Technical Brake Information

June 29th, 2011


  1. Use a test light by attaching a clip to a positive contact on the vehicle and touch the point of the test and the electrical connection of the combination valve. If the light does not come on, the valve system is operating correctly and further testing is not required.
  2. If the light does appear, this indicates that the pressure differential valve is stuck in the front of rear position.
  3. Bleed the brake system to determine which is blocked off. Set up one front wheel and one rear wheel for bleeding at the same time. Crack both the bleeder screw and gently pump the brake pedal a few times. The blocked side will trickle fluid out when the bleeder screw is cracked and the pedal is pressed. An unblocked line will squirt fluid from the bleeder.
  4. The lines that are clear must be left open and the blocked lines should have the bleeder screws tight to cause pressure to build up on that side. Be sure to use that standard bleeding procedures to prevent air from entering the brake system.
  5. Slowly press the pedal with steady pressure a number of times until the light goes out. This will center the differential valve. You may also hear a pop come from the proportioning valve. This is the metering valve returning to its equalized position. When the light goes out, close the bleeder screw.


It is rare that one of our kits will contain a defective power booster but, if you suspect that your booster is not functioning correctly, perform the following tests:

Basic Test:

  1. With the engine off press and release the brake pedal several time to eliminate vacuum from the power section.
  2. Press the pedal and hold down with light pressure, 15 to 25 pounds.
  3. Start engines.
  4. If the power unit is operating, the pedal will drop slightly. Less pressure will be needed to hold the pedal down.


  1. Disconnect the vacuum hose from the booster check valve and check the vacuum level at this point with the engine running with a vacuum gauge. You should have at least 18″ vacuum to the booster. Anything lower will begin to give a hard pedal. If the vacuum level is below 18″ you may be able to tune that engine and bring the vacuum level up to the level. If the vacuum level is around 16″ the addition of a vacuum reserve canister will improve braking. If the vacuum is below 16″ you will need to add an electric vacuum assist pump to supplement the engine vacuum.
  2. If the vacuum level at the check valve is 18″ check that the booster check valve is working. Disconnect the vacuum hose at the check valve and attach a piece of tubing. Blow into the valve. If the air passes through the valve, it is defective and needs to be replaced. Also look into the hose attachment neck on the check valve and be sure there is no obstruction inside the valve.
  3. Check your booster for a vacuum leak. With everything hooked up, run the engine at moderate speed. Release the accelerator and turn the engine off. Wait 90 seconds and apply the brakes. If the brake applications are power assisted there is no leak. If there is no assisting, the booster is defective and must be replaced.


  1. Your combination valve may have tripped shutting off fluid to the front or rear brakes. This condition will produce a very hard pedal. Check that fluid passed through the valve to both the from and rear by cracking a bleeder screw and observing a good flow of fluid. If one half of the system does not have flow, recenter the valve.
  2. You may have frozen rear cylinders of frozen caliper pistons. If these components freeze you can get a very hard pedal.
  3. Your pedal ratio may be too low. Check your pedal ratio. The pedal ratio must be in between 4:1 to 5:1. Some of the older cars that had power brakes used a ratio of almost 1:1. If you add a vacuum booster to this type of car you will have a very hard pedal. typically we are talking about late 50’s cars. Adjust ratio as necessary.
  4. Your booster may be undersized for the weight of the vehicle or the bore size of the master. If you try to use a small diameter booster such as a 7″ street rod booster for a heavy car you will get a very hard pedal. Compounding the problem is an attempt to use a large bore master (1-1/4″ or larger) on a small booster.


  1. Your pedal ratio may be to high. Power brakes will require a 4:1 to 5:1 ratio. If your ratio is around 6:1 you are getting to much mechanical advantage making the brakes extremely sensitive. Adjust the ratio to the correct level.
  2. The booster may be too large for the weight of the vehicle. Light weight vehicles with large boosters give you “touchy brakes”. This effect may dampen somewhat by going to a larger bore master.
  3. Too large booster for front drum brakes. Drum brakes do not require as much pressure as disc brakes (500 psi vs. 1000 psi). If your booster is very large (11″) and you have drum brake you are over-boosted. Do a pressure test to determine what you have.
  4. The booster has a cracker internal hub. When there is a crack in the phenoli hub inside the booster it will either be totally on or totally off. Any slight pressure to the pedal will cause the brakes to lock up. The booster must be replace.