When pressing on your vehicle’s brake pedal, there should be ample resistance that pushes the brake pedal back to its original position. But sometimes, a brake pedal can feel spongy, which is usually an indication of air in the brake system. A spongy or squishy brake pedal travels more and requires more pressure before it engages the brakes.
But there’s a difference between a spongy brake pedal and a sinking brake pedal. A sinking brake pedal slowly drops to the floor as you exert the same pressure. You’ll be able to feel the pedal sinking when you’re stopped at a stoplight or intersection.
These two symptoms are different, as having a sinking brake pedal is significantly riskier than a spongy one.
When you press on the brake pedal, you’re pushing a piston that compresses the vehicle’s brake fluid into brake lines leading to the brake calipers. Once the fluid reaches the calipers, the fluid pushes pistons that cause the brake pads to clamp onto the brake rotors, which creates friction and slows the vehicle down.
During a panic stop, the pressure in the brake lines, calipers, and wheel cylinders can surpass 2000 psi. Rusty brake lines can give way suddenly so that the pedal feels normal once, and the next time, it suddenly goes to the floor as the fluid escapes from the breached and weakened rusty line, a blown wheel cylinder, or a damaged hose.
Maintaining brake fluid pressure inside the braking system is critical to maintain proper function. A leak somewhere along the braking system can cause brake fluid pressure in the system to drop, which can cause the pedal to sink and your vehicle’s brakes to fail. That being said, most braking systems are diagonally split to prevent total brake failure.
The brake master cylinder is located right below the brake fluid reservoir. When you press the brake pedal, the piston inside the master cylinder pushes brake fluid into the brake lines and the calipers.
As the master cylinder is regularly used during braking, the piston and bore can eventually wear out. When this happens, the piston will have an internal leak that will prevent it from holding pressure. This won’t immediately cause brake failure because a brake master cylinder is composed of a primary and secondary piston assembly, which means there’s a second piston that acts as a backup.
A sinking brake pedal caused by a faulty master cylinder can mean that there’s an internal leak causing fluid to leak past the primary piston. This causes the pedal to drop and engage the secondary piston instead.
Continuously riding the brakes can cause brake fluid to heat up and boil, creating air where there was none previously and also heats the pads, shoes, discs and drums to the point that they can’t convert kinetic energy to heat energy. This is called “brake fade.”
When this happens, the brake master cylinder will require more input from the pedals to apply brake pressure. You might end up having soft brakes that can be mistaken for a sinking brake pedal or the pedal might feel okay and yet the vehicle isn’t stopping well.
The brake booster is responsible for amplifying the pressure from the brake pedal to better push the brake master cylinder. Without the brake booster, you’ll need a lot more pressure on the pedals to slow down your vehicle. If your vehicle has a faulty brake booster, then you’ll need to push down on the pedal with more force to slow your vehicle down. A malfunctioning brake booster can also cause the pedal to travel lower than normal.
Driving with a sinking pedal isn’t safe. If your brake pedal goes to the floor but the vehicle still stops, you should have your vehicle checked immediately. A sinking brake pedal can make braking difficult to gauge, which can cause you to brake too little and get into an accident. A sinking brake pedal can also mean that your vehicle could be nearing brake failure.
If you find that your pedal has sunk on a stoplight or intersection, check if your vehicle still has its brakes before accelerating. Sometimes, brake failure can occur at the same time the pedal sinks.
If the brake pedal falls to the floor, you shouldn’t drive at all. Lock the park brake and put the automatic transmission in park as soon as you can leave the road and get to a safe place.
If you notice that your pedal is sinking, you should immediately ask a trusted mechanic to check your braking system. However, you should be cautious when doing so because brake failure might be imminent. If you’re confident in your DIY automotive skills, you can also perform some basic troubleshooting.
Brake fluid can leak from the brake master cylinder, brake fluid reservoir, brake caliper, brake line, seals, or bleeder valve. The ABS brake module is also a common location where brake fluid can leak.
You can check underneath your vehicle if there’s brake fluid leaking onto the ground. Sometimes, the brake leak can be on the brake assembly itself and cause brake fluid to spread onto the tires and wheel well or drip from the frame.
If there aren’t any leaks in the system, then it’s very likely that a faulty brake master cylinder is to blame. Sinking brake pedals are a key symptom of a worn-out brake master cylinder with internal leaks. A brake master cylinder with internal leaks is the most common culprit for a sinking brake pedal.
The brake master cylinder is also known to develop external leaks that can cause corrosive brake fluid to drip onto the adjacent brake booster and damage it. That being said, a bit of fluid seepage (dampness) at the spot where the master cylinder is bolted to the booster isn’t always a cause for concern.
Since air bubbles can compress, it requires the driver to press the pedal more before the brakes engage. Sometimes the problem is so severe that it can also cause the brake pedal to sink.
If you can pump the pedal up and it feels better but then feels low and soft after you release it and wait a moment before trying again, you probably have air in the lines. You can remove the air bubbles inside your braking system by bleeding it. Bleeding the brakes involves opening the bleed valve and pressing the brakes to let out the air inside the system.
If your car has a drum brake system, the problem could also be the rear brake shoes. If the brake shoes are worn, the air gap between the shoe and the drum widens, causing the brake pedal to travel more than normal. If this is the issue, the brake shoes need to be replaced.
Your vehicle’s braking system is its most important safety feature. If you feel that your brake pedal sinks to the floor, you should immediately have it checked. Once the cause of a sinking brake pedal is accurately identified, then this issue can be fixed.
Once you’ve identified the cause behind your brake pedal sinking, you should replace it as soon as possible. Driving with a sinking brake pedal can make your brakes unpredictable, making it hard to stop your vehicle. For your own safety, it’s best to avoid driving until you replace the faulty brake line, master cylinder, or whatever’s causing your brake pedal to sink. Thankfully, getting replacement parts is fast and easy with CarParts.com.
As a one-stop shop for aftermarket parts, CarParts.com has a wide selection of high-quality brake lines, brake master cylinders, and brake shoes. We only source our products from the most trusted manufacturers in the industry, so you can rest assured you’re only getting the best. However, if you aren’t completely satisfied with your order, you can return it within 60 days of purchase. Just reach out to our customer service team for more information.
Don’t wait until your brake system suddenly fails before replacing faulty parts. Check out our wide selection of high-quality brake lines, brake master cylinders, and brake shoes at CarParts.com and get one today!
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