My 1970 Cutlass 350 transmission slips out of drive when cold. I can pull out of my garage and drive off while the carb is still on the last high idle step. When the engine settles to low idle it won’t go into drive or will slip out of drive when the car comes to a stop or is moving very slow. A slight increase in engine revs puts it back into drive. Within a mile it seems to operate normally. Shifting from gear to gear is always normal and very smooth. It only does this on a cold start in the morning. Fluid was changed at 59,000 it now has 79,000 miles. Fluid occasionally will be down a pint but has never gone lower. This was a very intermittent problem but now it has done this two weekends in a row. If it’s time for a rebuild then my two options will be to park the car until I get it fixed maybe next year or junk it. Any ideas?

 

Sounds like the front pump is worn - flushing with the thicker type 'F' thicker fluid may help!
Not just change the filter and gasket - a complete flushing!! All 12 qts.
Many oils change/repair places now do this - $100 - $150, but cheaper than a rebuild!

 

Motor mounts any good?

 

Please explain what the motor mounts would have to do with a slipping transmission!!

 

Maybe he wants the motor mounts before I junk the car?

 

Linkage!!! You said it goes in and out of drive. Linkage could be right on the edge of neutral and drive and any give in the mount could move the linkage slightly. Just a suggestion.

And no, I don't need the mount!

 

Possibly internal seals. Cold, letting fluid by, line pressure isn't making it where it needs to be to engage the clutches. Warm, they swell and you are good to go, for now.

 

Again, the thicker Ford fluid, or B+M Trick Shift might work!
They're both thicker and might overcome the pressure problem, not to mention cheaper than an overhaul!!

 

Lucas transmission treatment worked, no more slipping.

 

Then I'd have to say it is most definitely your internal seals. Don't know anything about Lucas, I don't use any additives in my ATF, but most if not all are just seal conditioners (swellers). I'd guess the seals on the clutch apply pistons, or the pump.

 

I really can't do a transmission rebuild this year. I'll be happy if I can go another year. I'm sure it's age related not miles, being over 40 years old.