Th350 not shifting

[Rich77]

Hey Guys,
I have a ‘77 c10 400/th350. The transmission is newly rebuilt. The 400 has a pretty aggressive cam. I’ve checked my vacuum hose and fluids.
The transmission won’t shift 1st to 2nd.
I can drive in R or 1st, but it won’t shift into 2nd.
Is it possible I need a power vacuum booster?
The brakes are also a bit soft.

 

[Matt69olds]

Who did the rebuild? Before tearing into it, I’d get with the builder if possible.

If that’s not an option, make sure the vacuum supply to the modulator is good. No vacuum= very late shifts. If the vacuum to the modulator is good, make sure the kickdown cable is free to move. You should be able to pull it and have it snap back freely. If not, make sure the cable isn’t kinked or melted on the exhaust.

If that checks out, next thing to check is the governor. It’s under a round cover towards the rear of the trans on the pass side. It’s a press fit, use a chisel or wide flat blade screwdriver and a hammer to lightly tap the cover off. Once the cover is off the governor slides out. Make sure the weights are free to move and the gear is in good condition.

 

[Ricko1966]

Make sure the modulator itself is good and you aren't running a rubber line all the way from the engine to the modulator.

 

[NickTransmissions]

Hey Guys,
I have a ‘77 c10 400/th350. The transmission is newly rebuilt. The 400 has a pretty aggressive cam. I’ve checked my vacuum hose and fluids.
The transmission won’t shift 1st to 2nd.
I can drive in R or 1st, but it won’t shift into 2nd.
Is it possible I need a power vacuum booster?
The brakes are also a bit soft.

How much vacuum is the engine making and how much at the modulator? Also check the governor gear as Matt suggested - make sure the gear isnt stripped. If governor is fine and vacuum is sufficient, call the builder. Could be valve body or issue with the intermediate clutch itself.