primarily determined by the type of valve and the
shafts, and springs for distortion, and all check
available facilities. Some valves are not repairable.
valves for proper seating.
Replace all defective parts that are not repairable,
R e p l a c e a l l d e f e c t ive p a r t s t h a t a r e n o t
including all kit parts and cure-dated parts at each
repairable, and replace all kit parts and cure-dated
disassembly. Before reassembly, lubricate all internal
parts at each disassembly. Before reassembly,
parts with the specified type of clean hydraulic fluid.
lubricate all internal parts with the specified type of
After you reassemble a valve, test it on a test machine.
clean hydraulic fluid.
The tests normally include flow control, pressure
Because of the many different versions of pumps
settings (for relief valves and regulators), and internal
and the complexity of most piston pumps, refer to the
leakage. Consult the applicable technical manual for
applicable technical manual for repair limits,
maintenance, testing, and repair information.
procedures, and testing information. Pay particular
attention to the Source, Maintenance and
Recoverability code (SM&R code) to determine the
lowest level of repair that is authorized on these
Reservoirs are fairly simple tanks that require
periodic flushing and cleaning. Since the reservoir
Testing hydraulic pumps after repairs is a must.
the drain valve in the bottom of the tank should be
This should be done by activities that have proper test
o p e n e d t o a l l ow a ny s e d i m e n t t o b e p u rg e d .
machines. AIMD airframe hydraulic shops usually
Additionally, most reservoirs are designed with
have the correct testing machines and trained
clean-out covers, illustrated earlier in figures 8-5 and
personnel to test these pumps, along with other
8-6, to assist in inspection and maintenance.
accessories, such as relief valves, directional valves,
and actuating cylinders.
Accumulators, designed like cylinder actuators,
are similarly repaired by using the same techniques.
Maintenance of cylinders in general is relatively
Caution must be exercised to ensure that the pneumatic
simple; the most common trouble is leakage. As with
pressure has been relieved prior to disassembly of an
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
Maintenance of filters is relatively simple since it
mainly involves cleaning the filter housing and
replacing or cleaning the filter elements. Replace the
element on filters with micronic (paper) elements, and
clean the elements on filters using porous metal
Hydraulic valves, like most other hydraulic
elements. Clean the porous metal elements with
units, normally require little maintenance if the fluid
ultrasonic cleaning equipment. Completely test the
is kept clean. However, they do occasionally fail.
filters that have been cleaned and repaired before
Internal leakage and control adjustments are the
reinstalling in the system. This includes pressure
most common valve problems.
s e t t i n g o f t h e b y p a s s va l ve , o p e r a t i o n o f t h e
Generally, the maintenance of hydraulic valves
consists of disassembly, inspection, repair, and testing.
pressure tests. Consult the technical manuals for the
The amount of maintenance that can be performed is
equipment or the filter design for test information.