904 loses top gear in 2500 mi

HI all,
I have a few questions after taking a stab at rebuilding a 904 tf myself only to have it last 2500mi.

First thing I noticed going wrong was 2nd to third shift started getting almost un-noticeable but once engaged no slip at all.

Then did its first filter and fluid change and saw a little to much friction material (fine, no big chunks or metal)
100mi later trans will not up shift to 3rd gear 1st & 2nd are still very good as is reverse, still strong.

Pull trans from Javelin / front pump sealing rings look good /direct drum frictions look intact But signs of heat in the steels some light bluing coloration and they feel hard as glass like slick to the touch.

Ok, can someone tell me if the VB calls for the up shift into third but the direct drum is slipping will the trans just stay in 2nd gear? Or will it drop down to first since the forward drum is always applied in forward gears.
Just trying to figure out if the valve body also has problems . or I just over powered the clamping force of top gear drum.

Car is an AMC Humpster (or should I say Dumpster) with a Complete Mopar drivetrain 360 Magnum also early c body 83/4" sure grip

Sorry haven't yet disassembled the complete trans . Thanks Jimi

Do you have the kickdown linkages on it and correctly adjusted, if not that will kill a T/F high clutch in short order.

In addition to what M said ......

Did you pressure check it WITH atf before attaching the valve body ? How’s the fluid ? How is reverse gear ?

Did that myself one time. Wait, it was the tranny shop that screwed it up and I fixed it. Had a full rebuild, ran fine for a few thousand miles, 1 and 2 were fine, 3rd started to feel like it was slipping. Eventually 3rd felt entirely like neutral. Ripped it down, found wasted front clutches and steels. Kept looking and found the inner hub of the front drum gouged from a burr on the input shaft. It caused a leak between the valve body and the clutches that eventually got big enough that they didn't engage.

Don't hold me to how technically correct that is - it's been a lot of years since I've been inside a torque flight.

Thanks for your answers,

Well as far as the throttle pressure linkage/ had to use a Loker brand cable since the car is a hybrid drivetrain .
the lever on the trans is fabricated (little longer then a stock lever) so that it is linear with the carb ratio . might be off some but the trans lever is all the way back at full throttle .

I guess should have squirted ATF in the case holes before air testing is that what you mean Doc? I did however air test the trans before and after and with 35lbs it sound like there is no significant air leaking.

Does everything have to be spot on for a 904 to live behind a 350 HP 5.9 L in a 3600lb car maybe the throttle pressure was low at cruise speed in top gear. by the way the V/B was stock from the Amc torque command 6 cylinder before the 8 conversion.

Thanks Jimi

It's really easy to cut or fold the outer lip seal on the piston of the front clutch drum. The rear drum goes together much easier. Have you pulled the piston out? Also check to be sure the inner lip seal protrudes from around the hub. Of course any wear where the seal rings ride in the drums or support would also be an area I would look at closely. The 904 should more than stand up to that HP and weight.

Won't upshift out of 2nd or slips/freewheels once it shifts to high?

I made that mistake once. I will never rebuild with out replacing them.

T... I pressure checked at about 80 psi or so. And those front clutch sealing rings are CHEAP ENOUGH (considering the alternative!) .... I would have changed those.

Drop the pan and report back on the condition of the fluid and if there’s any trash in the pan

Next time do the full series of hydraulic pressure tests before taking a trans apart.

Now that it’s apart, pull the front servo apart and check all the seals. The front servo gets line pressure from the valve body - same pressure and feed as front clutch apply - but only on the 2-3 shift and while in third.

You state you have a good solid reverse. If so, then the front clutch drum has clutch capacity.
Even though the front clutch drum runs at higher pressure in reverse, if there was an issue with it’s internal lip seals or the two metal seals between it and the reaction shaft support you would likely have some slippage in reverse.