Some details: 69 chevy c20 with a 350 sbc and a th400. Neither the motor nor the transmission are original.
Problem: when starting the truck everything shifts fine from 1st to drive. Say I drive a few miles to home depot. If I spend about 30 minutes or so, and come back to the truck and start it up, it will not shift from 2nd to drive. If I play with the shifter a bunch I can eventually get it to shift into drive, but as soon as I slow down enough for it to down shift again (say a stop light) it will not shift back into drive.
I have checked the fluid levels and they seem to be fine. I know quite a bit about engines and other mechanical stuff, but not as much about transmissions. I am thinking that this may have to do with a vacuum modulator or perhaps the solenoid. But I do not know how to check these, or even if these could be the problem. I would hate to waste the money on parts that were not needed.
Does anyone think it may be one of the two parts I mentioned? If so how would I verify they are broken. Any help would be appreciated.
Thanks Andrew