Brake Pedal Is Hard When Car Is Off Then Soft When On

Brake Pedal Is Hard When Car Is Off Then Soft When On

You could be relaxing in your driver’s seat and suddenly realize that you cannot push the brake pedal down.

This problem could make you wonder what the problem is.

Brake pedal hard when car is off then soft when the car is on

This article will discuss the leading causes of this problem and the solutions you should implement to correct the error.

In some other instances, your brake pedal could be soft, which means it goes down the car floor when you push it down.

If you wonder what the issue could be, kindly read this article to understand the possible causes and solutions.

The Brake Pedal Gets Hard When The Car Is Off, Then Soft When The Car Is On

There are various reasons why this happens. Here are some potential causes and the solutions:

Vacuum Pressure

Lack of enough vacuum pressure is the fundamental cause of a hard pedal when the car is off.

This aspect should be the first thing you should look into when identifying a hard pedal.

For a brake pedal to function efficiently, you need to have enough vacuum pressure.

This aspect will ensure that braking efficiency.

In some cars, especially the engines powered by gasoline, the vehicle can provide a self-vacuum required for boosting brake power.

Causes Of Vacuum Loss

Use of a vacuum booster with an improper vacuum level. If you don’t get this aspect right, you will get a hard pedal.

The car engine will require an estimated 18″ for brake efficiency.

Use Of A 3/8″ Fuel Hose

When someone decides to use this size of a hose, then most likely, this will cause a hard pedal.

Note that the primary purpose of a fuel hose is to decline expansion, hence the need to use a bigger-sized hose.

An Obstacle On The Vacuum Hose

This problem could limit the amount of vacuum pressure, resulting in a hard pedal.

Location Of The Vacuum Source In The Engine

This aspect is another crucial cause of the loss of vacuum pressure.

When small fittings are used, the chances are that the engine will not have the ability to acquire enough vacuum pressure.


  1. Use an 11/32″ fuel hose instead of a 3/8″ fuel hose.
  2. Use larger, large vacuum fittings in the engine to generate enough vacuum pressure.
  3. Ensure that all obstacles on your 11/32″ fule hose are removed to ensure proper vacuum pressure generation.
  4. Use a vacuum booster with a proper vacuum level. Remember to use an 18″ vacuum level booster.

Valve Problem

In many cases, when the brake pedal gets hard before you start your car engine, the combination valve could be the main problem.

Inside the combination valve, we have another valve called the Pressure Differential Valve.

It is a crucial valve, but please note that it could be a massive problem if it is not working correctly.

Cause Of Valve Problem

As explained before, the Pressure Differential Valve is essential.

It moves when there is a reduction of vacuum pressure on the front part of that valve instead of the rear side of the valve.

When the opposite happens, you expect the fluid from the master cylinder to hit a wall.

Understand that the brake fluid does not have compression properties. Therefore, the brake pedal is hard when the car is off and soft when on.

This concept can be described as a tripped valve.


If you notice that your valve is fully tripped, then you need to implement a solution as soon as possible.

You have to solve this problem by re-entering the Pressure Differential Valve. You have to replace the brake booster and your Mass cylinder.

How do you do this? You need to exert pressure on the two sides, the rear and front sides of the valve.

The amount of pressure exerted should be equal. You will be able to move the valve to its preferred position.

Reasons why brake pedal hard when car is off then soft when the car is on

Pedal Ratio

If you wonder what this means and how it causes a hard pedal when the car is off, let’s have an overview of what it means.

A pedal ratio explains the relationship between the pivot points of the pedal and the entire length of your brake pedal.

Please note that the brake pedal is only used to apply motion to your brake booster, and this sorely depends on the size and length of your pedal.

Causes Of Incorrect Pedal Ratio

If there is an incorrect error of about 1/4″ in the Ratio, this problem will cause some little pushrod to divert to the brake booster.

This problem hinders the brake booster from pushing the piston to its required destination, the master cylinder.

When the brake pedal is hard in such a case, You can bottom it out.

The movement of the pedal means that the master cylinder will be left in shock, which will entirely brake a lot of pressure on the wheels of your car.


You should relocate the connecting point of your pushrod, majorly in between the brake pedal and the brake booster.

Air In The System

In many cases, the air in the system is the leading cause of why you have a soft brake pedal when the car is on.

Cause Of Air In The Brake System

Air penetration in the brake lines is a massive contributor to soft pedals once you start the car. Air penetrates during repairs or when you are replacing brake fluids.


The best method to control this is to press the brake pedal more gently several times.

When you do this a few times, you will notice that your brake pedal will get slightly more firm than before.

If this works, you should find a way to bleed the brakes. Because we can’t see air, we have to develop a solution to get it out.

Fluid Leaks

Fluid leaks sound like an obvious problem but are one of the leading causes of a soft pedal when the car is on than hard when off.

This one causes serious harm to your holidays, and the bra brakes collapse entirely with time.


Leaks may result from hitting obstacles.

When you have leaks in any of your brake lines, this will result in less brake fluid, and eventually, it will cause soft pedals when your car is on.


  1. You need to repair the leak.
  2. As the user, you have to conduct a complete blending of the car’s brake system.

Master Cylinder’s Bore Size

When the vehicle has calipers with many small pistons, you will require a master cylinder with small bore size. The bore size is tied to the system.

If the bore size is minimal, the vehicle will need a more considerable distance to give the required volume and pressure needed to obtain a good piston journey at the caliper to yield the necessary clamping force.


  1. The master bore size should be correct. Do not mix sizes to ensure the safety of your brake system.
  2. Every size should be in the required form to ensure efficiency always.

Final Thoughts

In many cases, you experience a hard brake pedal when the car is off than a soft when it is on. You need to diagnose the problem, then proceed.

If you cannot do that, you can contact your dealership or the mechanic of your choice to assist you in drafting the problem.

The car user should always check the brake system regularly to ensure safety. As a vehicle user, you should check on your brakes regularly.