Brake boosters

A brake booster is a component that sits in front of the master cylinder, and assists the driver to press on the brakes.  It does this by using vacuum pressure from the plenum, and makes the pedal feel 'spongy'.

It is a very basic component with not much involved.  It is made up of a large housing that sits on the firewall, with the push rod attached into the center from the driver's side of the firewall.  When the throttle is closed, it creates a strong vacuum pressure which pulls the push rod towards the master cylinder.  There is also a check valve mounted to the booster, which is a one way valve, which only allows vacuum to come from the manifold to the booster, and not the booster providing vacuum to the manifold.  This means that if the engine suddenly turns of during driving,  the brakes can still be operated with brake booster assistance.

If a driver complains that the brakes feel very stiff, check if the brake booster is working, and that there are no vacuum leaks in the intake manifold.

• Turn off engine
• press the brake pedal 5-6 times
• it should feel very stiff. Press the brake pedal and simultaneously turn on the engine. As the engine runs and vacuum pressure builds in the intake manifold, the pedal should soften. 

A very common issue is a vacuum leak.  Check that there are no vacuum leaks within the plenum.  Also check that the hoses running to the booster are not damaged or being stressed in any way.  If vacuum and hoses do not show any issues, the booster will need to be bench tested.