•Position of piston is balanced between variable pressure chamber and vacuum chamber.
553 Brake Systems Technician HandbookBooster Air Valve – Release Position Technical Training 83When the brakes are released, the valve operating rod and air valve are moved to the right by the air valve return spring and the master cylinder. •The air valve contacts control valve, blocking atmospheric pressure. •The control valve moves away from vacuum valve. This opens the variable pressure chamber and constant pressure chamber. •The booster piston is pushed back to the right by the diaphragm spring.
553 Brake Systems Technician HandbookIf there is no pressure difference between the two chambers, the booster cannot create any assist. The brakes will be applied mechanically, requiring greater driver effort. Booster Air Valve – No Vacuum 84 Technical Training
553 Brake Systems Technician HandbookThe tandem brake booster design is a compact and powerful unit. Its two constant pressure chambers and two variable pressure chambers provide a large surface area in a smaller package. Tandem Brake Booster – Brakes Released Technical Training 85
553 Brake Systems Technician HandbookThe booster air valve operates the same as previously described. Tandem Brake Booster – Brakes Applied 86 Technical Training
553 Brake Systems Technician HandbookBrake Pressure Conversion Unit Technical Training 87The brake pressure conversion unit, located between the brake master cylinder and the brake booster, enables the master cylinder to generate higher pressure in the event of brake booster malfunction. If a booster malfunctions on a conventional brake booster and master cylinder assembly, the driver must exert much more effort on the brake pedal to cause enough pressure at the master cylinder to slow the vehicle. This is found on vehicles such as the Sienna, Tundra and Tacoma Double Cab but with a remote reservoir. If the master cylinder was disassembled or if the reservoir became empty, bleed the air out of the master cylinder. Note that it is necessary to run the engine at idle while bleeding. The No. 3 chamber must be bled during brake system service. Check the appropriate Repair Manual for the exact procedure to follow. A typical procedure includes these three steps: •Bleed air from the brake system at each wheel. •With the engine running, bleed air from No. 3 chamber. •With the engine off, bleed air at each wheel a second time. SERVICE TIP
553 Brake Systems Technician HandbookNormal Vacuum Operation 88 Technical TrainingSERVICE TIP The conversion unit has a large-diameter piston located in the No. 3 chamber and positioned between the primary master cylinder piston and the brake booster push rod. When the booster push rod pushes the No. 3 piston, it causes hydraulic pressure to build on the primary piston. Referring back to basic hydraulic theory, the apply piston (No. 3) is larger than the working piston (No. 1), so the force applied to the working piston is reduced for normal braking.