553 Brake Systems Technician Handbook Portless Master Cylinder Portless master cylinders use a slightly different approach to compensation that allows them to send more pressure to the wheel cylinders. Instead of using separate inlet and compensating ports, there is a single non-restrictive passage from the reservoir to the piston. A machined passage in the piston handles compensation. • When the brakes are at rest the valve is open, allowing fluid to move freely between the reservoir and chamber. • When the brakes are applied the spring loaded valve closes, sealing the passage and allowing pressure to build in the circuit. 16 Technical Training