87 supra brake booster problem

[Rob1]

ok one day while driving my brakes began to feel a bit soft but would firm up again when pumped, now when car is running the brake petal is very soft and brakes are not very responsive and if i pump the brake hard a few time my engine begind smoking! i figured i had a leak spraying onto exhaustbut i looked closly and the smoke is all coming from my exhaust leak between manifold and cat, and also pouring out of the tail pipe. i pump breaks hard it smokes for a few min then stops untill i pump break again, i disconected vacumm line from booster and pluged the line going to engine to avoid vacumm leak and the problem stopped. any ideas??? thanks