I have a '63 Pontiac Star Chief with an automatic trans. I believe it is a Roto Hydramatic 4 speed. Might be a Super Hydramatic. Sometimes it won't shift into high gear. For example, I drove it 50 miles the other day towing a small trailer and everything was fine. I parked the car for a couple hours and when I started home it was over reving in second, shifting hard into third, wouldn't shift into fourth. I drove it home 50 miles in third. I parked the car for 10 minutes and when I drove it again (still towing the trailer) it shifted properly. It's done this a couple times. I've changed the fluid and filter. It didn't help. I took it to a local shop but it wasn't acting up at the time so the guy couldn't do much. I was towing a small trailer each time this happened but I've been towing that trailer for years without incident and I towed it 300 miles after the last incident without a problem.
No I don't. I thought about that. But sometimes I can tow it hundreds of miles with issue. And the last incident happened immediately after the car was sitting for a couple hours.
There's something floating around in the valve body. Pull it down & clean it out. Keep track of where everything goes & put it back together EXACTLY like it was.
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