I decided to dig into my own 904 instead of hiring it out to a shop. It was
not as hard as I guessed, I got it torn down and back together with only
this one question.

The transmission was assembled including the Torque Converter and
then the pan and valve body were removed for the air pressure tests
described in the FSM. This is a 65 A904-G TorqueFlite 6.

During the air pressure bench test when air was put into the "front clutch
apply" port - the clutch applied with a thud as described - there was also
air coming out the "rear clutch apply" port.

Is it normal to have the rear and front clutch apply ports cross
pressurize each other? Or did I miss something when reassembling it?

The 63 904 has the same issue - the air put into the rear clutch apply port
leaks to the front clutch apply port. I traced the passages through the front pump to the reaction shaft. The front clutch port comes out
on the reaction shaft between two seal rings and the rear clutch
passage comes out on the bore inside the reaction shaft.
My guess is the air leaks past the seal ring toward the
rear of the reaction shaft, then around the end and into the inside bore
in the reaction shaft. I am going to surmise this is a normal behavior
based on the premise it is unlikely both transmissions would be
broken in that same manner.

Is posting replies to your own questions as bad as talking to
yourself?

Why do I get logged out while I post ?

I'm about to tear down my 65 A-904 would any one happen to have a copy of the factory service manual?