It can be frustrating and concerning if you’ve had or are bleeding your brakes but still experiencing a spongy brake pedal. Spongy brakes can reduce your stopping power and make driving more dangerous. Understanding the causes and following the suggested solutions in this article can help ensure your brakes work correctly and keep yourself and others safe on the road.
Before you read on, if you do not know how brakes work and the different components, brakes are one item a novice should not mess around with. If you’re having trouble with spongy brakes after bleeding, have a qualified mechanic rectify the issue. This article is to be used as an educational reference only.
Signs air is still trapped in the brakes
The signs air is still trapped in the brakes, the most obvious being the pedal sinking straight to the floor. Not to try and teach you to suck eggs, but if this isn’t something you’ve thought of, after bleeding the brakes, the brake pedal will need to be pumped again to push the piston out, so the pads meet the brake rotors.
However, a good indicator that air remains in the brakes after bleeding them is with the ignition off if you pump the brake pedal and hold it pressed. If the pedal slowly sinks almost to the floor, the most probable cause is air remains in the brakes. If you were to start the vehicle with the brake booster/servo operating, the pedal would almost certainly pump up and begins to sink when pressed.
Causes of a spongy brake pedal after bleeding
Air still in the brakes
The most obvious, but if the pedal doesn’t feel as it should, the most common cause is air remaining in the brake fluid. Start by pumping the brake pedal for a minute or so to try and force any remaining air closer to the bleed nipple and then carry out a full bleed again.
When bleeding the brakes, if the fluid is still bubbly or has a milky color to the fluid coming out of the brake bleed nipple, the brake fluid still contains air. You should continue bleeding the brakes.
If the brakes have been run completely dry of brake fluid during repairs, getting the brake pedal back is tricky without an automatic pressure bleeder. You may even need to bleed the master cylinder by cracking off the brake pipes one at a time to remove any trapped air.
Bleed nipple not tight
This is silly, but many people forget how important it is to double-check that all bleed nipples are properly closed tight after bleeding. This is essential to ensure no air can be drawn into the brake system when releasing the brake pedal. Depending on the repairs to the brake system that have been carried out, which resulted in the need for a brake bleed in the first place. The brakes should also be checked to ensure any component carrying brake fluid is tight. For example, if brake pipes have been made, check the unions are tight. It only takes a tiny amount of air drawn into the brake fluid to cause an issue.
Leaking brake fluid
On a car with brake calipers at all four corners leaking brake fluid is a bit easier to see. But in a vehicle with brake drums at the rear, it is impossible to know the brake cylinders are leaking until the brake fluid drips out the bottom of the brake drum. As you’ve probably now guessed, removing the brake drum and inspecting the brake cylinders for leaking fluid is the only way to check.
It’s not uncommon to see a weak cylinder that wasn’t leaking before bleeding after applying too much brake pressure, which can happen when bleeding the brakes. Be sure to check for fluid down the rubber seals on either side of the cylinder. Any signs of dampness or fluid will cause a problem when bleeding the brake fluid.
Just as a note, remember brake fluid can leak from any component that brake fluid passes through and not just at each corner of the car. So remember to double-check the brake pipes, which run the length of the vehicle, brake calipers, hoses, brake fluid cap, reservoir, brake unions, biased valves, and so on, for any indication of a leak.
Master cylinder failed
You can’t specifically see inside the brake master cylinder; it may have been acceptable before bleeding the fluid. Still, if the master cylinder was weak, the pressure of pressing and holding the brake pedal could force the seals to burst inside. The pedal will never come back until the master cylinder is replaced.
A master cylinder is relatively straightforward to diagnose as the standard brake system on daily road cars uses a diagonal split system. To explain, when the pedal is pressed, half of the master cylinder distributes the pressure to opposite corners of the vehicle, i.e., the front right wheel and the rear left.
This means you will always have control of the vehicle’s brakes if it fails. For this example, if the master cylinder has failed, you will typically find with the pedal pressed one set of opposites corners, the brakes are applied, and the other the wheels spin freely. i.e., the front right and rear left wheels brakes are on but not on the front left and back right wheels.
ABS modulator requires bleeding
The ABS modulator typically has all the valves closed and remains closed until it’s in operation, as the ABS kicks in. However, if the brake pedal feels fine after bleeding and then on the road test, it goes spongy; air may be trapped in the ABS modulator.
The other issue that might occur is even by applying a small amount of brake pressure; you might hear the ABS modulator valves chattering as they open and close. You may even feel some fast pulsing through the pedal; this is a sign the ABS is playing up, and a simple problem such as air trapped could be the cause. The downside is to carry out an ABS bleed requires the use of a scan/diagnostics tool to carry out the bleed.
Bad brake fluid
Brake fluid goes past its best in a couple of ways. One is contaminated the other is it’s below its minimum boiling point. Brake fluid is hygroscopic, which means it absorbs moisture, bearing in mind that the brake fluid cap needs to be open to top up the brake fluid; this is when it’s most susceptible to absorbing moisture. It can also be contaminated with dirt and debris, which will force tiny air bubbles from the debris surface into the brake fluid, only a tiny amount, but maybe enough to be cause an issue.
The final problem with contaminated brake fluid is the boiling point of the brake fluid will be reduced; this won’t be noticeable while the vehicle is stationary after bleeding the brakes. But, if test driving the car and the brakes are warm, and then the pedal becomes spongy, this could be a sign the brake fluid is below its minimum boiling point level; and you are experiencing brake fluid fade; you can read more about brake fluid boiling points in this article.
To properly check the boiling point of brake fluid, a sample of fluid should be taken from the caliper furthest from the master cylinder so either the rear left or rear right wheel and use a brake fluid boiling point tester which will indicate if the fluid is problematic and should be flushed.
How to fix a spongy brake pedal after bleeding
If you have already bled the brakes and the pedal still feels spongy, there are several potential causes and fixes that you can apply, and the below is a good order in which to carry them out:
- Carry out a complete brake fluid flush: Carry out a full brake fluid flush again; you may remove every last bit of air and fix the issue. If not, continue…
- Look for leaks: Check the brake lines, hoses, calipers, and rear wheel cylinders for any signs of leaks. Even a small leak can cause air to enter the system.
- Check the master cylinder: The master cylinder is responsible for pressurizing the brake fluid, so if it malfunctions, it can cause a spongy brake pedal. Jack up all four wheel corners of the car and try to rotate the wheels with the brakes applied to check the evenly distributed pressure.
- Get another opinion: You may need to have the vehicle recovered to a garage to have a mechanic inspect it and potentially carry out another brake bleed. It may just be something simple your missing.
Will air eventually bleed out of the brakes?
The only reason air will not eventually bleed out of the brakes is if something draws more air in. If you continue to bleed brake fluid through, the air will have no choice but to be forced out with brake fluid.
Using the correct tools will also help; trying a one-way brake bleed tube and bottle to reduce any air drawn back through is the best tool, alongside a pressure bleeder, which will continually apply the brake pressure and keep the fluid constantly topped up.
A spongy brake pedal isn’t something everybody should attempt to fix without correctly understanding how the brakes and each component that makes up the brake system work. However, if you are trying the repairs yourself, the most common cause of a spongy brake pedal after bleeding is that air is still in the brake system, and the best cause of action is to carry out another full brake bleed. If possible, use a brake pressure bleeder. If the pedal does not come back, check all of the items listed in this article, and if you can’t find the problem, have a mechanic look for you.