I've had it with this 904, about to give up!

I totally agree with Trailbeast and his suggestions.

Nothing in the accumulator circuit will cause your problem because the only time it's even pressurized is when the trans. is in 2nd gear. And the throttle pressure linkage adjustment won't cause it to not go in gear, it can cause the shift flare issue but it'll always go in gear no matter where it's adjusted.

For some reason the front drum isn't being pressurized or it'd go into gear. Like TB said it could be a front drum seal (your air test should have revealed a problem there but maybe it let go on fire up??), worn stator support where the sealing rings ride, etc.

Do some pressure tests like recommended above and see where it's at. That'll tell you a lot.

BTW: I assume when you rebuilt it you checked and adjusted clutch plate clearances as needed?