904 "braking" in 2nd gear

Steve your 904 is just fine for your application and plenty strong. But it's got a definate shift overlap problem that is detrimental to your transmission. The rebuilder didn't do something right apparently or you wouldn't have this problem. He should've pressure tested each component (clutch packs and servos) during the rebuild. It's also possible that he mixed up the selective snap rings when rebuilding the clutch packs and didn't check the clearances, which should allways be checked and adjusted if necessary. Another possibility is he did something wrong when installing the shift kit and/or put a valve in wrong in the valvebody. Nevertheless the rebuilder should make it good. Have you talked to him about it?

Whatever you do don't drive it until it's fixed or it will burn it up very fast!!