also leak, causing an indication that affects the cycle of the regulator. If the bypass leaks, part of the fluid from the pump, which should be going into the system, bypasses and returns to the reservoir. This bypass causes the regulator to take longer than usual to cut out. Once the regulator has cut out, the bypass opens; therefore, it does not affect the regulator cut-in cycle.

Maintenance

Hydraulic systems maintenance includes servicing, preoperational inspections, periodical (scheduled) inspections, repair, and test/check following repair. The key to hydraulic system dependability is the attention given to the cleanliness of the repair facilities. Externally introduced contaminants are credited for more component failure than any of the self-induced contaminations during normal operating conditions. Hydraulic contamination is discussed in great length later in this chapter. The various repair procedures for the more common hydraulic system components are addressed in the paragraphs below.

HYDRAULIC PUMPS. - All hydraulic pumps have one thing in common - precision construction. In general, damaged or worn pump parts should be replaced, as they do not lend themselves readily to repair; however, some manufacturers do allow restoration of sealing surfaces to their original flat plane if it can be done by lapping. Also, very minor scratches, scoring, and corrosion can be removed with a crocus cloth.

Generally, the maintenance of hydraulic pumps consists of disassembly, inspection repair (including replacement of parts and reassembly), and testing. After disassembly, thoroughly clean and critically inspect all parts for nicks, cracks, scratches, corrosion, or other damage that might cause pump malfunction. Inspect all threaded parts and surfaces for damage; inspect pistons, piston shafts and springs for distortion, and all check valves for proper seating. Replace all defective parts, and before reassembly, lubricate all internal parts with the specified type of clean hydraulic fluid.

Because of the many different versions of pumps and the complexity of most piston pumps, refer to the applicable technical manual for repair limits, procedures, and testing information.

The test after repair of hydraulic pumps is a must. This should be done by activities that have proper test machines. Hydraulic shops usually have the correct testing machines and trained personnel to test these pumps along with other accessories, such as relief valves, selector valves, and actuating cylinders.

ACTUATORS. - Maintenance of cylinders in general is relatively simple-the most common trouble is leakage. As with all other hydraulic units discussed in this chapter, consult the technical manual for the specific cylinder for all maintenance information.

Maintenance of hydraulic motors is generally the same as that discussed earlier for hydraulic pumps.

HYDRAULIC VALVES. - Hydraulic valves, like most other hydraulic units, normally require little maintenance if the fluid is kept clean; however, they do occasionally fail. Internal leakage and control adjustments are the most common valve problems.

Generally, the maintenance of hydraulic valves consists of disassembly, inspection, repair, and testing. The amount of maintenance that can be performed is primarily determined by the type of valve and the available facilities. Some valves are not repairable; in this case, return them to supply or scrap the valve and install a new one.

Replace all defective parts that are not repairable, including all kitted parts and cure-dated parts at each disassembly. Before reassembly, lubricate all internal parts with the specified type of clean hydraulic fluid After you reassemble a valve, test it on a test machine. The tests normally include flow control, pressure settings (for relief valves and regulators), and internal leakage. Consult the applicable technical manual for maintenance, testing, and repair information.

RESERVOIRS. - Reservoirs are fairly simple tanks that require periodic flushing and cleaning. Since the reservoir collects much foreign material contaminants in the bottom, the drain valve in the bottom of the tank should be opened to allow any sediment to be purged.