Not even new brake pads can save you from the dreaded soft brake pedal. Maybe you noticed that you still have to press your brake pedal far into the floorboard to stop even after swapping out your pads. Fresh, thick pads should have brought that pedal back up where it should be, but it didn’t.
Your pedal probably feels spongy and it’s often hard to feel exactly when the brakes start to bite. These are the telltale signs of air in the braking system, which happens over time with use. It’s time to replace that old brake fluid and bleed all of the air out of the system.
Why Your Brakes Get Spongy
There are a few different ways air can enter your braking system, and all of them involve brake fluid. Your brakes are a hydraulic system, which means that fluid makes them work. Pressing the brake pedal shoves brake fluid out to the brakes at the four corners of your car. In a disc brake, fluid pushing into the brake caliper is what moves your brake pads inward to clamp down on the rotor and slow the car using that friction.
Older-style drum brakes work using the same principle, although the chunks of braking compound that wear down in drum brakes are in “shoes” instead of “pads.” Press the brake pedal down and the brake fluid pushes the shoes outward against the metal drum, thus also using friction to slow the car. Many cars still use drum brakes in the rear since the front brakes do most of the work. (For simplicity’s sake here, we’ll refer to both shoes and pads as a “pad,” as they both have the wear material that makes your brakes stop.)
Prestone Prestone DOT 4 Synthetic Brake Fluid
Prestone Prestone DOT 4 Synthetic Brake Fluid
As pads wear down, it takes more fluid to press them onto the braking surface. If you let your brake pads wear so thin that the brake fluid level drops too low in the master cylinder reservoir where you fill the system with fluid, this can introduce some bubbles into the brake lines. Air is much easier to compress than liquid, so bubbles in your brake lines act like a very soft spring in that solid column of brake fluid between your foot and the brake calipers or drums. Bleeding the brakes will flush that air out.
While you’re bleeding out the air bubbles, it’s best to flush all of the old fluid out of the system and replace it with new stuff. Over time, brake fluid becomes contaminated with atmospheric dirt and abrasive metal wear particles from moving parts in the brake cylinders and calipers.
Brake fluid also absorbs moisture from the air, which can lower its boiling point enough to make it boil at the end of a long downhill grade or some other scenario such as a track day where you’re using the brakes repeatedly or for a long time. Just like with a pot on the stove, boiling fluid makes springy air bubbles.
High temperatures also degrade the alcohol-based fluid itself. Eventually, brake fluid that was relatively clear when it went in may start to look more like coffee.
Antilock braking systems are even less tolerant of contaminated fluid and air than systems without ABS. The ABS hydraulic pump operates at several thousand psi, forcing brake fluid through very small valves. This can whip air and brake fluid into a latte-like foam, which makes bleeding air out of the system even more difficult. Worse yet, the ABS pump and valves can easily be damaged by those tiny abrasive particles.
Fortunately, air that enters the ABS controller can be bled out. However, some vehicles require the use of a special ABS scan tool to cycle the pump and valves to purge all the air inside. You can avoid these ABS problems by regularly remembering to bleed and flush your brake fluid, which is easy enough to do at home.
Let It Bleed
To properly bleed the brakes, start by acquiring about sixteen ounces of fresh brake fluid. An unopened can has a long shelf life, but an opened can should be discarded within a few weeks. Put the vehicle up on jackstands, removing all four wheels.
Next, make sure the bleeder valves can be loosened. You’ll need a box wrench that fits the bleeder bolt, as a crescent wrench or vise-grip could strip off the edges of the bolt and make it impossible to loosen. It may help to squirt a little penetrating oil on the bolts the day before you bleed your brakes, especially if rust is common where you live. Loosen these bolts, but leave them closed for now. If they’re still not budging after using some penetrating oil, try tapping the wrench very lightly with a small hammer.
If you can’t turn the bleeder valves without breaking them off, you may need to replace the brake calipers or wheel cylinders. These are tiny, hollow bolts, so be careful with them! They’re easier to break or strip than you think.
Next, you’ll need a kitchen tool: A small turkey baster. Remove the top of the master cylinder reservoir and suck out as much of the old brake fluid as you can. Clean any sediment out of the now-empty reservoir with a clean, lint-free rag.
Brake fluid is caustic and will destroy the paint and finishes on your car, so remember not to let it drip on your wheels or bodywork. Lay some extra shop towels around where you’ll be working with the brake fluid just in case, and wipe off any fluid dripped onto a painted surface immediately.
Once the reservoir is clean, get a piece of clear plastic tubing that fits snugly over the end of the bleeder bolt. Aquarium tubing is perfect for this, and it's cheap. Push one end of the tube over the brake bleeder bolt at the passenger-side rear corner of the car (or whichever corner is furthest from the brake fluid reservoir). Put the other end of the tube into a small, clear bottle with an inch or two of clean brake fluid in it, which will keep air from being sucked back into the brake cylinder or caliper.
Put a piece of 1x4 lumber or some other spacer under the pedal to prevent it from traveling too far and overextending the brake master cylinder when the brake system is opened and the line pressure is released. Refill the empty master cylinder reservoir with fresh fluid up to its maximum fill line and put the cover back on the reservoir. Fluid can squirt out of an open reservoir every time the pedal is released.
Cue the Helper
Your helper needs to be someone who can follow instructions exactly, but even someone wearing a white dress could do this job, as they won’t have to crawl around or get dirty at all. Have your helper sit in the driver's seat and await your orders.
Here’s the drill: You say “down.” Your helper depresses the brake pedal all the way down with about the same amount of force needed to keep the car from rolling forward at a traffic light. Then your helper says "down" to confirm but keeps pressing the pedal. Warn your helper that the brake pedal is about to lose pressure and sink to the stop you’ve put behind it on the floor, but they must keep pressing it regardless. Then crack the bleeder bolt a quarter-turn.
Some of the old, contaminated fluid will flow down the tubing into your bottle, and you’ll probably see a few clear bubbles in the line. When the flow stops, close the bleeder. Then you say “up.” Your helper says “up” to confirm, and removes their foot from the pedal.
Repeat this process until fresh, clear fluid comes from the bleeder screw with no bubbles in it. Any out-of-sequence moves here can suck air into the caliper. While the end of the tubing is submerged in fluid, air can still travel past the threads on the bleeder bolt into the caliper if there's negative pressure in the system while the bleeder screw is cracked open.
Top off the brake fluid reservoir with fresh fluid every six or so pumps. Do not allow the reservoir to get more than half empty as air can be sucked into the master cylinder if the fluid level drops too close to the bottom of the reservoir. Most reservoirs have a “Minimum” line for this reason, so if yours has one, watch and make sure that your brake fluid never dips below that line.
Once clean fluid is coming through the tubing, tighten the bleeder bolt and move your operation to the left rear brake. There, start doing the same routine with your helper all over again until clean, bubble-free fluid comes out of your tube. Repeat the process with the right front brake and finally with the left front brake. The idea is to work from the calipers farthest away from your brake fluid reservoir inwards to get the gross fluid in the furthest reaches of the system out first.
You may want to double-check your work by going back around to each caliper in the order that you flushed them, reattaching the tube and giving the system a few pumps to ensure that a solid stream of clean fluid comes out. Close the bleeder screw once you’re done with each corner and make sure you’re still topping off the brake fluid reservoir as you go.
Finally, fill the brake fluid reservoir to its maximum fill line, put the wheels back on the car and go for a drive to make sure that fixed the issue.
Air Breather
For an older vehicle without an antilock braking system, this process will do a great job even if you introduced extra air in the system after replacing a caliper or brake cylinder. This even gets most of the big bubbles out of a newer system with ABS. However, if there’s still some sponginess on those ABS-equipped brakes, you may want to bleed the ABS controller itself.
You’ll need to consult your car’s service manual to find the appropriate way to bleed your car’s antilock braking system. There may be a bleeder bolt right on the ABS controller, or you may need to beg, borrow or steal an ABS-capable scan tool.
Stef Schrader routinely breaks and attempts to take project cars on race tracks. She enjoys fancy cheeses, good coffee, fast Porsches, traveling to new places and rare, weird cars. She lives with a large collection of Fisher-Price Puffalumps and an overloaded parts shed.
