My 973 blew the o ring seal on plug of sync valve twice in a day. Disassembled valve and found 2 spools in speed valve stuck. Having to take out welch pugs to free spools, I Had to wait over a week for new plugs to fit up valve. Loader would not move at high throttle. I assumed it was air in system and after an hour of idling it worked well. Unit had normal power to bulldoze a new bench in quarry for 3 days then blew another o ring behind a test port on sync valve. I decided to change oil and filter, but suspecting the speed valve problem may have returned, pulled it off and alarmed to find the plunger in sync valve was now broken. What could bE causing such an increase in pressure to eventually cause this damage? I hope the plunger is available from CAT. My serial# is 90L0601. Since buying unit the transmission was overhauled and replaced busted seals in a track motor to solve drive problems. Previous owners didn't seem to have appreciated need for proper servicing. When I figure out how to access photo library on my iPad, I will show photo of valve. Peter
Photo Photos of damage.
I'm not the guy with the answers but thought is the check valve in the pump stuck.
I can't say for sure what caused your sync spool to break, but if you're blowing O rings then it sounds like an overpressure problem. Drive pressure is controlled by two relief valves, one for the left side, one for the right side. The valves are located in the left and right charge/relief valve bodies, which are the things your sync and sync cutoff valves are bolted to. I would suggest reassembling it and checking your main relief pressures. It sounds like you have the manual, but if you're not sure on the procedure, shout out.