Slant Six stalling when foot on brake & engine warm - transmission issue?
... this is a huge dip, from say 1000RPM to 500RPM, and with "pu[t]ts" that dive down to 200 it seems.
Without giving hypothetical numbers ("say 1000RPM"), can you tell us
exactly what engine speed it runs fine at and what engine speed it begins to drop at, and what engine speed it is actually going when it drops?
We know that this doesn't directly involve the brake, as you have manual brakes with no connection to the engine.
SO, can you duplicate this failure mode without the brake, such as by adjusting the idle speed downward?
Is the transmission exhibiting any other signs of misbehavior?
The brake holding the transmission back
may provide some back force against the engine through the converter, but we do not know whether the problem is that, or whether it is just the resulting drop in RPMs.
– Eric