About the Master Cylinder
A complete visual inspection of the entire hydraulic system should be performed to rule out any external leaks. Then if we take a look at the master cylinder it’s easy to see why this part must be considered any time a customer complains the pedal is fading to the floor or they must keep pumping the pedal to keep from creeping at red lights. This is a common master cylinder symptom caused by leaking cup seals.
The master cylinder consists of two pistons that work in tandem to apply pressure to the brake 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 master cylinder internal cup seal leak is a common problem with all types of vehicles. These seals can wear or become contaminated. Always use the proper brake fluid.
