Air checking 904 question

I'm building a 904 and may have a problem during air checking the clutches. The direct seems fine,engages firmly and no air escaping. The rear clutch is a different story. The clutch engages but air escapes from the port next to the port that I apply air pressure, this is the port that goes to the converter? Anyone run into this?

Sounds like you might have a broken sealing ring on the input shaft, and have air leaking into the pump housing.

I wish it was that simple. The rings look fine but I have swapped to the old ones with no change.

Teflon or steel rings? Do they have grease/assy lube on them? How is inside surface where they seal against?
Doug

The rings are metal and well lubed. The drum had a small amount of wear,I gently cleaned it up with 400 grit.

Are the pump parts and input shaft original to that trans?

The original pump was replaced with a transtar rebuilt pump. Input shaft,drums are original to the trans.

You might have to disassemble the pump and make sure there's nothing going on where the clutch hub contacts the pump body. I think that is just a metal to metal seal, correct? Maybe there is a crack or piece of debris in that area.

There was a change in the support and rings, might have a mismatch of parts.

I swapped the single ring on the input shaft to a teflon ring from the small end of an accumulator piston and no air leakage can be heard. There is a tan plastic ring that has flat cut ends that seems to work as well. What ring would be the correct choice in that location? Thanks for the help.

Square cut seals (usually "plastic") should have the ends butted when placed in their respective bore.

The metal variety with locking ends should have a gap between the locking tabs when placed in their bores.

Thanks, I knew that but I don't do many trans builds so it never hurts to give me a reminder.