[ss3011]

Does the trans actually shift, from 2nd to third, or do the wheels just turn at the same speed. Even in third gear, intermediate clutch is applied. The 2nd gear roller clutch or sprag just overruns. If the trans locks up in 1st gear, then you have the forward clutch, low/reverse clutch, and either the intermediate clutch or the high gear clutch on. Suspect cross leak or a missing check ball. Most race trannies would not have the low/reverse clutch on in 1st gear. Street manual VB's put it on in 1st to give low gear braking. You should focus on figuring out how oil pressure is getting into the intermediate clutch or high gear clutch circuits, when you are in 1st gear. It is also possible your intermediate band is coming on, in low and holding the high gear drum, just like the intermediate clutch would be when in 2nd gear. Only thing is, it has low capacity and would probably start to slip if you got into the throttle, when "locked up" in 1st gear. When the transmission was rebuilt after being overheated, it is possible that some plastic parts, changed "shape" and were not caught or replaced during the rebuild. Those parts like an accumuator piston, or band servo, could be a source of a cross leak.