This 400 is in my 81 gmc crewcab with a jegs 2000 stall converter I just had it rebuilt a year ago and it doesnt even have 200 miles on it yet. when i drive it first thing it doesnt want to shift out of first. then when it does shift it slips in 2nd and 3rd but when this happens you can stop put it in reverse punch it and when you put it back in drive it shifts out fine and doesnt slip again unless you kick it down into passing gear or kill it and start it back up. I have already changed the vaccum modual filter and fluid and this didnt change anything.

thanks for any help

Damon

I should add it worked fine then its been parked for 2 months then last week when i got it back out this started happening

Check your shift cable or linkage ? Had a cable melt recently - the B&M cables are krap.

Check your shift cable or linkage ? Had a cable melt recently - the B&M cables are krap.

^This

If that doesn't work, check the vacuum modulator. Cheap part, easy fix, and it could very well be causing the problem you're having.

my truck has a shift rod no cable. but i did check it looks fine

yea i replaced it and the fluid when it started. I will change it again though could have got a bad one

fluid level checked? Vaccum to mod. valve working?

Fluid level is full and yes I have vaccum to a new module I just installed today still no change. This trans has me stumped if it was burnt up clutches why would it clear up after punching the throttle in reverse?