There’s usually a little bit of resistance when you step on your brake pedal to give you a little feedback on how the brakes are performing. You know you’re in trouble if you suddenly step on the brake pedal and it feels spongy as it fades to the floor or your foot crushes down.
In a moment like this, you need to quickly pop the transmission into neutral and do your best to safely coast to a stop with the emergency brakes bringing the car to a full stop. With some luck, you might get the car into a parking lot or wayside rest area, where you can safely figure out what to do next.
A brake fluid leak is the most common reason why a brake pedal goes down to the floor. Though there are certainly other things like a bad master cylinder, sediment in the brake lines, or air in the brake lines are also in play.
To figure out what’s wrong with your brakes, we’ll have to inspect the major system components like the brake booster, brake lines, and the master cylinder.
How Does a Car’s Brake System Work
When you step on your brake pedal, it supplies mechanical force to the brake booster, which is amplified and passed on to the bake system’s master cylinder. The master cylinder then translates that force into hydraulic pressure, which passes through the hydraulic fluid of the brake lines connected to each brake caliper.
The pressurized brake fluid causes the caliper pistons to press the brake pads onto the rotors. This creates friction, which slows the car down relative to how hard you press the brake pedal.
Anything that affects the mechanical or hydraulic force at any point in this system will affect both the brake pedal’s resistance and the brake caliper’s performance.
Is It Bad if Your Brake Pedal Goes to the Floor?
Anytime your brake pedal goes to the floor, it’s a sign that the mechanical force of your foot isn’t being correctly sent to the brake calipers. Sometimes it’s a progressive problem, where the brake pedal initially feels a little spongy. Then it starts going down a little farther and a little farther with every press. Suddenly it crushes down to the floor, and you’re left with no direct understanding of what your brakes are doing other than looking out the window to see if you’re indeed slowing down.
Usually, the red brake warning light comes on in the dash at some point in this rapidly progressing brake failure. It might flash when the brakes are still spongy only when you step on the brakes, or it might come on and stay on glaring at you to let you know you need to get to a safe spot as soon as possible!
Causes of a Brake Pedal That Goes to the Floor
The loss of hydraulic pressure from a fault in the master cylinder or the brake lines is the most common reason why a brake pedal will sink to the floor. Though there are a lot of different things that can cause this. It’s also not the only brake system fault to worry about.
1. A Leak in the Brake Lines
A small puncture, severe rust, or other damage that causes a brake line to bleed hydraulic brake fluid will gradually depressurize the brake system diminishing the resistance on the brake pedal. Of course, this also reduces the force applied to the brake calipers making them increasingly weak until they fail completely.
In a scenario like this, the loss of pressure in the brake trips the fluid level sensor in the master cylinder, which turns on the brake warning light on the dash. Different automakers have slightly different tolerances for when the light comes on, but it’s usually very early in the brake failure process, which might give you three to five steps on the brake pedal before the system is too weak to compress the brake caliper pistons.
In a case like this, you should be able to spot hydraulic brake fluid leaking out of the lines when you pull over. Usually, it pools near a wheel or in the undercarriage. If you can’t immediately spot it when stopped, have someone else step on the brake pedal, and you should see a spray of fluid somewhere.
2. A Bad Master Cylinder
If one or both of the brake circuits or the piston cups inside the master cylinder fail, the mechanical force of your foot will not be directly translated to hydraulic pressure in the brake system, allowing the brake pedal to sink to the floor. However, most modern brake systems have tandem brake circuits, which allow the brakes to still work even if one circuit fails.
If you’re dealing with a single brake circuit failure, you’ll still have a little brake power to work with. If the first brake circuit failed earlier, and you didn’t notice then the entire brake system will be inert from the moment the brake pedal sinks to the floor, and you’re in an emergency situation.
It’s also possible for a replacement master cylinder to fail or blow a connection to one of the brake lines at one of the ports. This will rapidly bleed the pressure out of the brake system which you’ll feel when the brake pedal crushes down.
If the master cylinder has blown a connection or has a cracked seal, you’ll see a lot of brake fluid leaking out under the car near the firewall.
Though it’s also possible for the cups to fail inside the master cylinder without any obvious brake fluid leak, in a case like this, you’ll also notice the resistance of the pedal fading more and more as you step on the pedal.
3. A Bad Brake Booster
Suppose the casing, internal diaphragm, or other components inside the brake booster fail. In that case, it won’t be able to transmit the mechanical force of your stepping on the brake pedal to the master cylinder. This will cause the brake pedal to fade to the floor while also hampering the hydraulic pressure needed to apply and activate the brake calipers fully.
Some cars have a special pump or motor that maintains the vacuum. If it fails, the brake booster won’t be able to pull the necessary vacuum to amplify the mechanical force of the brake pedal to the master cylinder even though the brake booster itself might be excellent.
Symptoms of a bad brake booster can manifest subtly as increased braking distance or the brake pedal sitting a little higher or lower than usual. The brakes might feel spongy, to the point that you have to stomp on the pedal to get the brakes to respond. As the brake booster continues to fail it will eventually turn on the ABS warning light.
4. Air in the Brake Lines
If even the smallest air bubble gets into your brake lines it will dampen the performance of the hydraulic brake fluid causing the brake pedal to feel spongy, to the point that it might even fade to the floor. However, your entire brake system from the master cylinder to the calipers and back is a completely sealed hydraulic environment So, the real concern here is how the air got into the brake lines in the first place!
If you recently had a brake job or changed your own brakes and the lines weren’t bled properly, a small air bubble could be in the system. Though this usually only affects one or perhaps two brake calipers. The more serious concern is that there’s a leak in a brake line or the master cylinder that is allowing the hydraulic system to suck in outside air.
Extremely worn brake pads that cause the calipers to contract to the max can also such a small amount of air into the system through the brake fluid reservoir. This will show up as low brake fluid with a spongy brake pedal. You’ll likely also hear squealing or grinding noises coming from the front brakes, which are common signs of severely worn brake pads.
How to Diagnose and Fix It
Diagnosing why your brake pedal goes down to the floor starts with checking the brake fluid reservoir. This will help you better understand where the fault might be.
1. Interpreting the Brake Fluid Reservoir
It’s usually located near the firewall inside the engine bay and is connected to the master cylinder. If the fluid level is low, it’s strongly suggesting that the problem is in the hydraulic elements of the brake system. If it’s at the high mark, then the fault is more likely with the mechanical components of the brake system such as the brake booster.
2. Looking for Brake Fluid Leaks
you want to take your time looking for any potential brake fluid leaks. Brake fluid usually looks light yellow or brown. Check around the master cylinder and the connection to the brake fluid reservoir. You might have to feel around with your hands or a clean shop rag in some of the hard-to-see areas.
Continue looking for brake fluid leaks dripping from under the car. They’re most likely to happen at the metal connections near the wheel wells, on the brake hoses, or within a brake caliper. Make sure to check the back side of each tire, where the brake fluid might leak and pool under the tire tread.
If possible, have someone else step on the brake pedal with the engine running. This will cause any leaks in brake lines or calipers to spray pressurized brake fluid.
How to Fix
If you do find signs of a brake fluid leak the brake line, hose or leaking component will need to be replaced. If you’re absolutely desperate to get home or to the mechanic, you can try a very short-term quick fix.
Wipe the line dry, or buff off any loose rust from the line. Then wrap it tightly with several layers of duct tape and refill the reservoir with fresh brake fluid. If the leak is in one of the front brake lines this should get you 3 to 5 presses of the brake pedal before the system fails again. If it’s one of the rear brake lines, you might get 5 to 7 presses of the brake pedal before failure.
Though the smart thing to do is to have the car towed to the nearest mechanic to replace the brake line and any other leaking brake components.
The cost of a mechanic replacing a leaking brake line or hose can range from $75 to $350. The price can vary widely depending on how damaged the line is, and if any other brake lines look like they’re at risk of imminent failure.
3. Diagnosing Brake Calipers, Pads & Air in the Brake Lines
If you’re somewhere that you can feasibly take a tire off, you should give the calipers a good once over. Look closely at the dust boot for any signs of wetness from a minor fluid leak within the caliper itself.
While you’re there check the brake pads to make sure they’re firmly attached to the caliper, and that they aren’t overly worn. If the brake pads are down to the squealer brake indicator tabs or lower, and the
If your car has rear drum brakes, you’ll have to go an extra step to take the drum off and inspect the wheel cylinder for signs of brake fluid leaking into the drum or shoe.
If you find the brake pads are severely worn, and the brake fluid level in the reservoir is low, without any obvious leaks in the brake lines, then air has probably been drawn into the brake fluid lines.
How to Fix
In the case of a brake fluid leak in a caliper, you’ll need to get the car towed to a mechanic where both calipers on that axle will need to be replaced. They might also recommend replacing the brake hoses.
The cost to replace both calipers on an axle will cost around $300 to $500. Then you’ll likely also need to replace the brake hose, which will add another $50 to $75 to the final bill.
If you have air in your brake lines, you’ll need to bleed and refill the reservoir. If you’re a reasonably capable DIY mechanic, you might be able to handle this on your own. Though there’s no shame in having a mechanic do it. Especially since you’re probably going also to need to have the brake pads replaced!
The cost to have a professional bleed your brake lines will only run you about $80 to $125.
The cost to have your brake pads also replaced will add another $115 to $300 per axle.
4. Diagnosing a Bad Master Cylinder
If you have brake fluid coming out of the master cylinder, with the brake light on and the brake pedal fading to the floor, then the master cylinder itself likely has a serious failure. There’s nothing you can do on the side of the road, and most of the time, it’s better to replace the master cylinder simply.
How to Fix
If you’re a reasonably skilled DIY mechanic, you might be able to handle a master cylinder replacement on your own. Though considering you’re working with precision hydraulics and some special wrenches, you might want to simply take it to a mechanic.
The cost to have a master cylinder replaced with a full brake inspection will run around $175 to $300. Though in some more sophisticated models, the price could go as high as $500.
5. Diagnosing A Bad Brake Booster
Most cars have a vacuum-assisted brake booster that is married to the brake pedal and sits in the engine bay between the firewall and the master cylinder. It usually looks like a super-thick pancake turned vertically.
A bad brake booster will start showing itself with a brake pedal that is either very soft or increasingly still which also increases the braking distance. You might also hear a hissing noise when you step on the brake pedal. Sometimes a bad brake booster can also cause the engine to stall when you press down on the brake pedal. This is because the brake booster shares vacuum pressure with the engine itself.
If there’s a significant vacuum leak, you might get a check engine light. If this happens, you can hook a code reader up to the ECU.
A code P0577 indicates a pressure problem within the brake booster.
A code P0171 might also be thrown to indicate a lean mixture in the engine from unmetered air entering the engine via the same vacuum leak.
How to Fix
If you’re a reasonably capable DIY mechanic, you might be able to replace your own brake booster.
Though with some models, the brake booster is hard to get at and requires you to work with the master cylinder. So there’s nothing wrong with bringing it to a mechanic.
The cost to have a bad brake booster replaced usually runs around $300 to $500. Though if your car has a hydraulic brake booster, the cost skyrockets to as much as $750 to $900.
Frequently Asked Questions
What To Do If Your Brake Pedal Goes to the Floor While Driving?
If your brake pedal suddenly crunches or gradually fades to the floor, you need to assume that your brake system has failed. Even if it seems to be slowing the car down, you should then slip the car into neutral to begin decelerating and turn the hazard warning lights on to let other drivers know you’re in trouble.
The emergency brakes can stop the car, as they don’t use the same hydraulic lines as the normal brake system.
Though, if possible, you want to let the car slow down as much as possible before pulling the E-brake. The faster you’re going, the harder it will be to steer the car with the emergency brakes on, and the force can be jarring.
How Far Can I Drive with a Brake Line Leak?
Ideally, you shouldn’t drive at all with a leaking brake line or hose. If you’re in a desperate spot, you might be able to wrap the leaking brake line in duct tape and refill the brake fluid reservoir to get another 5 to 7 stops before the system bleed out again.
A leaky brake line or hose is the most likely reason your brake pedal fades or crushes to the floor. However, the air in the system that’s drawn in from the reservoir due to overly worn pads, a bad master cylinder or dying brake booster could also be to blame.
If you have a check engine light that throws a code P0577 without any obvious signs of a brake fluid leak or a brake warning light, you should strongly suspect your brake booster has failed.
If you see brake fluid anywhere under the car, then a line, hose, a seal has failed and needs to be replaced before the car is truly safe to drive again. A red brake warning light on the dash will also help confirm this as the reason why your brake pedal fades to the floor.
If you have low brake fluid in the reservoir, badly worn brake pads, and no obvious brake fluid leaks, then air might have gotten sucked into your brake system. This will cause the brake pedal to feel spongy and/or fade to the floor.
If you have a red brake warning light on the dash, and no obvious brake fluid leaks or check engine light, then chances are good your master cylinder has failed and needs to be replaced.
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.