TM 5-3810-300-24 & P2
2. If the system has been partially drained of oil: (valve, pump, reservoir, lines but not cylinder).
a. Fill reservoir to recommended level.
b. Start engine and run at idle, adding oil to reservoir to closely maintain recommended oil level.
c. After several minutes, steer gently and slowly from stop to stop at first with engine idling and as the oil clears at
moderately higher speeds.
d. When the oil clears, adjust oil to recommended level and secure system.
1. Loosening fitting connections at the cylinder or valve generally does not aid in air elimination from the circuit.
2. It is always desirable to have the cylinder fitting connections on top to aid in purging the air from the system. If
necessary, and if possible, rotate the cylinder ports to the top for bleeding. This may be helpful in some stubborn
3. When a system contains a large amount of air, as during the filling cycle, an attempt to steer will result in the pump
building up pressure in the system which compresses the air. Several interesting, and sometimes disagreeable, things
may result from this air in the circuit:
a. There will be a time delay in building up pressure due to the compression of the air and this will show up as a
"hump" or "lump" when rapidly changing steering directions.
b. Rapid steering action results in pressure surges and may cause overflow of the oil to spout from the reservoir in
c. If the pump is stopped, oil may also gush from the reservoir due to decompression of the air.
d. Attempts to steer with large amounts of air present may result in erratic action or "chatter".
4. In general, when filling the circuit, try to avoid churning small air bubbles into the system by keeping enough oil in the
reservoir so the pump does not suck in air and pump it along with the oil. Small bubbles require the longest bleeding
5. If the system will not bleed itself and the oil remains aerated, the suction line from the pump may be the cause of the
trouble due to a leak, (broken, cracked line, loose fittings, etc.).
6. Always fill the reservoir to the level recommended.
In some pumps the pressure relief valve and the flow control valve are separate, and in some, these valves are in a single
unit. These valves must be free of dirt and stickiness. Check for leaky pump shaft seal (causes oil aeration).
A hydraulic pump requires finely machined surfaces and closely fitted mating parts to maintain sealing for pumping.
Check for scoring or damage to teeth, rotor and vanes. Check clearances.
Follow service manual for the specific make and model of pump being serviced.