brake booster

[Bob@MorganBingham.Com]

Originally Posted by Bob@MorganBingham.Com

2010 Yaris Sedan - orignial purchaser.
Dealer has brake booster on order to replace under warrenty. Symptom was a sound the Toyota mechanic had "never heard before from the transmission side of the engine" when pulling to a stop. This was a type of intermitent hum.
Note: A British classic sports car mechanic thought it was a "vacuum sound."
The Yaris brake booster on my checking the forum, does seem to work on a vacuum principle.

FOLLOW UP - noise persisted after Toyota replaced brake booster SEE "Humming Yaris"

[Geoff Peace]

Originally Posted by CTScott

The brake booster works off vacuum. Shut the car off, press the brake pedal to the floor. Repeat. If on each additional pump the pedal goes less far the booster is fine. If you can continue to press it all the way to the floor either the check valve or the booster has an issue.

You can check the check valve by removing it and blowing through it. You should be able to blow through the threaded side (that screws into the booster), but not the hose nipple side (that accepts the vacuum line).

The hissing may be a vacuum leak in the hose, the check valve or the booster. The above will help figure out which...

Agree with the above. Also with the engine switched off press the brake pedal several times to exhaust the servo. (brake booster in US I think) Then with firm pressure on the brake pedal start the engine, the brake pedal should go down a little further as the servo assists the foot pressure. If this is so all is fine. The servo is simply two chambers with a diaphragm between. When in use, one side is in vacuum, exhausted by the inlet manifold, the other is normal air pressure which provides the assistance.

Regards Geoff Peace.