MF 255 Aux Hydraulic Problem

Hi,
I have a MF 255 that is having hydraulic problems. I just bought this tractor so the history of the hydraulic system is unknown. Here"s what I know: The loader cylinders have new seals installed. The lift cylinders work for a few minutes if the fluid is cold but it becomes less and less responsive with use. The system was low on fluid so I added some universal hydraulic fluid. I later found that the fluid I used was a 20 weight fluid and the 255 needs an 80 weight fluid. I drained 5 gallons of fluid out and added 5 gallons of 80w90 to the sump. The system worked beautifully for about 5 minutes and then gradually got less and less responsive again. I put a pressure guage on the aux line while the engine was warm and get about 500 psi when I initially kick in the selector lever. After about 3 seconds the pressure is nearly 0psi. I know that the pressure should be above 2300psi but I"m not sure if my test is valid. My return line is still hooked up to the loader valves which are in a closed position. Would this affect my pressure test? I also haven"t checked/cleaned the filters. Does this sound like a problem that filters could cause? To me it seems like filter problems would be worse when the oil cold/thick but maybe I"m missing something. Any help/advice would be appreciated.
-Jason

I should also add that the 3pt lift is working fine. Its lifting 1000lb hay bales with no problems.

A 255 DOES NOT use 90w fluid, the 20w is correct! You need to make sure that the fluid you put in is OK for a wet brake system. If soemone has been using 90w in the past, you will be looking at installing a new pump, but based upon the symptoms, you probably already need one.

You should be using MF Perma-Tran oil.

On my MF 245 there are separate pumps for the 3pt and the aux. They use oil from the same sump. Massey Permatran 3 is the correct oil.

AUx hydraulics, right? As previously mentioned if 90w has been used the aux pump would/may be shot. The 3 point works off the main hydraulic not the aux. I would change the aux filter first but I doubt it will solve your issue. Since both systems use the same fluid the main may be failing also. Sorry to be the bearer of bad news.

if your tractor has factory remotes it has a separate pump ,the 3 point has it's own pump, best clean the 90 wt out of the system it will cause problems,your system uses permatran if you can stand the price,universal tractor fluid works fine, first clean the pump strainer and replace the external filter if it's equipped with one then hook the test gage directly to the remote, make sure your gage can handle the pressure,that way when the valve is opened momentarily to read the gage the fluid can't be escaping thru another valve, the relief valve should squeal and gage will read the amount of pressure being delivered,if it's low and the relief is squealing the relief is set wrong or damaged,if low and no squeal check relief valve for broken parts,if it's ok then the problem could be loose or broken lines inside the case that connect the pump to the external lines, i have run into that a few times,if the lines are ok pump is all that's left,if the pressure is good the the problem is in the loader system