Posted by Owen Aaland on September 11, 2009 at 17:31:08 from (207.13.114.55):
In Reply to: 656 Hydro pressure posted by theponyroper on September 11, 2009 at 05:42:07:
Most likely your problem is scoring on the brass and steel plates for both the hydro pump and motor. Scoring allows oil to get between the plates and force them apart causing loss of oil pressure. They are highly machined surfaces to cleanly seal the oil as the plate turns and blocks off the holes the oil flows through.
Scoring on the hydro motor plates can cause the difference in pressure. The pump always turns in the same direction so the pressure loss should not change between forward and reverse. The motor reverses direction for reverse so it is possible to have different pressures before the plates separate.
The high pressure relief valves are pilot operated, controlled by the Foot-N-Inch valve (clutch pedal). Generally the valve will fail because of scoring and stick open. It is unlikely there is anything wrong with them. A cracked pressure line will surely cause a loss of pressure in one direction. It would be unusual to have both line cracked. Leaking check valves can also cause loss of pressure but there again it would be unusual for both of them to have failed. Both relief valves and both checks valves are the same for forward and reverse so it won't hurt to switch them. A positive check for cracked lines can be made by capping the fitting at the relief valves. You wont be able to get a pressure reading but if you drive it you can see if it makes a difference. WARNING!!! IF YOU DO THAT THE ONLY CONTROL OF THE HYDRO IS THE FORWARD/REVERSE LEVER.
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