951C transmission failing in first gear

Make sure it is slipping 1st in both directions, this will isolate it to 1st gear pack and trans. hydraulic system, if it is only one direction and not in any other gear then it is in the transmission hydraulic system Could be clutch pack worn out, failed piston seals, maybe fault in selector valve or blown 'O' ring or hook ring in the plumbing directing oil to clutch. If in more than one speed in one direction it is a direction clutch. Lift floor plates, apply brakes and check each gear and direction, see which ones the prop shaft stops spinning and which ones it spins (hope it is not the D4/D5 style with the converter in the transmission housing). EDB possibly will have a comprehensive fault isolating check list, I am just a dumb arse who does it by progressive elimination.

I am pretty sure it does it in both directions but I can make sure tomorrow.

It does the same thing in forward or reverse.
What pressure should this transmission be at when it is in gear? The gauge on it goes to 300 and it exceeds that in 2nd but only hits 300 cold in first and deteriorates as it warms up.

What's the s/n?

69H 5082
I found my book on it and it look like there is a modulating relief valve spool that could be stuck partially open. I just dont know where it is located.

The modulating relief functions to control clutch engagement for all speeds. If it works for one it will work for all. That's not your source of problem.

Ok, Just looking at the schematic It looked like it was only in the first gear circuit. I feel like it is a pressure loss issue. Is an internal loss in the transmission the only way it could affect only first gear?

You can see if there are any pressure leaks by unbolting the transmission cover .leave it on with out bolts. Lock the brakes ,and start machine ,put the transmission in 1st gear at idle and remove the cover to see if there is oil leakage at the spool and pressure valve .Also check for leakage at the oil supply tubes entering the front and rear of the spool valve .tubes going to the rear of transmission case supply oil to the # 5 rotating clutch which is the frist speed clutch .Oring seal failure sealing those tubes could be starving the frist speed clutch .Another problem could be the Teflon sealing rings leaking oil at the rotating clutch .Trans has to be removed ,and disassembled to replace the teflon seals .Heat causes the seals to get hard over time ,and not seal properly

Back to your pressure question..
Minimum setting is 275psi, maximum setting is 305psi +/- 15 psi checked at filter cover or pump. Pressure should hold in all forward and reverse speeds. (controlled by the modulating relief valve)