I"m at the same point with mine. Going to check the valve first, but I have an early "65 powershift without the BR port to check. Can you post a bit more about the PTO. Does it run all the time? Does it run when the tractor is first started? Will it ever shut off, certain RPM"s, below above? Did you try to see if it had "pressure" on it, or was it possibly "creep" from oil drag? How many hours are on the engine/PTO.
Mine is going to be split anyway to fix a leak on the trans, but we are going to go through the valve first as it has about 9000+ hours on it. Freshen up any springs/seals and o rings as needed.