A few years ago I drained, flushed, and refilled the hydraulic system with universal trans-hydraulic. Lift system worked poorly afterwards, with a lot of oil foaming. Suspecting pump seals, (and that I should have left well enough alone,) I put new bearings and seals in the (vane type) pump, which didn t help. As I recall, the pump internals looked quite good. I tried switching to straight hydraulic fluid, no difference. After some time, the oil foaming stopped, but now the lift works while the oil is cold, but won t lift anything heavy once warm. When it is working, it seems just fine. I later realized that it probably just took a long while for the air to work out after draining it. By the way, I didn t have to do anything to prime the pump.
I found this site, and began learning from the wealth of knowledge here. I inspected the system in operation, finding no leakage from the cylinder, relief valve, or anywhere other than some around the back pressure valve. Finally resigned to the fact that the pump is the problem, I decided to ignore the prevailing wisdom and go with the easiest repair, and have ordered a vane pump kit, rather than converting. (I hope I won t regret it, at least for awhile!)
Here s what puzzles me: When I had a pressure gauge on the test port, I found that with the control neutral, the pressure is purely proportional to engine speed - from around 20 to over 100 psi. Suspecting some kind of problem with the back pressure valve, I pulled the lift cover, first finding the seal washer to be squashed and deformed, explaining the leakage I saw. The valve itself appears free and functional, however takes less than 20 psi air pressure to begin to open it. Other than rust and dirt around the lift arm, everything else under the cover seems clean and in good shape (except half worn cam follower pin.) I am trying to talk myself out of going into it any further, but wonder if somehow a sticky unloading valve could cause what I see, while still allowing proper operation with cold oil. Any ideas? Since it seems to work ok, should I just ignore it?
I thought I d pull the back pressure valve apart and clean it and replace the o-ring, but I can t for the life of me see how to get it apart. I see a circlip in there, but I can t get to it. I tried just prying between the sleeve and body, but not too hard. What s the trick?
An interesting note: I found the back pressure dump tube routed right into the pump suction hole. It is obvious that the tube was factory bent to go there. I read the paragraph in the I&T manual stating "The open end of the back pressure valve discharge tube must not be located near the pump suction tube opening.." I m guessing Ford found this was a cause for foaming and later changed the routing. Mine must be an early one, but I don t see that it has been a problem.
I am also toying with the idea of trying to incorporate a suction screen into the sump. Has anyone tried this?