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How to Test And Fix Spongy Brake Pedal

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Tags: Tech, Brakes
How to test your brake booster

After you’ve successfully bled your brake system, does your pedal continue to feel spongy or are you still not getting the performance you need out of your brakes? One of the first things you can check is the integrity of your booster and master cylinder.

*The valve on your booster where the vacuum line plugs in is a one-way valve and it’s the first thing to check.

*Next with the engine off, pump the brakes several times to remove any residual vacuum in the booster.

*Now push the brake pedal firmly down and hold pressure on the pedal, start the engine and when the vacuum reaches the booster the pedal should go down slightly. If it doesn’t move at all or if it goes down over an inch or more there is probable cause for concern.

*Next install a vacuum gauge in the line at the booster, start the engine and check the available vacuum. For full assist manufacture requirements vary between 17-22” of vacuum.

*When using a more radical lift cam with lower manifold vacuum supply, one option is to use a 12 volt vacuum pump available from Speedway under part number 91028146.

How to test a dual feed master cylinder

Note: This test will require a complete re-bleeding of the system due to the brake lines being removed.

  • First remove the brake lines from the master cylinder ports. Then block off the master cylinder brake line ports using the correct size inverted flare plugs or bolts. Dual feed master cylinders may have ports on both sides that need to have all four outlets plugged off.
  • Note the protruding male cone of the inverted flare seat in the master cylinder port is made of a soft material and can easily be damaged.
  • After all the ports have been plugged, apply constant pressure to the pedal, the pedal should be firm, and should not drop over time. If the pedal quickly becomes squishy there may be air in the master cylinder. You may need to bench bleed the master cylinder. If the pedal is firm and then drops over time under constant pressure, the master cylinder should be replaced.
Common pedal ratios

There are many variables that can come into play when determining what your pedal ratio should be. Input force, the master cylinder bore size, and of course the pedal ratio. The line pressure achieved is a direct result from each of these variables and every system will require an optimal line pressure. From our experience, on a typical street rod application, what our technicians typically recommend as an excellent starting point is a 6:1 ratio when using a 1" or 1 1/8" bore master cylinder, power or manual setup. Since pressure range outputs can vary, it’s a great idea to install a pressure gauge to verify what your maximum pressure achieved is.

Many of our customers experience spongy or hard brakes due to incorrect line pressures, which are caused by incorrectly matched components. Sometimes it's impossible to identify the problem until you have some data to work with. The brake pressure testing gauge we offer, part number 91001704 is a must have tool for anybody designing their own brake system. From our experience, typical line pressure in a panic stop can be upwards of 1200-1300 PSI, while routine pressures can be around 800 psi or even less on drum brakes.

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