I have a 4250 that seemed to be jumping out of gear when backing up hill. All of the 15 forward gears worked fine.
One mechanic suggested it might be a blown gasket in the control valve housing. I replaced all O rings and gaskets in that and modulating valve. It didn't change anything.
After this I was 'messing' around with it to try to get some rhyme or reason to when and why it was doing this, and I discovered that when it appeared to jump out of gear, all I had to do is step on the clutch for a little while and then let it out and it would go again.
Then it suddenly quit working entirely and now it won't move in forward or reverse.
I put in new oil filters, sump screen cleaned out.
175 psi at 2000 rpm at system test port
8 1/2 - 10 psi at 1000 rpm on the lube pressure test port on the return oil manifold when testing circuits.
Any help would be much appreciated!
We finally found the problem after consulting many mechanics. It was the tow disconnect. We had to build a gallows to raise the cab, with the help of an engineering friend, and the actual repair didn't take long.