Our C4 AAN turbo S-cars do not have a conventional power brake system. Conventional power brake systems relay on a vacuum-based assist, with the vacuum coming from the engine and stored in a large vacuum accumulator (see diagram below). Since our cars are making boost when under more than cruising throttle, vacuum-based boost was thought not to be an option. As a result, our power brake system is based on hydraulic pressure created by a high pressure channel in the power steering pump. This hydraulic pressure is stored in a pressure accumulator (aka, "The Bomb") that is located in the left fender well, underneath the washer bottle and behind the stock intercooler.

When you step on the brakes, the brake pedal moves the brake servo which controls pressure release from the Bomb, which then moves the piston in the Master Cylinder. Subject to the electronic control of the ABS brake system, the fluid from the brake reservoir is pushed out to the front brakes and, via a rear brake bias valve, out to the rear brakes. Here is a link to Fred M.'s Brake Accumulator and Servo Test info.

Here are some diagrams that hopefully illustrate the above.





Link to Steve M. (boxflares/UrScubed)'s Power Steering/Brake pump rebuild DIY

Here is a photo showing the location of the "Bomb".


Link to a "Bomb" test to see if is bad
Link to a "bomb" removal write-up
Link to David K's "Bomb" recharging website



Here is what the Brake Servo looks like (Courtesy of D. Fifield):

LINK TO D. Fifield's Brake Servo R&R DIY




And here is an image of the Girling G60 Front Brakes the North American UrS-cars came with (Europe got a version of HP-2 after first having UFO brakes):


And here is the general arrangement of the rear brakes



This *could* be on the exam. ;>)