D7E Transmission pressure

I have a serial no 75E335 military cat which I just purchased Mid summer this year. It is a low houred machine and i have put on about 50 hours on it with no problems till last week. I started to notice presure drops when I shift the tranny between all 3 gears forward and reverse. I checked the oil level and it was just on the add mark with the engine at idle. Oil looks to be yellowish in color not burned or bad smell . I continued to run the machine and a few hours later noticed the gauge to flucuate more, to the point it kick the tranny out to neutral and the machine stopped. I would wait a moment and shift it back and away it would go again. air temp around 40 degrees, and I wasnt working the machine hard at all, just pushing small dirt piles and filling tile line holes. Yesterday I decided to bring it home and look it over closer and maybe change the oil on the tranns. I started the machine up and it did not build trans pressure for 3 to 4 minutes. Finally the gauge came up and stayed up so i shifted it into gear to go and imeadeately the presure dropped and it would not go. Finally after a couple more tries it stayed up and I was able to load the machine with no hesitation. I got the machine home and started lookin it over. I have 4 hydraulic pumps on this mahine, I am not 100% sure what they all do but there are two under the floor pan on the right side ( left smaller one runs the winch and the right larger one is for the dozer. there is one in front of the dozer pump that I believe is the tranny pump. I did a pressure check on it and at a idle it had 305 psi. There is another pump hanging on the back bottom of the bellhousing, not for sure on this pump? I want to change trans oil but not for sure on drain plugs and magnetic screens? There is a plug on the bottom of the bell housing, I assume it drains the torque converter and just to the rear of that plug there is a housing cover with a bulge on it and it has 6- 3/8 bolts holding it - i think it is the screen that can be pulled out and cleaned. The tranny drain plug looks to be under the rearend of my machine but there is also a plug about one foot to the left and another one foot to the right of the rearend drain plug. What do they drain? There is also one under each final drive which I know drains them. I know the winch has its own oil supply seperate of the machine. I have drained it and put new oil in it when I first got the mahine because I do use the winch for pulling a tile tractor through wet ground. Any ideas and help would be appreciated! Doug

Well Doug, at first I would have suspected a worn pump but you have 305 at idle so pump sounds good. Do you notice a sound like the pump is full of marbels when it doesn't drive? That could be the pump drawing air, might explain gauge fluctuating too. Take the rear skidpan down and inspect the pump suction line. Not sure if your trans has a suction line coming off of the front bottom of the trans or not. If it does there is a screen inside the trans to inspect and clean.
If all of that is good and problem presists keep a gauge in the trans pump and come on back with the pump pressure when it will not drive. I might be a clogged orifice to the load piston behind the modulating relief valve. That would keep pump pressure at initial pressure (app 75 PSI) too low to drive.
Later Bob

May have a broken spring behind modulating relief valve.

I think you just have a couple of stopped up trans filters if it's not building pressure for a couple of minutes. Look under the seat for it. Sometimes there's another one under the floor plate to the right of the decel pedal. There's a magnetic screen also to check, I think it's close to the oil cooler.

75E335 is equivalent to a 48A 6393 -up.
Has two magnetic strainers, one on torque converter outlet going to oil cooler and one on the suction side of the hydraulic oil pump. Also has oil filter on discharge side of the same pump.