904 "braking" in 2nd gear

Odd problems like yours is usually caused by the trans trying to shift into two gears at once. I would suspect that the rear band may be trying to apply when you are in second. (or maybe direct clutch pack appling instead of rear band, either one would cause your problem) Second gear should have the front band and the forward clutches applied only. The sprag should be free wheeling. If there is a oil passage in the valve body that is bleeding over to the rear servo while in second gear, the rear band will apply and try to stop the sprag. You need to connect a transmission pressure gage to the test port on the rear servo. There should be 0 psi on the gage except when in reverse and when in manuel low gear. If this is your problem a new valve body should fix it, Good luck, Ray