Phil Krolick – AutomotiveBack to Instructor Webpages
Brake pedal free-play is a measurement of how far the pedal moves before the master cylinder first begins to apply.
Free play is critical on all braking systems in order to keep the Vent Port open inside the master cylinder. If this vent port is not open, brake fluid pressure will increase as brakes heat up. This will “self-apply” the brakes and cause more heat in the brake fluid.
As brake fluid heats up it expands. With the vent port blocked, the brakes will apply with more pressure, causing more heat and pressure. The driver may feel the brake pedal getting harder to apply or notice the vehicle seems to lack power. This is due to the vehicle operating with the brakes held ON.
To make sure the Vent Port is not blocked with the brakes OFF, you should check to ensure there is a swirl or disturbance in the reservoir of the master cylinder while the brakes are applied.
Free-Play is measured with a ruler or tape measure. Too little free play is not good! Too much is rarely a problem as long as there is still plenty of pedal reserve (distance to the floor while brakes are fully applied).
Free play should be checked any time the master cylinder is replaced. It should also be checked if the Power Brake Booster has been replaced. Also be sure to check brake pedal free-play after replacement or adjustment to any brake switch.
Brake Pedal Reserve
Brake Pedal Reserve is a measurement from the floorboard of the vehicle to the TOP of the brake pedal when the brakes are applied.
Check the brake pedal reserve distance with the engine running. This allows the power brake system to operate. Apply the brakes with a moderate force (about 20 lbs of force).
Make sure there is room between the brake pedal and the floorboard with the brakes applied.
Not all manufacturers specify a pedal reserve distance however there should be at least 2 inches of travel before the brake pedal hits the floor.
Many factors affect pedal reserve.
...Drum brakes can be out of adjustment.
...There can be air in the brake fluid.
...The master cylinder can be worn and “bypassing”
...Pedal linkage can be worn or mis-adjusted
...Disc brake rotor can have too much runout causing “knock-back”
...Wheel bearings can be loose or worn causing “knock-back”
Knock-Back occurs when the spinning rotor wobbles and pushes the piston too far back into the disc brake caliper. When the brakes are applied the piston must travel further than normal and will be noticed as a low brake pedal.
Knock back only occurs when the vehicle is moving.
A pedal that has good pedal reserve with the vehicle parked, but a low brake pedal while driving should have the rotors and wheel bearings inspected for lateral runout.
Instructors can Logon here