A cutaway view of the Goodyear master cylinder is shown in figure 12-23. Fluid is fed by gravity to the master cylinder from an external reservoir. The fluid enters through the cylinder inlet port and compensating port and fills the master cylinder casting ahead of the piston and the fluid line leading to the brake actuating cylinder.

Application of the brake pedal, which is linked to the master cylinder piston rod causes the piston rod to push the piston forward inside the master cylinder casting. A slight forward movement blocks the compensating port, and the buildup of pressure begins. This pressure is transmitted to the broke assembly. When the brake pedal is released and returns to the OFF position, the piston return spring pushes the front piston seal and the piston back to full OFF position against the piston return stop. This action again clears the compensating port. Fluid that was moved into the brake assembly and brake connecting line is then pushed back to the master cylinder by the brake piston as the piston is returned to the OFF position by the pressure of the brake piston return springs.

Any pressure or excess volume of fluid is relieved through the compensating port and passes back to the fluid reservoir. The compensating port assures against dragging or locked brakes.

If any fluid is lost back of the front piston seal due to leakage, it is automatically replaced with fluid from the reservoir by gravity. Any fluid lost in front of the piston from leaks in the line or at the brake is automatically replaced through the piston head ports, and around the lip of the front piston seal when the piston makes the return stroke to the full OFF position. The front piston seal functions as a seal only during the forward stroke. These automatic fluid replacement arrangements always keep the master cylinder, brake connecting line, and brake assembly fully supplied with fluid as long as there is fluid in the reservoir.

The rear piston seal seals the rear end of the cylinder at all times to prevent leakage of fluid. The flexible rubber boot serves only to keep out dust.

Provision is made for locking the brakes for parking by a ratchet-type lock built into the mechanical linkage between the master cylinder and the brake pedal. Any change in the volume of fluid, due to expansion while the parking brake is on, is taken care of by a spring incorporated in the linkage. The brakes are unlocked by application of sufficient pressure on the brake pedals to unload the ratchet.

Brake systems employing the Goodyear master cylinder must be bled from the top down. In no case should bleeding be attempted from the bottom up, because it is impossible to remove the air in back of the piston seal. Bleeding operations are covered later in this chapter.

The Gladden master brake cylinder consists of a cylinder body, valve, piston, piston rod, return springs, and a stop assembly, as shown in figure 12-24. The piston rod extends through the valve, the piston, the stop assembly, and the return springs, and is connected by an eyebolt to the broke arm on the rudder pedal. When the cylinder is in neutral, the valve is not seated. Fluid from an independent brake reservoir enters the cylinders reservoir port. Fluid entering this port is allowed to flow through the piston and fill the lower chamber.

When the rudder pedal is depressed by toe pressure, the piston rod is pulled downward, causing the valve to seat and close the piston orifice, This movement also forces fluid into the brakes pressure line to the wheel brake assembly, thus applying the brakes.

When the pedal pressure is released, the springs return the valve and the piston to their neutral position. The retracting brake assembly piston forces the return fluid back through the piston orifice to the brake reservoir.