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3 Common Causes for a Soft Brake Pedal

35 years of building customer relationships at an automotive dealership helping them understand how cars work and what it takes to fix them.

This brake caliper is leaking past the piston o-ring and is causing a soft brake pedal.

This brake caliper is leaking past the piston o-ring and is causing a soft brake pedal.

The Three Most Common Reasons for a Soft Brake Pedal

  1. The brake master cylinder is leaking internally or externally.
  2. There is a leak in the system someplace other than the brake master cylinder.
  3. One or more of the calipers has a frozen or rusted moving part that isn't moving as designed.

There are other reasons why your brake pedal might feel soft, but in this article, I only discuss the three most common, in hopes of helping you diagnose the issue quickly, and give you some basic checks you can run.

Is It a Leak? Or Something Else?

The first step in diagnosing this problem is to check the master cylinder reservoir fluid level. Is the master cylinder reservoir low on fluid, or is it completely empty?

If the system's brake fluid reservoir is empty, most likely the system has a leak and you'll need to visually locate the failure. If the system is low on fluid but not completely empty, you're more likely to have a component that is frozen or rusted and needs servicing.

The brake master cylinder and booster, located under the hood.

The brake master cylinder and booster, located under the hood.

Notice About Low Brake Fluid

If you find that the brake fluid is low, adding brake fluid to the reservoir will not fix the issue. Your brake pedal will still be soft. If you want to keep driving safely, you must find what is causing the fluid to be low.

What You See When the Brake Master Cylinder Is Leaking Internally or Externally

If the brake master cylinder leaks internally, usually the only sign you will have is that the brake pedal sinks to the floor slowly when you're applying light pressure to it. For example, when you are sitting at a traffic light with your foot on the brake, eventually the pedal will reach the floor. Unfortunately, you can't see the leak from a visual inspection, because it's inside the master cylinder, between the shaft seals. No fluid is being lost from the reservoir; it's lost between chambers internally.

If this is something you're starting to notice, pull into a parking lot and put the vehicle in park or apply the parking brake, and check the brake fluid level in the master cylinder first. If it's empty, you have an external leak in the system, if it's within the full and low line, the master cylinder may be leaking internally.

A quick check to see if your master cylinder is leaking internally would be to apply light pressure to the brake pedal and notice if it sinks slowly towards the floor. If it does, you know you have a problem with the master cylinder leaking internally.

Visually Inspecting the Brake System for Fluid Leaks

If the brake fluid reservoir is empty or extremely low on fluid, you'll need to do a visual inspection of the entire brake system to find the leak.

  • Start by looking behind each wheel with a flashlight, because if a caliper or wheel cylinder has a leak it will usually drip onto the inside of the wheel or tire.
  • Look at all the brake lines, starting at the master cylinder, and follow them to each wheel. Some brake lines are hidden by plastic shields or go up and over components, so look for wet oily fluid on the underside of the floor. When the line or component failed, it would have been under pressure, and may have sprayed fluid onto the floor.
  • Check inside the vehicle where the brake pedal connects to the master cylinder. It's usually connected by a steel rod and a cotter pin. Follow the steel rod to the firewall. If you notice any fluid dripping from the rod, or the carpet is wet by the brake pedal, the master cylinder is leaking past the rod seal inside the vehicle.
Look for leaks at the brake pedal master cylinder rod located at the top of the brake pedal under the dash.

Look for leaks at the brake pedal master cylinder rod located at the top of the brake pedal under the dash.

Removing and Inspecting the Calipers

See the photo gallery below. Caliper failure is very common because the intense heat generated when braking destroys rubber components and melts high-temperature grease into a liquid, leaving the caliper components exposed to the elements and lacking lubrication.

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  • Complete a thorough inspection by removing each caliper bracket and inspect all the slide pins for deterioration or rust. If any of the slide pins are rusted, jammed, or frozen, you'll need to restore them to normal operating condition or replace the caliper.
  • Squeeze each caliper piston with large channel lock pliers or a C-clamp, checking the piston's ability to ease back into the caliper housing without getting hung up. If any piston refuses to move back into the caliper housing completely, it will have to be rebuilt or replaced.
  • Inspect for any leaks around the pistons, line connections, or castings. If you find any leaks, the caliper will need to be repaired, rebuilt, or replaced.
  • Remove the brake pads from the caliper brackets and inspect the condition of the pad material. If the pads are jammed, frozen, or rusted into the caliper bracket and you need a hammer and pry bar to remove them, inspect for rust build-up under the caliper anti-rattle shims. This rust build-up is very common and causes pressure on the pads resulting in premature wear.

Note; If you do find the pads jammed into the caliper bracket I recommend you remove the anti-rattle clips and remove the built-up rust under them with a metal file. Or remove the rust gently with a grinding wheel, but be careful not to remove any metal from the caliper bracket or the pads will not fit properly. Once all the rust is removed, apply some brake pad lube to the contact surfaces, then reinstall the pad anti-rattle clips.

It's okay to lightly remove some rust from the pad holder tabs, but I don't recommend grinding off the metal on the pad holder tabs because it doesn't fix the actual problem and the condition will get worse over time.

Caution: Don't Drive a Vehicle With a Soft or Sinking Brake Pedal

A soft or sinking brake pedal is a serious problem and can be extremely dangerous resulting in a vehicle accident or even death. If you notice the brake pedal in your car is getting soft or is sinking to the floor while you rest your foot on it, there is a problem and you need to either park the car immediately or have it towed to a destination for repairs.

Caution: Brake Repairs on Newer Vehicles With Electronic Emergency Brakes

Keep in mind that newer vehicles have electronic emergency brakes that are built into the calipers and are controlled by the car's computer and ABS systems. These systems can affect the pressures in the system as well. So if you have one of these newer vehicles I highly recommend you have the brakes serviced by a professional who has the tools and software to perform the job properly.

Soft Brake Pedal FAQs

Is It Safe to Drive With a Soft Brake Pedal?

No. A soft brake pedal means there is a problem in the system that should be repaired before driving the vehicle.

Can Low Brake Fluid Cause a Soft Brake Pedal?

If the brake fluid is low enough for air to enter the brake master cylinder, that could cause the brake pedal to feel soft, but if the brake fluid in the master cylinder reservoir is above the low line, there may be a different problem.

Can You Drive Without a Brake Booster?

If the brake booster is disconnected, the brake pedal will feel rock hard, and it will be very difficult to stop the vehicle. I don't recommend ever driving a vehicle with the brake booster disconnected unless you consider using it in a legal smash-up derby.

Can the ABS Cause a Soft Brake Pedal?

Yes, there are a lot of moving parts inside an ABS modulator, and an internal leak or malfunction can cause the brake pedal to be soft.

What Causes a Sinking Brake Pedal?

A sinking brake pedal is usually caused by either a small leak in the brake hydraulic system or the brake master cylinder itself.

Why Does My Brake Pedal Go All the Way to the Floor Sometimes?

When a brake pedal goes to the floor sometimes, either the master cylinder is starting to deteriorate internally, or some other component in the system is not working correctly and needs diagnosing.

How Do You Test a Brake Booster?

A car's brake booster is activated, once the engine starts, by the engine's manifold vacuum, so if the engine port has vacuum at the brake booster and the brake booster assists the brake's stopping power, the brake booster is working as designed. If the brake pedal is hard and has no assist from the brake booster, it's possible the vacuum diaphragm is damaged inside the brake booster and is leaking vacuum.

Here is a quick brake booster check:

  1. Sit in the driver's seat of your vehicle. With the ignition key off and the engine not running, press the brake pedal three times to release all vacuum from the brake booster. At this point the brake pedal should be very difficult to press down.
  2. With your foot on the brake pedal applying light pressure, start the engine. The brake pedal should drop a few inches and the brake pedal should feel normal.
  3. If the pedal doesn’t drop once the engine is started, there may be a problem with the brake booster.

Why Is My Brake Pedal Soft After I Changed the Pads?

  1. If the brake pedal is soft after changing brake pads, it's possible that one of the rubber brake lines is twisted or that one of the brake calipers has an issue. Double-check the calipers to be sure all moving parts are not frozen and there are no leaks in the system.
  2. If you just replaced a brake caliper, line, or wheel cylinder, you will need to bleed the air from the brake lines.
  3. If you just replaced the brake shoes, you’ll need to adjust the rear brake shoe to drum clearance.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

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