Brake Master Cylinder: Description and Operation
MASTER CYLINDERDESCRIPTION
Two different master cylinders are used on this vehicle. Vehicles without Antilock Brakes (ABS) use a standard compensating-port master cylinder, while vehicles equipped with ABS use a center-valve design master cylinder.
ABS vehicles, equipped with rear disc brakes, use a master cylinder with a 23.82 mm (0.937 inch) bore diameter, while non-ABS vehicles, equipped with rear disc brakes, use a 22.23 mm (0.875 inch) bore diameter master cylinder.
The ABS master cylinder is a two-outlet design and the brake tubes from these primary and secondary outlet ports lead directly to the Integrated Control Unit (ICU) before going to each wheel brake (Fig. 94).
The non-ABS master cylinder is a four-outlet design (one for each wheel brake) with two screw-in proportioning valves (one for each rear wheel brake). One is attached directly to the inboard side of the master cylinder housing while the other is attached to the bottom (Fig. 96).
Both type master cylinders mount to the power brake booster using two nuts. They both have a seal on the rear of the mounting flange to seal vacuum in the booster.
The master cylinder body is an anodized aluminum casting. It has a machined bore to accept the master cylinder piston and also has threaded ports with seats for hydraulic brake line connections.
The master cylinder has the brake fluid reservoir mounted on top of it which gravity feeds brake fluid to the master cylinder when it is required. The reservoir is made of see-through plastic and it houses the brake fluid level switch. A removable brake fluid level switch is mounted in the left side.
OPERATION
When the brake pedal is pressed, the master cylinder primary and secondary pistons apply brake pressure through the proportioning valves (on non-ABS vehicles) and chassis brake tubes to each brake assembly. The brake fluid reservoir supplies the brake hydraulic system with the necessary fluid to operate properly.
The non-ABS master cylinder's primary outlet ports supply hydraulic pressure to the right front and left rear brakes while the secondary outlet ports supply hydraulic pressure to the left front and right rear brakes. ABS equipped master cylinder outlet ports supply hydraulic pressure to the ABS Integrated Control Unit (ICU) where it is distributed to the individual wheel brakes.
The master cylinder reservoir cap diaphragm is slit to allow atmospheric pressure to equalize on both sides of the diaphragm.