C4 slipping from 2nd to 3rd

[66Fairlane500]

Hi there!

Im having some trouble with my friend's C4
We rebuild it with new clutch plates, gaskets and seals, and we made air-checks throughout the process.
Brakebands were as good as new.

The the reason why we took it apart, was because it was spilling oil and slipping from 2nd to 3rd.

After the rebuild it is running perfect while it's cold, but after a while it starts slipping when shifting to 3rd.

Could it be the Oil-pump? Or do you guys have any ideas?

 

[40 Deluxe]

Have you put a pressure gauge on it? If you have good pressure in reverse, it is not the pump as reverse pressure is much higher than forward pressure. How about the valve body-are any valve bores worn and letting fluid bypass, thus dropping the pressure when the fluid thins out as it warms up? Maybe a foolish question, but did you reseal the piston in the direct drum? Did you use Teflon sealing rings or the OEM type hook rings on the rotating parts? End play within spec?
What type fluid are you using: Type F or Dexron? Most C4's need Type F to shift properly-check what's stamped on the dipstick. Dexron has different friction modifiers and causes a soft shift in Fords. Note: Parts stores and speed shops sell a special fluid called Trick Shift by B & M Transmissions that makes your trans shift firmer and quicker. Look closely at the can and you will see that it says this fluid "Meets Type F specifications"!
Sometimes an air check can fool you. High air pressure (150#+) can make a poor fitting seal hold better than the usual 40-60# trans pressure.

 

[LOWRIDER]

If it was a unit with a lot of miles on it I would say you have a glazed band or drum. Being just rebuilt points to new concerns. The drum wasn't overheated with hotspots (like a brake drum) was it. Chucked up a few drums on a brake lathe to remove hard spots on rebuilds. Drum needs to be clean and not real shiny or the band won't grab correctly and shift right through to 3rd. Also make sure it's adjusted correctly. I agree with 40 deluxe you can't check with 150#'s of air pressure. I used to bench check them with 15psi.

 

[66Fairlane500]

Have you put a pressure gauge on it? If you have good pressure in reverse, it is not the pump as reverse pressure is much higher than forward pressure. How about the valve body-are any valve bores worn and letting fluid bypass, thus dropping the pressure when the fluid thins out as it warms up? Maybe a foolish question, but did you reseal the piston in the direct drum? Did you use Teflon sealing rings or the OEM type hook rings on the rotating parts? End play within spec?
What type fluid are you using: Type F or Dexron? Most C4's need Type F to shift properly-check what's stamped on the dipstick. Dexron has different friction modifiers and causes a soft shift in Fords. Note: Parts stores and speed shops sell a special fluid called Trick Shift by B & M Transmissions that makes your trans shift firmer and quicker. Look closely at the can and you will see that it says this fluid "Meets Type F specifications"!
Sometimes an air check can fool you. High air pressure (150#+) can make a poor fitting seal hold better than the usual 40-60# trans pressure.

All seals were replaced, with the same kind we took out. I Would say they match oem.
The Oil we put on was Sunoco ATF 153.
It was my first ****** rebuild, so im a rookie on that area.