T-400 slips in drive but has reverse after rebuild
Last week I rebuild my 400TH, it had been slipping in 2nd and 3rd before the rebuild. I found plenty of burnt clutches that I replaced along with a few bushings and seals. Hard parts were alright with little signs of wear. I put the transmission back in yesterday put 6 quarts in it and started it up. Went into reverse (on jacks) and it took off after a few seconds, then in drive it didn't go. I checked fluid and added more for a total of 10 quarts. After that the the trans rotated in both ways.
I dropped the truck and backed it out of the garage, however when I drop it into 1st you can feel it engage but it slips with the slightest hit of gas. I drove it backwards around the yard trying to get fluid moving however I could get more than a roll foward in 1st gear and when it went into 2nd it acted as if it was in neutral.
Any thouhgts on where to start with this? Reverse is solid and I am confused at this point. this was my first go at a rebuild but really didn't see any reason why it wouldn't work.