I have a 1975 GMC Sierra 25 (3/4 ton) pickup with 50K miles, 350ci, 4bbl, AT. Today when pulling out of the barn, it was very sluggish. I drove down a hill to the house where a put it in reverse and nothing - as though it was in neutral. The forward gears still worked but no reverse at all. Later, it acted the same but I let it warm up at idle speed for 5 minutes or so and I was able to back up the hill to the barn where it is now. I’m decent with engines but a dunce on transmissions. Any help would be greatly appreciated.
Your describing a transmission that is neutralizing. Or isn’t building the hydraulic pressure needed to engage the clutches. When automatic transmissions start acting in this manner, think transmission rebuild/replacement.
The transmission has an internal seal that is failing. I had a 1980 Chrysler with a 727 trans that had been sitting for over a year when I bought it that did this. I never did get it fixed. The transmission lasted the 4 years I had the car, but for the first year or so, wouldn’t go into reverse until the fluid warmed up. I learned to back into parking spots and park on slight inclines so I could coast out. After some time, the shrunken seal apparently revived enough that there was only a slight pause in going into reverse.
You may be able to live with this indefinitely, but you will probably have to have it rebuilt to fix it. One of the cures that claims to revive leaky seals may help, but use at your own risk.
The first thing I would do is check the fluid level with the dipstick. If the barn has a level floor start the engine; shift through all the gears with the brake ‘on’; place the shifter into park; pull the dipstick; wipe; reinsert; and check the level. It should be at the ‘cold full level’. If you have to add fluid, check underneath the truck for leakage from the transmission. You might slip some cardboard under the transmission and see if you can spot where the oil is dropping.
It appears that this truck sits for long spells. This allows the transmission fluid to drain out of the torque converter. This raises the stationary level above running level. Thus seals that ordinarily only see splash are exposed to the fluid level. So seals like the manual shifter shaft, speedometer cable seal, speedometer gear adapter, rear seal, etc can leak. Then when you start the truck, the level is too low once the torque converter is filled again. The pump starts to draw air and cannot develop enough pressure to clamp the clutches and bands.
BTW, do you have the TH-350 or the TH400 in this?