Bleeding the brakes stinks, but it's an essential safety precaution

bleeding brakes

There were a lot of excellent reader comments about my piece a couple of weeks back on why you shouldn’t cold-start a long dead car. I completely agree with everyone who said that, ideally, you should pull the spark plugs and oil the cylinders before trying to rotate the engine, and that the more you oil and the longer you wait before spinning the engine with the starter, the better.

In my defense, I listed “make sure the engine is free to rotate” first because I was blurring the line between looking at a long-dead car as a potential purchase versus dealing with one you already own. That is, if you’re thinking about buying a long-dormant car and want to assess its status, you don’t necessarily have the time or the access to oil it and rotate the engine in 12 degree increments for 30 days. But the more valuable the car and its engine are, and the more you care about them, absolutely, oil before rotating.

And now, on to the matter at hand: Bleeding brakes.

Let me be clear about this from the get-go: I don’t like bleeding brakes. I hate the feel and the smell of brake fluid. When I bought my precious 1973 BMW 3.0CSi 33 years ago, the car had been sitting dormant, the clutch master cylinder had leaked inside the car and soaked the carpet, and the smell of brake fluid (combined with the rodent urine) was enough to gag a maggot. Whenever I smell brake fluid, it takes me back to that, and not in a soft-focus Pepperidge Farm way.

But I bleed brakes—a lot—because it has to be done. Any time you open a brake system to replace a hydraulic component, such as caliper or a flexible rubber hose, you introduce air into the brake lines, and that air has to be bled out by running brake fluid through the system, which carries any air trapped in the lines along with it.

If you’re never done it, bleeding brakes is, in theory, a very simple operation. At each wheel, there’s a bleed valve. On disc brakes, they’re on the calipers. On vintage cars with multi-piston calipers, sometimes each caliper has several bleed valves that you need to bleed in a certain order. On drum brakes, the bleed valves are on the wheel cylinders. In either case, you go to one wheel at a time in the bleeding order prescribed for your car. Generally, this order is most distant from the master cylinder to least distant, which on most cars is right rear, left rear, right front, left front, but you should check service information for your car to be certain. (My Lotus Europa specifies left rear, right rear, left front, right front. Bloody Brits.)

Generally, you jack up the front or back of the car, set it on jack stands, and pull the wheels off to access the bleed valves. You press a rubber hose over the little nipple at the end of the bleed valve and stick the other end of the hose into a receptacle to catch the brake fluid. You use a small wrench to open the bleed valve, run brake fluid through the system to bleed it, and then bleed the brakes at the other jacked-up wheel. You let the car down, jack up the other end, and bleed there. You need to make sure that, while you’re doing all this, the brake fluid reservoir doesn’t run dry, because if it does, you’ll have waaaaaay more air in the line than you had before. And you need to be very careful about getting brake fluid on paint. If you spill or splash any, wash it off with soapy water, stat.

That’s pretty much it, but the key phrase is “run brake fluid through the system.” There are several different ways of doing this. They’re all variants of either pushing fluid (pressurizing the system) or pulling fluid (applying a vacuum). In addition, there’s the Zen-like approach of letting gravity do its thing.

If you’re using one of the pushing methods, if your catch receptacle is clear, and you submerge the end of the hose in brake fluid, you can watch for a blast of bubbles followed by no bubbles as the air in the line for that wheel is purged. For the pulling methods, you don’t get the visual confirmation of air being passed, so instead, you can count for a fixed number of seconds, or pump a fixed number of times, or do it until a fixed amount of fluid has run through.

The following is not intended to be an exhaustive list of every bleeding method and device, but it’s a pretty good overview. Photos are of actual bleeding tools in my garage.

Pressure Method #1: The Old Tried-And-True Spousal Method

In the spousal method, a lot of time is spent opening and closing the bleed valve while your true love pumps the brake pedal.
Rob Siegel
In the spousal method, a lot of time is spent opening and closing the bleed valve while your true love pumps the brake pedal.

You’re not really the spouse or the best friend of a car person until you’ve been roped into helping them bleed brakes. In this tried-and-true method, you ask your ever-patient spouse/friend to sit in the driver’s seat while you move from wheel to wheel. You open the bleed valve, say “down,” and he/she depresses the brake pedal. It’s usually advised not to push it all the way to the floor, as that may send the piston in the master cylinder into regions it hasn’t traversed since it was assembled and may cause the seals to leak. When spouse/friend says, “It’s down,” you close the valve, reply “And… up,” and they let up on the pedal. You repeat this 20 or so times per wheel. Once you figure out about how many down-up cycles it takes to drain, say, 75 percent of the reservoir, you can then do that many pumps, stop if it appears that the bubbles have stopped, refill the reservoir, and move onto the next wheel.

Advantages: No additional equipment is required beyond a wrench for the bleed valve, a rubber hose, and a catch container.

Disadvantages: It requires a second person. That second person is usually your spouse. There may be a fixed number of times she/he consents to being summoned out to the cold greasy realm of your garage. Plus, in certain braking systems, pumping may not work well because air bubbles can get sucked back up the line when the pedal is retracted.

Pressure Method #2: The Big Pressure Bleeder/Reservoir

My Motive power bleeder sees a lot of use. Note the quick-release connection for changing caps.
Rob Siegel
My Motive power bleeder sees a lot of use. Note the quick-release connection for changing caps.

Several companies make a power-brake bleeder that screws onto the reservoir in place of its cap and pressurizes it with air. The most convenient-to-use ones also have their own brake fluid reservoir, thereby making it so you can fill them with several quarts of brake fluid and not have to worry about refilling the reservoir at the master cylinder after each wheel is bled. The best known of these is probably the one from Motive Products. It has a form factor that looks a bit like a garden sprayer, a big plastic jug with a big T-handled push-down pump. Most folks pump them up to 10–15 psi. A flexible tube comes off the tank and has a quick-release fitting that snaps onto a reservoir cap that fits your reservoir. Different caps are available for different makes.

Advantages: They provide one-person operation and let you run a lot of brake fluid through the system very quickly and efficiently. Mine sees a lot of use. The only reason I’d use something else is if I don’t have a reservoir cap that fits.

Disadvantages: They’re a little pricey. On Amazon, the cost of a “black label” Motive with a nice knurled aluminum cap for my vintage BMWs is $71. Plus, you may need several different caps if you own vintage German, American, and British cars, and each additional cap with a quick-release adapter is about $25. If your cap is not available (as turned out to be the case with my Lotus), Motive will sell you a “make your own” kit that you can use with a spare cap. Another disadvantage, as I found out after owning the Motive for 10 years, is that if the tubing splits from age while the tank is pumped up to 15 psi, it’ll dump a lot of brake fluid all over the garage floor very quickly.

Pressure Method #3: The Small Pressure Bleeder

My 30-year-old Gunson’s Eezi-bleed. Once in a while, I break it out.
Rob Siegel
My 30-year-old Gunson’s Eezi-bleed. Once in a while, I break it out.

In addition to the Motive, for over 30 years I’ve owned a Gunson Eezi-bleed. This venerable product uses air from a tire (usually the left front, as that’s the one that’s normally closest to the reservoir) to pressurize the brake reservoir.

Advantages: It’s simple, moderately priced, provides one-person operation, comes with several different caps, and has no moving parts.

Disadvantages: It doesn’t have an integral fluid reservoir, so you have to stop, unscrew it, and refill the reservoir on the master. Also, by connecting it to a tire, the pressure in the reservoir will be that of the tire, which initially is probably 30–40 psi, which is pretty high. Then the pressure drops off pretty quickly. In order to do all four wheels, you need to have an air compressor to re-inflate the tire. These days, I only use the Eezi-bleed if there’s some problem with the Motive.

Vacuum Method #1: The Power Vacuum Bleeder

This vacuum bleeder gets around the problem of fitment of the reservoir cap.
Rob Siegel
This vacuum bleeder gets around the problem of fitment of the reservoir cap.

Instead of pushing fluid through the system, it’s possible to pull it through. There are a number of moderately-priced vacuum bleeders that connect directly to the bleed valve and run off an external air compressor, using the venturi effect to create a vacuum inside a coffee carafe-shaped container and sucking brake fluid into it.

Advantages: In addition to one-person operation, the main advantage is the fact that, because it’s pulling not pushing, it doesn’t need to screw onto the reservoir, so if you can’t find a pusher solution with a compatible cap (as was the case with the Lotus), this is another option. And they’re cheap; I bought this one on Amazon for $25.

Disadvantages: As soon as you crack open the bleed valve, air tends to be pulled in through the threads; you may need to seal them up with grease or Teflon tape. Because of this, it doesn’t move nearly as much fluid nearly as quickly as a pusher system, and you don’t get that satisfying visual confirmation of “YES! THERE’S that pocket of air!” There’s also the downside of the lack of an integrated fluid reservoir. Some vacuum systems mitigate this by coming with a separate fluid bottle that you can suspend on top of the master cylinder’s reservoir, but this is little different than simply refilling it yourself.

Vacuum Method #2: The Hand-Pumped Vacuum Bleeder

Not much to recommend these hand-pumped units these days.
Rob Siegel
Not much to recommend these hand-pumped units these days.

Before the advent of other methods such as power bleeders and power vacuums, there were small hand-held pistol-grip vacuum bleeders. These typically had a short hose with a fitting that would slide over or into the bleed nipple, and a small container to catch the evacuated fluid.

Advantages: These days, the advantages are few—one-person operation, low cost, and that’s about all.

Disadvantages: Your hand gets tired from all the pumping, they have the same vacuum leak issues as the power vacuum bleeder, and because they’re not drawing a constant vacuum, they can have difficulty extracting air—pulling it and fluid down the brake lines only to have it go back up when the hand pump is released.

Gravity Bleeding

Some folks swear that no pumping, pushing, or pulling is necessary at all, and that you can simply let gravity have its way, open each bleed valve, be patient, and wait for fluid to run out, bringing air along with it. While I have gravity-bled clutch slave cylinders, those are in the center of the car and thus tend to be a straighter, less-circuitous shot than the wheels at the four corners. Plus, on the Lotus I’m working on now, there are two hydraulic brake boosters that are essentially remote master/slave cylinders positioned high in the engine compartment between the master and the wheels, and you’re not going to gravity-bleed through those. But, hey, if it works for you, great.

How Do You Know You’re Done?

When you’ve bled all four wheels, if the pedal feels firm, you’re done. If it doesn’t, you try it all again. If you’re bleeding a car that had a hard pedal before you installed new components, there’s no reason that it shouldn’t return to having a hard pedal. But sometimes brakes are maddeningly difficult to bleed. Just when you’re ready to beat the master cylinder to death with a brick, you bleed it one more time, and the pedal firms up. This need to bleed several times is part of the big advantage of a power bleeder like the Motive that has its own fluid reservoir. It also means that if you’re smart and have the equipment, it’s best to put the whole car in the air, either with four jack stands or a lift.

However, if you’re resurrecting a long-dead car whose brakes are in unknown condition, it’s quite possible that the pedal may never firm up because the bore and seals inside the master cylinder are shot. My rule of thumb is four attempts at bleeding. If I use my Motive power bleeder and, for each wheel, nearly drain the reservoir, that’s a bit under a cup per wheel, or about half to one quart per attempt. Four attempts would be half a gallon to a gallon of brake fluid. That’s more brake fluid than I want to encounter in a lifetime. If, after that, the pedal still doesn’t firm up, call the time of death of the master cylinder and replace it… at which point, of course, you need to bleed it all again.

Did I mention how much I hate the smell and feel of brake fluid?

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Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel magazine for 30 years. His most recent book, Just Needs a Recharge: The Hack MechanicGuide to Vintage Air Conditioning, is available on Amazon (as are his previous books). You can also order personally inscribed copies here.