newbee brake problem

okay recently I noticed that when the brakes are depressed it started making a funny purring noise. now i have really noticed that the care is losing power as the brakes stay depressed. if i accelerate right after having ther brakes depressed the car will studder before gaining power. now i know it has to do with vacume pressure. i just don't know which part to replace. i think it has to be the the booster or the master cylinder. there are no other symptoms or panel lights on. help please.

pick up a brake booster tthe diaphram has a split in it causing a vaccum leak