Hello, I have a Ford 3400 and am having problems with the rear hydraulic lift. It only rises a couple inches and work sporadically. I performed a hydraulic pressure test as outlined in the I&T shop manual. I removed the top accessory plate and rotated it 180 degrees and made a block off gasket. When I start the tractor and slowly close off the added return line the pressure goes up to about 2000 psi for about 15 seconds and then starts to fluctuate all the way down to zero. Within a minute the pressure drops to zero and stays. If I open the return line and start the process over the same thing happens again. Does this sound like a faulty relief valve or perhaps something else? Thanks