My 1979 Catalina has an intermittent fluid leak from under the rear of the engine, in the area of the rear main seal and transmission/ crankshaft hookup. I suspected it's motor oil; it wouldn't be surprising for a 42 year old rear main to be drying out, especially after sitting outside for 6 years. I'm beginning to think it may be transmission fluid that is intermittently leaking, not motor oil. It's not valve cover gaskets, they were replaced probably 6 months before my girlfriend's parents parked it, and the whole top of the motor is dry as a bone. The oil also hasn't dropped noticeably on the dipstick.
A while back I noticed the tranny would slightly disengage itself if sitting still, and the rear end would lift up slightly like you took it out of gear. Pushing on the gas if it's like that, it would accelerate with a slip, until a pressure rise made it grab better, or it would eventually reengage on its own if you coasted, then it drove fine as long as it's moving. I checked the dipstick a bit over a month ago based on the instructions and it didn't show fluid when hot. I poured in a quart, level came up to the right amount and the funky behaviors stopped. It just started to come back a bit the other day and it was down a pint. What would be causing intermittent loss of fluid in that area? Revving the engine seems to make fluid come out sometimes, other times it doesn't do anything.
Your transmission input shaft seal is probably shot. You'll have to pull the trans and torque converter and replace the seal.
That's what I was thinking it is. A friend of mine at work is a retired auto tech. He says he pulled a tranny out of a car around the same era when he was 14 to replace that same seal and put it back on in a few hours. I may try to do that when I'm not working in my side street anymore. Any chance something like AT-205 might rejuvenate it for now, or do those tend to crack and rip, not just shrink?
The AT-205 is just a temporary fix. While you have the trans off, check the rear main crank seal behind the flex plate. It's easier to do it all at one time.
I have this very problem on a '65 Marlin, it's a good bet that as @Doc says the input shaft seal on the transmission is shot. (At least you don't have to deal with a torque tube in order to drop the transmission!)
I figured this was the problem, but wanted some more experienced opinions, before I tear stuff apart. Haha. I'll be moving into a house with multiple garages here soon, so this will probably be a garage job, not a side street job.
On an unrelated note, where would you find a brake master cylinder for a 1960 Rambler? Getting ready to trailer one from Waverly to Lebanon, Ohio and that, along with tires need replaced at minimum. Emergency brakes still work!
Yeah, fortunately it's relatively easy to drop the trans on your Pontiac. The Marlin has torque-tube drive so the rear axle and torque tube assembly have to be pulled out of the car to drop the transmission. Or the engine could be pulled out to access the front seal. (Marlin is also unusual for the time in coming standard with power front disk brakes for 1965.)
On that 1960 Rambler (I think it was a Rambler Six, not the small American?) it would be a single circuit master cylinder since AMC didn't go to a dual master until 1962. Does it have power brakes? (Most don't.)
Galvin's (ramblerparts.com) lists master cylinders for both power and non-power 1960 Ramblers, the latter having a four bolt mount to the firewall. They also have repair kits. (You can always send the unit out to be sleeved if the bore is badly pitted.)
https://www.ramblerparts.com/catalog.php
There are also rebuilding services that advertise in Hemmings.
By the way, that car also has the torque tube setup so pray you don't have to drop the transmission for anything!