•
Position of piston is balanced between variable pressure chamber and
vacuum chamber.
553 Brake Systems
Technician Handbook
Booster Air Valve –
Release Position
Technical Training
83
When 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 Handbook
If 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 Handbook
The 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 Handbook
The booster air valve operates the same as previously described.
Tandem Brake
Booster – Brakes
Applied
86
Technical Training
553 Brake Systems
Technician Handbook
Brake Pressure
Conversion Unit
Technical Training
87
The 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 Handbook
Normal Vacuum
Operation
88
Technical Training
SERVICE 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.
