AP Racing master cylinders use small cut-off ports to ensure that pressure is relieved from the brake system when no travel is applied to the brake pedal. As the brakes are applied the seal travels over this cut-off port. In normal operation the seal has travelled past this port before high pressure has built up in the system. However, when used in conjunction with ABS, depending on how the ABS operates, pressure can be built up earlier in the travel or during the return stroke. This can then result in heel nibble where the seal is partially extruded up the cut-off port. The pulsing nature of ABS can also make this effect worse.

It is possible to run AP Racing cylinders with ABS by allowing sufficient travel before pressure is built up and limiting the pressure during return, but as AP Racing do not control the ABS we cannot guarantee successful operation. Typically 6mm of travel will allow all seal sizes to be past the port and the maximum pressure up to this travel should be approximately 10 bar maximum. If this is exceeded the life of the seal will be compromised and re-sealing should be carried out more frequently