Power Brake Solutions

For decades “power brakes” has referred to that large round can the brake master cylinder bolts onto. That can is the vacuum power brake booster. How does it work and does it have a future?

The metal can is hollow with a flexible diaphragm dividing the can into two air chambers. A spring retains the diaphragm in place. The mechanical linkage between the brake pedal and the master cylinder runs perpendicularly through the center of the can and diaphragm.

Engine intake manifold vacuum is applied to both sides of the diaphragm until the brake pedal is pushed. Pressing the brake pedal opens a valve to let outside air into the air chamber on the brake pedal side of the diaphragm. That air is at regular atmospheric pressure and that pressure compresses the diaphragm into the side of the can that still is in a vacuum. As the diaphragm compresses, it helps push the rod connected to the brake pedal linkage into the master cylinder. This help from the compressing diaphragm is the “power” in power brakes.

If the engine is turned off or stalls, then the intake manifold vacuum disappears. Air at atmospheric pressure no longer can compress the diaphragm. The brakes still work because the mechanical linkage between the brake pedal and master cylinder is still there. But the driver must push hard on the brake pedal because not only has the vacuum power assist gone away, but the diaphragm and its retaining spring is now likely resisting rather than assisting the movement of the brake pedal linkage.

Vacuum power brake boosters usually last a surprisingly long time before the can, rubber diaphragm or valves start leaking or there is some other problem. But the future of the trusty vacuum power brake booster is still uncertain because new hybrid and even regular cars now intentionally turn off the engine when the car is coasting or at a stop light. The driver cannot be expected to push much harder on the brake pedal every time the engine stops and the vacuum assist disappears.

Luckily power brake solutions have been around for years to take care of diesels and turbocharger equipped engines that do not have reliable intake manifold vacuum. The solution has often been to just use a vacuum power brake booster but add an electric or belt driven vacuum pump to provide consistent vacuum.

Higher production volume diesel and now hybrid engines often use a hydraulic power brake booster pressurized by the power steering pump and/or an electric pump. Hydraulic fluid pressure helps push the rod into the brake master cylinder. “Hydroboost” type power brake boosters have been on diesel trucks for years and new and remanufactured hydraulic power brake boosters by ACDelco, A-1 Cardone, and Motorcraft are already available in the DBC catalog.

For now, it might be a good idea for prospective owners to consider the power brake booster system while shopping for a hybrid car or a vehicle with a “stop-start” engine. If a similar system has already proven itself on older vehicles or it is currently being installed on high production volume vehicles, then it is less likely to be expensive to maintain and repair down the road. If it is a system installed on relatively few cars, then it could be expensive to repair. Let me tell you about the hydraulic windshield wipers in my ’67 Lincoln sometime…