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.
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 commonprecision 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
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.