Causes of Intermittent Hard Brake Pedal


Hard brake pedals aren’t something you ever want to deal with. Diagnosing the problem and dealing with a hard brake pedal early can make a huge difference in the cost of the repair. 

Here are some of the most common causes of hard brake pedals to help you diagnose them and get the right kind of repair. 

Not Enough Vacuum

Hard brakes are often the result of not having enough vacuum in the hose. When the problem is intermittent, you might have a good vacuum at times and not enough at others. Unfortunately, this problem can be hard to predict, so it’s worth getting your brake lines checked for vacuum the first time you have hard brake problems. 

When your engine runs it acts as an air pump that creates a vacuum for your car’s brake system. The vacuum is created by the air pulled by the air intake manifold. Sometimes the intake runner that helps create that vacuum can also be run through the carburetor.

From the intake manifold to the brake booster you need a working vacuum hose. 

All that vacuum is important because your brake function relies on a valve in the middle of the booster that is designed to be in a neutral position when the brake isn’t in use. When it is in use, the movement of the diaphragm in the valve changes pressure, activating your brakes. 

Normally, pressing on your pedal is enough to cause movement in that diaphragm. But, if the vacuum isn’t strong enough, just pressing on your pedal won’t be enough. The valve might not be in a neutral position to start with, and it takes that much more pressure to get the result you need. 

That means that you’ll end up putting more and more pressure on the brake pedal to get it to move. The brake will also feel harder, and it might take more pressure just to get the brake pedal to move at all, much less to get the brake pressure you need to slow down.  

Read: Brakes Locking Up In Reverse?

Most of the time wrong hoses aren’t something you have to worry about in a car. Manufacturers and mechanics both work hard to make sure they’re using the right equipment in the right places, which means that its unlikely to have the wrong kind of hose on a new car. 

However, if you’ve bought your car used, do the maintenance yourself or pay anyone who isn’t a licensed mechanic, you might end up with a brake vacuum hose that isn’t designed for the kinds of pressure created by the brake system.

You might not have a vacuum hose at all, in which case it might not hold the required pressure or might not be able to create a true vacuum at all. 

At least 18” worth of vacuum is recommended for most brakes to operate properly, and they will start to have degraded performance if there is less vacuum available. 

Technically a bad hose or the wrong hose contribute to the same problem we’ve already talked about. If the hose isn’t working properly, chances are you won’t have enough vacuum in the line for the valve in the brake booster to work well. 

Of course, even if you have the exact hose your car needs, you may still have hose issues over time. The hoses in your car need to be replaced on a regular if infrequent, basis. Without regular replacement, they can dry out and lose flexibility. 

The temperature changes near your engine can make hoses wear out faster, and with a hose that needs to maintain a vacuum that can be a big problem. 

If you’re having intermittent hard pedal problems and you haven’t checked your hoses in a while, it’s a good idea to check your brake hoses now.

Not only will you be able to spot if the hose is losing flexibility or beginning to crack, but you’ll also be able to check on the hoses for other critical equipment and other parts of your brake function to make sure everything is in good working order. 

Read: Why Brake Pedal Sinks to the Floor?

Defective Brake Booster Check Valve

The brake booster check valve is the valve we’ve been talking about this whole time, the one that relies on the vacuum created by the engine to remain in the neutral position when you aren’t braking. 

In addition to having problems with other equipment that prevent the brake booster check valve from functioning properly, it’s also possible for the valve itself to be defective or get damaged. 

This valve works to hold the vacuum created by the engine in place. When you need the vacuum to work, the valve allows it. When you don’t need the vacuum, it’s supposed to hold that vacuum on one side, protecting the rest of the brake booster. 

Fortunately, checking for a bad valve is relatively straightforward. The valve is inserted into the brake booster, but it isn’t built-in to the main component. You can remove it. 

To test whether it’s working simply remove the valve and detach the vacuum hose. Blow into the vacuum hose (with the valve still on the other side). If air moves through the valve you know you have a defective valve and have found a potential source of your hard brake problem. 

However, just because you’ve found a bad valve doesn’t mean that that was the only contributing factor. Especially when your hard brake is intermittent it’s possible to have multiple causes all at once.  

Read: Brakes Squeal While Driving

Bad Brake Booster

While it’s more common for the brake booster check valve to go out than the brake booster itself, there are still plenty of other moving parts in your brake booster that can be damaged over time and might lead to an intermittent hard braking problem. 

Your brake booster is a highly precise piece of machinery with a front and a back, a valve that’s designed to hold vacuum, and several other moving parts. That kind of complexity means that there are a lot of little things that can go wrong and seriously affect performance. 

If your brake booster is bad, chances are you’ll need to get it replaced. But testing it isn’t that difficult and can get you one step closer to determining what kind of problem you’re dealing with. 

Turn your car completely off and pedal the brake several times to remove any vacuum that was created by the engine. The last time you pedal the brake, keep some pressure on it. Imagine you’re keeping the car stopped, but not as much pressure as if you were trying to slow down in a hurry. 

Turn the engine on. The brake pedal should move down about half an inch when you first turn on the car. If it moves, it’s working. If the brake pedal stays perfectly still it’s time to check the booster and valve to see where you’re having problems. 

Pedal Ratio

Issues with pedal ratios are more common in older muscle cars than in more modern designs, but it does still crop up occasionally. You should know about this problem if you like to drive vintage vehicles. 

The problem with the pedal ratio is exactly what it sounds like, the length of the pedal doesn’t work well with where the pedal pivots, making it feel hard to move even if all the other parts of the brake system are still working. 

That’s it. Those are all of the most common causes of hard brake pedals. Intermittent brake hardness can be even more difficult to deal with since you never know when you’re going to have issues with the brake system. That makes it even more important to get your brakes diagnosed and repaired as soon as possible. 

Once your brakes are operating at 100% again you’ll be able to drive with confidence, knowing that your brakes have your back. 

Related: Brake Fluid Leak – What To Do? How To Fix It

3 thoughts on “Causes of Intermittent Hard Brake Pedal”

  1. Hi very nicely explained. Thank you
    I have been trying to look into an intermittent fault I am currently experiencing, so your article has been very helpful.
    I have a 2013 mini cooper coupe S ( r58/n18 engine I think}
    The brake pedal goes hard “intermittently” for a few seconds. The pedal goes back to normal after I blip the throttle. This fault only seems to happen when parking, or upon start up in. morning. The pedal does not go hard while driving (thank god)

    If you have any other thoughts, and would like to give me your thoughts. You can e mail me>

    Many thanks

    • Hi Kian, The only way to really figure out what’s wrong is through the processes of elimination. In layman’s terms, the reason your brakes become hard overnight or after extended periods of parking is because the braking system lost some of its vacuum, which is “restored” when you start the car again. The vacuum is what makes pressing your brakes a non workout, so make sure to check everything listed in the article

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