1960s Allis Chalmer Tl-14D After 5 Minutes Won't Move

I traded work for a 1960s vintage Allis Chalmers TL-14D. After 5 minutes the forward clutch pressure drops from 200psi to less than 100psi and it won't move. For the first 3 minutes I can spin the tires. I've changed the transmission filter, clean the screen and put in new fluid and it made no difference. It won't move in forward or reverse. I suspect the first gear clutch pack is leaking. Before I tear down, any tests I should run. Also, where can I find piston rings and seals for the clutch packs. I've rebuilt a direction reverser on a John Deere dozer, but this tractor is new to me. Any help would be appreciated. Thank-you