OK. I have rebuilt a 1969 C4 as much to practice on as anything and here is my problem. After I assemble the forward AND the direct clutchs using either the square cut OR the round O ring in the inner piston seal I can get an air check but with a lot of air leaking. On the stator support again both forward and direct will air check. Lots of air leaking. But after I assemble the entire transmission I get a good air check of BOTH bands and the governer and NO air check on either clutch. This is driving me crazy. Anybody got some suggestions? Thanks

Pour a couple of ounces of atf in each air check passage and retry. If it doesn't improve to a dull thud with slight air loss,you have no choice but to pull it back apart and find out why.

Also, don't hit it with 125 psi. 60 psi should be sufficient. You will always get some leakage -- it's obvious when it's not sealing.

Are you testing them dry? Did you use assembly lube? What is your air pressure?

On my 1969 C4 air check. I have used air pressure from 35 psi ( will not apply the piston ) to 80 psi. I used Vaseline to lube the seals and a few drops of ATF to try for a better seal. I turned the clutch drums on the stator shaft to try for a better seal ( no better ). Where can I buy new rubber seals for the forward and direct clutch pistons WITHOUT buying a complete reseal kit? And how much more seal would I get with the transmission filled with ATF and in a running car? Poor idea or really BAD idea.

Remove the pistons, remove the seals from the pistons and drums.

Install each seal separately and test the fit in the drum.

If there is one that fits too loosely replace it.

There must be some resistance when test fitting each seal.

Remove the metal, I hope, rings from the stator support and test fit each one in the drums.

For replacement seals you will probably have to go to a transmission shop and root through their saved box of goodies.

Installing the transmission at this point would be futile.
If the drums won't apply with air they sure won't apply with hydraulic pressure.

First. Thanks for the answers. Here is what I have done and the results. I pulled the two clutch drums and the pump from the C4. With a lot of vaseline on the seal rings on the stator I get a GOOD air check on the direct clutch after I poured about 2 OZ of ATF into the hole in the pump face. The forward is still NO GO. The piston in the forward is D0AP-7A258-B which is a 1970 style piston. The 1969 forward piston is C4AP-7A258-C. Since the drum is cast I can not tell if it is correct for the piston I am using or not. Any thoughts? Are there any differences between the seals used on C4AP piston and the D0AP piston? I hope that is the problem as I am getting way too good at tearing a C4 down. Thanks again Pete

The early piston is .050" smaller I.D than the later ones Not sure on the seals being the same ,i doubt it.. What you need to do as was mentioned earlier fit each seal individually in place and check the drag on each. Same with the sealing rings .try each in their bores for snug fit. You shouldn't have to pack them in vaseline to get them to air check.

i am a full time trans builder at a nationwide chain.. most units will non air check because of assembly lube. back air press down to 50-60psi pour a little atf in the hole keep trying!! it will come around!

I am the guy trying to rebuild an early 66, Ford C4 and I could not get the clutches to air check. Turns out the wrong piston was in the forward clutch. That is replaced and is A OK. Now when airchecking the direct clutch on the stator support it will not air check. BECAUSE there is a hole drilled in the top of the piston (opposite the check ball). What is this? It looks like it was meant to be there and not a casting flaw. There is a boss for it and a slight relief in the outer part of the piston to access this. About 1/128 inch in diameter. Time for another new piston? If I get another reverse/high drum will the 1970 and up interchange? Thanks

Yes they will interchange... If you are planning to go to a 26 spline input I believe you need to change the Dir drum and the FWD drum as well. If you need any parts let me know, I am parting a 1968 C4 (24 spline) originally behind a 289ci in my Mustang and am NOT expecting high dollar profit.