Hybidder,
For giggles I would block beneath one side of the mast after tilting back, then tilt forward to lift one drive wheel off the ground. Start machine, shift F N R N. If the lifted tire tends to stay spinning in forward this can be an indication of a failed forward pack due to overheated/ warped transmission discs. There should be some plugs on the control valve or on the transmission case, that may have F R and P. If you were to unseat a gauge in the Reverse port and saw 140-155 psi, then would expect to see the same on the Forward port. If tire did not spin on previous test, and the F pressure is very low, and your valve is working correctly, then I would suspect a blown clutch pack piston seal OR a leaking oil distributor seal (end of the shafts).
As mentioned, general procedure is to remove the mast, then pull diff/trans together out of front them disassemble on bench. Good luck!