“Bleeding brakes” and “how to breed brakes” are phrases that lots of people google, and one or the other is probably why you’re reading these words. In this article, we’ll look at why having air in brake lines is a death knell for reliable brakes. We’ll look at how to bleed brakes by yourself or with a buddy. And then we’ll look at how to replace brake lines and hoses, as well as things like bleeder screws.  — Motor City Mitch

SPONGY BRAKES & THE BOX TRUCK

One Saturday years ago, I rented a box truck to move an apartment’s worth of stuff to the bungalow my wife and I had bought in Royal Oak, a suburb of Detroit. Royal Oak is known for being the epicenter of the annual Woodward Dream Cruise.  But my drive that day in the 15-foot rental was neither a dream nor a cruise.

I was turning left at an intersection after leaving the lot, when I realized the truck handled like a boat. (It seemed to hesitate before turning, as if making up its mind whether to do so.) But more concerning was the brake pedal. When I pushed it down, it went to the floor. The truck would slow and stop, but it took a while. If I’d been smart, I would have returned it immediately. But I wasn’t and didn’t — instead I drove slowly. Especially once we’d loaded our stuff aboard. When I needed to stop, I hit the brakes early and hard, my foot clamped to the floor.

We got our things moved that day and somehow I avoided an accident. But looking back, I realize how lucky I was. I hadn’t had to stop for a kid chasing a ball into the street, someone running a red light, or a biker swerving in front of me.

What I dealt with that day was spongy brakes, an extremely soft brake pedal. The truck’s brakes (probably drum brakes) were no doubt worn down. But when your brake pedal goes to the floor, you’ve generally got the problem of “air in brake lines.” So before we look at how to bleed brakes free of that air, let’s explore why that’s a problem in the first place.

PASCAL’S LAW

The brake system in a car or truck is a sort of cardiovascular system for stopping. There’s a heart-like device, the master cylinder, which pumps hydraulic brake fluid through through a network of brake lines and hoses (the arteries and veins). It’s a system of immense power, thanks to a law of nature called Pascal’s law.

Pascal's Law

Pascal’s law says that when you have fluid in an enclosed space, pressure from anywhere spreads evenly everywhere. The picture above shows how this works. Press your thumb on a ball of liquid — say, a small water balloon — and pressure goes equally in all directions. It’s the principle behind hydraulic machines, like presses and lifting devices.

If you look at hydraulic animation below, you’ll notice that the vertical pipe on the left is smaller than the one on the right. Let’s say it’s twenty times smaller, in terms of the surface area on the top. That means, thanks to Pascal’s law, that when the foot pushes down, the equivalent of twenty feet push up on the right. It’s a 20-fold mechanical advantage. That’s the magic of hydraulics, and the reason car lifts lift, dump trucks dump, and your brakes brake.

hydraulic power animation, Pascal's law

The key to this all working correctly is that fluid doesn’t compress — at least not enough to make a difference in hydraulics.

HOW AIR HURTS HYDRAULICS

The reason for bleeding brakes, or ousting air from any hydraulic system, is this: air compresses a LOT. There’s another natural law — Boyle’s law — that says when you increase the pressure on a air (or any gas), you reduce the space it takes up proportionally. So if you double the pressure on some air, you shrink its space to half. If you triple the pressure, you shrink its space to a third. And so on.

That’s why you can take a bike pump, for example, and force a gallon’s worth of air into less than a quart. And that’s why a standard scuba tank, with a pressure of 3,000 psi, compresses air into roughly 1/200th of its original space. (By the way, 1/200th is the difference in size between a Boeing 737 on the runway and a model plane on your kid’s bookcase.)

This compressibility of air, depending on the amount, can ruin a hydraulic system. In the animation below, you can see the difference between air and no air. When there’s air, instead of pushing the weight up on the right, the woman’s foot just shrinks the air into a tiny space.

Hydraulic animation with and without air, how to bleed brakes

THE PROBLEM WITH AIR IN BRAKE LINES

Because of all this, air in brake lines is a particularly big deal. When you step on your brake pedal moderately hard, you do so with 70 pounds or so of of force. Your brake pedal, which is itself a lever, amplifies that force up to six times. And if your car has a brake booster, it’ll add even more force.

That pressure goes from your master cylinder, through your brake lines, to your front calipers. And depending on the size of your master cylinder and how many pistons in each caliper, the force can be up to 3,400 psi per caliper. That’s a lot — more than the working pressure of a standard scuba tank. (For the preceding math, by the way, I’m indebted to Andrew Markel’s article in Brake & Front End magazine.)

The animation below, with the pedal removed, shows the flow of fluid in a disc brake system from the master cylinder through a brake line to a caliper and the brake pads.  Again, the pressures involved are enormous, easily going above 3,000 psi. And that’s why breeding brakes free of air is so important.

how to bleed brakes, bleeding brakes, air in brake lines

AIR & THE MASTER CYLINDER

Air can get into the system a number of ways, one of them being through the master cylinder. Below is a master cylinder for a Ford F-150 — the part by itself on the left, and installed under the hood on the right in an image from the F-150 Forum.

Ford F-150 master cylinder, how to bleed brakes

The illustration below, from Wikihow, shows the main parts — the reservoir on top, which you fill with brake fluid, the master cylinder itself, which pumps fluid into the brakes lines, and a sensor to alert you if the fluid’s too low. (This is different from an ABS sensor, although it will signal you with a warning light on your dashboard.)

master cylinder parts diagram, bleeding brakes

BLEEDING BRAKES & BRAKE FLUID

In the video below, YouTube mechanic ChrisFix flushes the brake fluid from a 2001 Mazda B3000 (Ford Ranger). In doing so, he shows how to bleed brakes by yourself or with help from a friend. He also shows how to work on drum brakes, which the Mazda has in back, and on disc brakes, which are in front. (His process for both is the same.)

Early in the video, he explains that a cause of air in brake lines is water. And that brings us to the topic of brake fluid.

Modern cars and trucks use DOT 3 or DOT 4 brake fluid for the most part. And these fluids are hygroscopic, meaning they absorb water. That can happen at the top of the reservoir, where the fluid is exposed to the humidity in the air.

Why does this matter? Because water boils at lower temps than brake fluid — 212 °F at sea level vs temps above 400 °F for new brake fluid. As your brake fluid absorbs water over time, its boiling point goes down and when it boils, you get vapor in your brake system. And that makes your brake fluid compress.

Another word about brake fluid: check your owner’s manual for the type you should use. DOT 5, for example, is silicone-based and repels water, but it doesn’t work well with anti-lock brakes (ABS).

BLEEDING BRAKES WITH A BUDDY

What ChrisFix shows above is how to bleed brakes with a friend. He starts by opening the reservoir and removing some old, muddy-colored brake fluid with a turkey baster. He then fills the reservoir to the top with clear, new fluid.  Here’s what he does next:

  • First, he heads to the passenger-side rear wheel — furthest of the four wheels from the master cylinder. As long as the ABS control module is located in the front of the vehicle, the passenger rear brake will probably have the longest brake line. (And the most fluid to flush out.)
  • Then he find the bleeder valve, removes the cap, and attaches a piece of clear vinyl tubing, roughly 30 inches long. It’s tubing with a 1/4″ internal diameter which fits both his disc and drum brakes.
  • Next, he secures the end of the tube to the bleeder valve with a medium zip tie — a step he says can be helpful if the fit onto the bleeder valve is loose.
  • He runs the other end into a plastic bottle sitting below the valve (an obvious but important point).
  • Next, he loosens the valve a quarter turn or so and tells his friend to press the brakes.
  • After fluid comes out, he tightens the valve and tells his friend to release the brake pedal.
  • He then repeats the previous step until he gets clear fluid coming out of the bleeder valve — a sign that he’s completely flushed old fluid out of the brake line going to that wheel.
  • Finally, he tightens the valve firmly before telling his friend to release the pedal. That way the upward pull of the release doesn’t pull any fluid back through the tube and into the system.

HOW TO BLEED BRAKES BY YOURSELF

Later in the video, as he switches to a disc brake, he shows how to bleed brakes by yourself. It’s a faster process:

  • First, he refills the reservoir with new brake fluid.
  • Next he heads to the driver’s side rear brake and attaches his tube to the bleeder valve with the zip tie.
  • Then he makes sure that his tube arcs up above the bleeder valve and then down to and into his plastic  bottle. It’s a matter of physics — he wants air to rise from the valve up into the tube.
  • He makes sure he has several inches of fluid in the bottom of his bottle, and checks that the end of the tube going into the bottle is submerged in that fluid. That way, when he releases the brake pedal, the tube will suck in fluid rather than air.
  • Next he cracks open the valve with the crescent wrench until fluid starts to come out into the tube.
  • Then he heads to the brake pedal and pumps it until only clear fluid — and no air bubbles — are coming out of the valve.

The key to the step is the brake bleeder he uses. In the next video, he shows how to makes a brake bleeder using a 20 oz. Powerade bottle:

HOW TO BLEED BRAKES WITH INJECTORS AND VACUUMS

The two methods above use the brake pedal to drive old air-filled fluid down and out the system. There are other ways, however, and the next video, by YouTube mechanic Scotty Kilmer, shows one of them. Although this video is from 2008, the product is still on the market and you get a good look at how Kilmer injects new fluid through the bleeder valve up to the reservoir.

The product Scotty’s using is an injector brake bleeder kit, which Phoenix Systems sells on Amazon, where it has mixed reviews. You can check out alternative  bleeder kits on Amazon and elsewhere.


The next YouTube video from the Canadian mechanic channel 4DIYers, shows how to bleed brakes by yourself with a vacuum pump and pull fluid out of the bleeder valve. The mechanic connects a vinyl tube to his bleeder screw and then creates vacuum pressure with a hand pump. Only after he has enough pressure does he open the bleeder screw. When he’s done, he tests his brake pedal to make sure it’s not soft and spongy.

HOW TO REPLACE BRAKE LINES

Another reason you might end up needing to bleed brakes is if a brake line fails.

In the next video, 4DIYers shows how to replace brake lines, going over different line materials: steel, coated steel, and copper nickel. (Carlson makes all three types — see our brake line page).  Copper resists rust and is easier to bend into shape. Steel is less expensive but rusts more easily. Coated steel helps with rust-resistance.

At the end of the process — which involves removing old lines, cutting new line, bending and double-flaring it before installing — the mechanic bleeds his brakes.

REPLACING BRAKE HOSES

Unlike metal brake lines, which run the length of your car or truck, brake hoses are made of rubber (sometimes sheathed with braided steel), located near your wheels. Their flexibility allows them to move with your suspension system.

In the next video, Eric the Car Guy, a YouTube mechanic from Ohio, shows how to replace brake hoses that are dry-rotted and cracked. One detail he emphasizes is using a line wrench to loosen and tighten connections, as opposed to an open wrench. “I can’t stress how important this is,” he says. “If you use an open wrench, there’s a high probability you’ll round the fastener.”

REPLACING BLEEDER SCREWS

In ChrisFix’s video on how to bleed brakes, the last brake he works on has a problem: the bleeder valve doesn’t let out fluid or bleed at all. He removes it, shows how rusty it is, and remarks how he’s had a bleeder screw actually break in half in the past.

This is yet another brake part — similar to caliper mounting bolts (which can get stuck) or caliper slide pins (which can seize) — where it makes sense to replace old parts. Carlson makes these parts — check out our sales support page if you need help.

CONCLUSION

At the start of this article, I wrote about the day I rented a moving truck to move stuff from an apartment to a Royal Oak bungalow. Although my memory isn’t perfect, I definitely recall the sick feeling in the pit of my stomach when I felt that brake pedal go all the way to the floor the first time. As I mentioned earlier, I spent the rest of my time in that truck driving on side streets as slowly as I reasonably could. Having reliable, functioning brakes is something we normally take for granted, so if your pedal begins going soft and spongy, take some action and be safe!