Master Cylinder: Fading Pedal

First a complete visual inspection of the entire hydraulic system should be performed to rule out any external leaks. The master cylinders cup seals or the cylinder bore itself may be worn. Internal leaking or bypassing cup seals must be considered any time vehicles brake pedal is fading to the floor. This fading pedal with no external leak found is a common master cylinder symptom caused by these cup seals. The result will be a loss of hydraulic pressure. The vehicle will creep at stop lights as the brake pedal fades toward the floorboard.

The master cylinder consists of two pistons that work in tandem to apply pressure to the braking system. These pistons use cup seals to seal fluid in the pressure chamber. When the brakes are not applied the seals rest between the "compensating" / replenishing port and inlet port. As the seal moves past the replenishing port it begins to create pressure in the pressure chamber, through the lines, and onto the braking units. If the fluid bypasses the cupped seal on either the primary or secondary piston, the peddle will fade to the floorboard or you will have to continually pump the pedal to maintain pressure. This internal leak caused by bypassing cup seals is a common problem with all types of vehicles. These seals can wear or become contaminated.