Brake Problems! Help!

[radford]

2005 X5 4.4i – I have to stand on the brakes when I come to a stop or when the RPM’s get low. It’s like they just quit working. The brake pedal becomes very stiff and nonresponsive when backing out of driveway or sitting at a red light. I can work the brake pedal like you would a clutch to keep the RPM’s up and the brakes work like normal. The only time the brakes do this is when the engine is below around 2000 RPM I guess. My mechanic replaced the vacuum pump but that had no effect. So now he thinks it’s the brake booster. So I am going to send the pump back for a refund. So I guess my question is – Do you think my problem is the brake booster or could it be the master cylinder? Please help!