BP:

If B1 were not releasing, and K1 were simultaneously applied, the trans would try to lock, and the car would decelerate.

What is the source of the modulator vacuum? If there is a vacuum control valve (VCV) is it providing vacuum to the modulator at small throttle pressure?

Also check the adjustment of the control pressure linkage; if it is too short the shift points will be delayed, and if way too short there will be a 1st gear start!