We started this 4-part series examining possible vacuum pressure as the cause of a hard brake by picturing a driver's inability to apply his brakes in traffic because the brake pedal is too stiff - requiring an inordinate amount of pushing power. An extreme scenario for sure, but the horror of a hard brake pedal can strike at any time. In power booster equipped vehicles, the issue is more common than you may think. Fortunately, you don’t need a degree in Rocket Science to diagnose and fix the glitches that are hardening your pedal.
Another culprit responsible for a hard pedal could be the combination valve, and in particular the Pressure Differential Valve within that valve. This valve is there for safety reasons but can cause headaches if things aren’t working properly.
The Pressure Differential Valve is designed to move should pressure drop on the front side of the valve versus the rear side and vice versa. When this happens, fluid coming from the master cylinder basically hits a wall.
Because the brake fluid can’t be compressed, the pedal becomes rock hard. You still can generate moderate pressure on the “good” side of the valve therefore allowing the car to be driven in a limp home scenario.
This condition is what is commonly known as and referred to as a tripped valve. If your pressure differential valve has been "tripped" it must then be re-centered by equalizing pressure on both sides of the valve. This is accomplished by getting the valve to move the proper direction and therefore putting the valve back in the center. Of course, the problem that caused the valve to “trip” must also be corrected at this time.
If your vehicle has OEM disk brakes it is highly likely that it also utilizes some form of valving within the brake system. In this case, the same valve that operates the warning light on your dash - the pressure differential switch - could be the problem creating a hard brake pedal. Follow our plan above to on the right path, but as always, be cautious and employ a professional if you are unsure. Properly operating brakes are essential for safety. We are always here to get you the right equipment or additional help if you need it.