Slowing and loss of force in hydraulic system

[VIPDesign]

I have a hydraulic system that slows and looses force as it 'heats' up (I use quotes because I do not suspect heat as the problem). I have isolated the pump from the rest of the system and hooked it to a pressure gage and needle valve. The pump (gear type) is capable of rated system pressure. I have not done a flow test and this test was done 'cold'. My return line from my test set-up is clear and it seems like there are air bubbles which I know will cause my symptoms, particularly if the entrained air volume increases. What I don't know is if there is air getting in or if I have the wrong viscosity oil?

Is there an easy way to detect inlet air leaks (kind of like sparaying starter fluid on an intake manifold of an internal combustion engine to find the source of intake air leaks). Is there a dye that I could subject possible entrance points to and look for its appearance in the clear hose of my test rig?

I still have to do some more tests to make sure the relief valve is not opening too soon, particularly after running for a while (aforementioned pressure test was run without relief valve).

 

[StoneCold]

Your relief valve could be the culprit but gear pumps do wear out and will "slip" oil as the viscosity drops.
A good machine shop or a gear pump repair shop should be able to tell you if your pump is still meeting it's clearance specs.
Do you have a strainer in-line. I have been bitten by them before. Make sure you take the whole basket out for cleaning.

Regards
StoneCold