Transmission

I do not know what you mean by "coming back". That does not describe any symptom or problem.

For a transmission this old, the rubber lip seals are going to be hardened from age. That alone will not cause problems. What is a bigger concern is the transmission fluid will have all drained down into the pan. That lets the fiber clutch plates dry out and the lip seals can dry-rot. It is standard procedure to soak new clutch plates in transmission fluid just before they are installed during a rebuild so they do not get chewed up from running dry on the clutch pack's initial application. I would be concerned that wear has already taken place. If there is air in some passages, that will bleed out in short order, then shift speeds should be normal.

Let me explain what I mean by "symptoms". I had a car with broken sealing rings on the input shaft. Those allowed the fluid to drain out of the torque converter when the car sat overnight. The symptom was when the transmission was shifted into reverse or drive, first time in the day, the engine didn't slow down and the car didn't move right away. As the torque converter gradually filled up, the car started to creep ahead within about five to ten seconds, and from then on, it clunked into gear like normal, and drove and shifted fine the rest of the day. THAT is a symptom I can analyze, thanks to my wondrous experience of owning that car.

One other potential problem you could run into is varnish build-up in the valve body. That is associated more with heat than with time, but it can cause some valves to stick. Those valves are typically spring-loaded governor and throttle valves, and those are the two that determine the speeds at which shifts take place. That varnish can be worn away by the movement of the valves. The symptoms would typically be late up-shifts under light acceleration, or a failure to down-shift when slowing down. Even though the transmission might stay in the wrong gear, that would not cause slippage in any clutch pack. Car speed would respond to throttle. It would just be lagging in the wrong gear.