a4 1.9 tdi engine sound lowers when brakes applied

[sbt]

hi all . just recently my car which is an 2004 a4 1.9tdi as been behaving very strange the car runs great except for when i apply the brakes . as i am coming to a stop the engine tone goes down for a second or two then returns to its normal tickover sound although you can hear it this does not show on the rev counter . the brakes work fine it sounds to me like a vacuum issue does anyone have any idea what the cause could be any advice would be very much appreciated . many thanks

[phil miller]

hi mate i would be tempted to check the brake servo vac pipe and hose's its a big black hose that goes thro the bulkhead, if you do this with the engine running and get someone to push the brake you should be able to locate the problem mate