My 1989 GXL will idle fine until I apply the brakes, car moving or not. The idle will drop very low, approaching 500-600 RPM, and will either recover and find it's idle again, or start to die, at which point if I can, quickly press my foot to the gas to keep it alive, or it dies. This kind of randomly happens, and occasionally happens when i shift into reverse as well. Manual tranny. I speculate it could be the BAC Valve? I hear that valve is the cause of many erratic idle problems.

 

Bac valve seems like it would fix the problem, I recently replaced my booster and it fixed the problem but ocasionaly the problem will come back.