Tech Talk with Master Power Brakes

How to: Diagnosing a Hard Brake Pedal

Posted by rick on Sep 18, 2017 11:28:21 AM

Do you have a brake pedal that feels hard to press?

You may have a bad booster, booster hose, or check valve. In this 'How To' post, we offer a simple checklist to help you diagnose potential causes of a hard brake pedal.

If you can’t get a pedal please perform these tests before calling! Click here to download a printable PDF version of these troubleshooting tests.  If you are unable to diagnose your problem and decide to give us a call, please have your customer order number ready. Our commitment is to guide you in selecting the right kit for your application and be here for you all the way through installation.


vacuum canister, hard pedal

The most obvious cause for a hard pedal is simply not enough 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 might help your situation. If below 16” of vacuum, an electric vacuum pump might be your best option.

wrong hose, vacuum hose

Check the hose running to your booster. It is not uncommon to ask for 3/8” vacuum hose at your local parts store and the kid behind the counter doesn’t have a clue that there is a difference between vacuum hose and fuel hose. He just knows it’s 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.

valve, brake booster valce, check valve

The brake booster check valve could be defective. This is something easily overlooked. To check, remove the valve from the booster and disconnect from the hose supplying vacuum. Blow into the valve on the hose booster side. If air travels through and comes out the hose side, then you have a bad check valve. If you are unable to blow through, reinstall the valve and look for a different problem.

bad booster, booster

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

A) With the engine not running, press and depress the brake pedal several times to remove any vacuum from inside the booster.

B) 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.

C) Start the engine and pay attention to what happens to the brake pedal.

D) If the pedal drops slightly, then the booster is working correctly.

E) If the pedal does not move at all, then there is a very good chance the booster is bad.

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 hard 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 hard pedal.

Need Help?

These are just a handful of possibilities that are very common when a customer mentions a hard 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!

We invite you to Experience the Difference of Master Power Brakes - The Leader in Model Specific Brake Kits.

 download our ebook


Topics: diagnostics, hard brake pedal, hard brakes, how to, How To Series

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