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How Does A Brake Booster Work & TECH



Is a power booster required with disc brakes?
No. Although a power booster gives a good pedal feel, manual brakes work fine.
What are the symptoms of a bad power booster?
A bad power booster will give a very hard pedal, it will feel like you need two feet to stop the car.
What does a booster do?
A power booster helps assist pushing the master cylinder piston when you apply the brakes.
How much pressure should I be getting to the wheels with a power booster?
Typically you should expect about 1000 psi. to the wheels for a disc brake system. A disc brake system requires this amount of pressure so be careful when using a smaller 7" booster that puts out only 900 psi pressure.
What size booster do I need?
For lighter weight vehicles like street rods a 7" booster will work fine for disc/drum set ups. For a 4 wheel disc system a 7" booster will not give enough assist. An 8" dual diaphragm booster is the ticket for 4 wheel disc.
Can I use a Ford Booster on a GM vehicle?
If you can mount the booster properly and link it to the pedal then any booster can be used on any type of car.
How much vacuum is needed to operate a booster properly?
For a power booster to function properly you will need at least 18" of vacuum. Anything lower will give you a hard pedal.
I have power drum brakes, can I use the same booster if I change to disc brakes?
Yes. Just be sure to use a disc brake master cylinder.
What happens if the vacuum is too low?
If your vacuum level is too low you will experience a hard pedal and it will feel like the vehicle won't stop.
I have a radical cam. Is there any way to supplement my low engine vacuum.
Yes an electric vacuum pump that works off the 12 volt system and gives 21" vacuum.
Will a reserve canister help?
Somewhat, but usually they will only show a moderate increase in braking performance since the engine doesn't really get to a sufficient vacuum level on a constant basis. You really need a constant 16" at least.
What are my options if I dont want to use the vacuum pump?
Not many. Without the supplemental pump the only choice is to eliminate the booster and go to manual brakes.
How will I know if my booster is bad?
A bad booster will give a very hard pedal and it will feel like the vehicle will not stop.
How do I check to see if my booster is operating properly?
Shut off the engine. Depress the brake pedal a few times to evacuate the booster Apply a steady pressure to the pedal and start the engine. The pedal should fall slightly.
Is there any way to decrease booster size and still maintain assist?
Yes. By using a dual diaphragm design booster you can maintain a high level of assist while keeping a small size. Pirate Jack offers an 8" dual diaphragm booster that fits most street rod and GM applications.
I have installed a big block and the booster hits the valve covers. Is there anything I can do?
We offer an 8" dual diaphragm booster that will work for most GM and Ford applications.
Will I need a special booster with 4 wheel disc brakes?
The main consideration with 4 wheel disc brakes is that you have plenty of power assist. Don't undersize your booster with a 4 wheel disc set up.
When I step on the brakes the pedal feels good but I get no braking.What could cause this?
One of the causes of this is a mismatch between the booster pin length and the depth of the master cylinder piston hole. Be sure the hole is not too deep for the booster pin.
I have installed a new booster on my car and now the brakes drag.
Again this can be due to a mismatch between the booster pin and the master cylinder piston. Too long a pin will cause this.
I have installed power brakes on my origional manual brake car and now the brakes are extremely sensitive.
There are two attachment points on the brake pedal.For power brakes you need to use the lower attachment hole. Using the upper hole will make the brakes too sensitive.
Power boosters keep going bad on my vehicles.
You are probably getting corrosive vapors back into the booster. Install a vapor trap and that should eliminate the problem.
CanI rebuild my own booster?
This is not a good idea. There are many parts inside the booster that will require special tools to assemble and re assemble.


How does a booster work?

A power booster uses atmospheric pressure to help assist the pushing of the master cylinder piston. Basically, a power booster is divided into two chambers internally with a rubber diaphragm seperating each. With no force applied to the booster there will be vacuum on both sides of the diaphragm. This vacuum is supplied by the engine either from the intake manifold or the back of the carburetor.

When you press on the pedal, the rod opens a poppit valve in the booster that allows atmospheric pressure on the firewall side of the diaphragm. This causes the diaphragm to push on the master cylinder piston thru a plastic hub, providing power assist to the master cylinder.