Engine Stalling when Braking

[Rob_M]

Hey Guys,
I have a 68 427 Biscayne w/original power 4 piston disk brakes that has an issue when braking. After the car is fully warmed up, when I brake the engine stalls. As an example I will put it in neutral going 30 mph hit the brakes, and it stalls. If I take my foot off the brakes quick enough the engine will resume. It's as if I turn the key off and on quickly. It will even do this when in gear. Of course it dumps raw gas in the exhaust and I get a *very* loud pop out the back when I let up on the brakes.
I have checked for vaccuum leaks, blocked the vaccuum booster, adjusted the floats, (has a Holly), checked for loose wiring near my brake pedal and engine bay, etc - Nothing.
I am at a total loss on what is happening. I plan to replace the check valve that is on the front of the brake booster but like I mentioned earlier it still does it when the booster is unplugged & vaccuum is blocked. I have noticed it doesn't do it when the car is cold and when I go backwards.
Any ideas or thoughts would be greatly appreciated.

Thanks,
Rob Mansfield