What Is a Brake Bleeder Screw?

When you think about it, the idea that putting a small amount of pressure on a pedal could bring a speeding vehicle to a halt sounds a little ludicrous. Luckily, we’ve developed braking systems that do exactly that (and more). One key to good brakes is surprisingly small: the brake bleeder screw. Do you know how to use it and what it does for you?

We Need a Brake

Brakes rely on hydraulic pressure because air is compressible, but fluid is not. This is key to understanding how a small amount of force can yield car-stopping results. Because fluid can’t be compressed, the force applied at the pedal is transferred almost instantaneously to the caliper piston, which presses the brake pads against the rotor. In drum brakes, a hydraulic cylinder presses the brake shoe against the brake drum.

The trouble comes when you get air in the system (as with a leak) or otherwise get low on fluid (as with bad brake pads that cause air to be drawn in through the master cylinder). Once air is introduced, your brakes will become spongy at best, or at worst, you could lose braking ability altogether. Obviously, if there’s a leak or some other underlying problem in the system, that must be fixed first. Then, it’s time to bleed the air out, and that’s where the bleeder screw comes in.

Screw It

A brake bleeder screw is as it sounds — a screw that enables bleeding. Its threads are tapered to fit snugly into the caliper, preventing fluid from bypassing it to get to the outside. The head is built to fit perfectly into a machined nook within the caliper, so that when tightened, the surfaces mate and fluid can’t escape. However, the screw is hollow with a small hole in the head, so that when it’s loosened, fluid and air make their way in through the screw, exiting the top. Because air naturally rises to the top of the fluid in the caliper (or is forced there via compression), loosening the screw first purges the system of air, followed by fluid.

The Purge

Bleeding air out from the brakes isn’t hard per se, but it does generally require a partner, unless you have special tools. Here’s what to do:

1. Put on goggles and have pans and rags at the ready to contain the mess.
2. With the car safely raised on jack stands, have a friend climb into the cab and pump the brake a few times, then hold it down. Pro tip: Constantly communicate to let them know when to push and release, and for them to tell you what they’re feeling at the pedal.
3. Once they’re holding down the brake, crack the bleeder screw a quarter- to a half-turn and you’ll get air, fluid or a spritz mixture of the two. While this happens, the pedal will go down.
4. Once the pedal reaches the floor, tighten the screw and tell your friend to release.
5. Have them pump the brakes again to see how it feels — you’ll probably have to repeat this several times at each caliper until all the air is out of the system.
6. Make sure to top off the fluid after.

Bleeder screws enable trapped air to be released, so you can travel with firm, responsive and safe brakes. If bleeding air out through the screws doesn’t solve your brake issues, see a trusted mechanic immediately.

Check out all the brake tools and equipment available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on brake bleeder screws, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photos courtesy of Blair Lampe.