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Tech Talk with Master Power Brakes

What Causes A Firm Brake Pedal and How Do I Fix It?

Posted by toby on Aug 6, 2014 4:44:48 PM

Do you have a brake pedal that feels firm or hard to press? If you do, the culprit may be a bad booster, booster hose, or check valve. In this post, we offer a simple checklist to help you diagnose potential causes of a firm brake pedal.

If you can’t get a pedal, start by performing these diagnostic tests before calling! Click here to download a printable PDF version of these troubleshooting tests.

1. The most common cause of a firm brake pedal is insufficient vacuum. Any brake booster requires a minimum of 18” vacuum to operate efficiently. Anything less than that will contribute to the pedal being harder to push. If your vacuum is in the 16” to 18” range, a vacuum canister may help. If below 16” of vacuum, an electric vacuum pump is probably your best option.

2. Check the hose running to your booster. It is not uncommon to ask for 3/8” vacuum hose at the local auto parts store and the guy behind the counter doesn’t understand the difference between fuel hose and vacuum hose. For some guys, hose is hose. Fuel line is not designed to resist sucking shut like vacuum hose. It will only resist expansion. Therefore, if you have the wrong hose and it is sucking shut, it doesn’t matter how much vacuum your engine makes if it is never getting to the booster.

3. Check to make sure the brake booster isn’t defective by removing the valve from the booster and disconnecting from the hose supplying vacuum. Blow into the valve on the hose booster side. If air travels through and comes out the hose side, a bad check valve is the problem. If you are unable to blow through, reinstall the valve and look for a different problem.

4. The booster could be bad. There is a quick and simple way to check this. Follow the steps below:

  •  With the engine not running, press and depress the brake pedal several times to remove any vacuum from inside the booster.
  •  On the last push of the brake pedal, hold moderate pressure on the brake pedal. Don’t push like you are panic stopping, simply hold pressure like you are sitting at a red light.
  • Start the engine and pay attention to what happens to the brake pedal.
  • If the pedal drops slightly, then the booster is working correctly.
  • If the pedal does not move at all, then there is a very good chance the booster is bad.

5. If all of these systems above check out, it’s time to look at some other areas. From here, the diagnostics are a little more complicated than the simple tests listed above. Here is a list of possible things to check:

  • Are you running a combination valve in your system? Is the valve still centered or has it tripped to one end or the other? If it has, this could be causing your firm pedal.
  • Do you have drum brakes? If so, make sure the wheel cylinders are traveling freely and not stuck.
  • What kind of pedal ratio do you have? On a power brake system, it is recommended that your ratio be 4:1. Many older vehicles in the 50’s used a pedal ratio of 1:1. This will not work on cars equipped with a vacuum brake booster.
  •  Is your brake booster correct for weight of the vehicle? Using too small a brake booster on a heavy vehicle will give a firm pedal.

These are just a handful of possibilities that are very common when a customer mentions a firm pedal. Take a look at your brake system and see if any of these are causing the problem.  Or, if you are ready to throw in the towel and need expert help, give us a call or drop us an e-mail and we’ll get your brakes working properly so you can get your classic out of the garage and back on the road where it belongs!

Topics: classic cars, hard brake pedal, How To Series, firm brake pedal, how to fix

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