Brake Bleeding

Brake Bleeding

Purging air and moisture from your binders

Brake bleeding is something that no sane person finds enjoyable. It can be messy and often involves bodily contortionism to access the bleeder screws. However, properly bled brakes are one of the most vital services you can perform on your car.

When & Why

Many brake problems can be traced to air or contamination of the hydraulic fluid. Leaks at fittings where lines and hoses connect to the master cylinder, calipers and/or wheel cylinders can allow air to enter the system, as can rusted-through brakelines. Also, air is introduced whenever a brake-system component other than the friction material (pads or shoes) is replaced. Air can also appear after moisture gets into the fluid and boils, giving off steam. Two signs of air in the fluid are a noticeable decrease in brake performance and a "spongy" pedal that quickly and nearly effortlessly goes to the floorboard (air compresses easier than fluid).

Fluid Facts

Although a properly functioning brake system is closed to outside corruption, brake fluid still degrades over time. Fluid must withstand heat that can exceed 650 degrees F at the wheel without boiling and also remain in a fluid state during the coldest winters. Furthermore, brake fluid lubricates and fights corrosion on internal parts.

Most brake fluid is hydroscopic: It absorbs water. On the positive side, any moisture that enters the system-through condensation, through rubber parts such as seals and hoses or through an unsealed bottle of brake fluid-is dispersed throughout the fluid. That way, chances of localized corrosion and fluid freezing are minimized. On the other hand, absorbed moisture lowers the fluid's boiling point, raises its freezing point and degrades its anti-corrosion additives. As little as 3% moisture can decrease fluid's performance by 30% and also negate its anti-corrosives. Additionally, standard DOT 3 brake fluid can only absorb up to 7% moisture, after which water droplets-and braking problems-begin to form. Anti-lock brake systems (ABS) are especially affected by moisture because they can cycle as many as 20 times per second. Spongy pedal feel is a sign of moisture in the brake fluid.

Depending on temperature fluctuations and humidity, a brake system can take on as much as 3% water in as little as eight months. It's usually recommended that the fluid be entirely flushed at least every two years or 24,000 miles.

Remember that brake systems are closed, so loss of fluid indicates a problem somewhere. When a master-cylinder reservoir is low, look for the cause instead of just topping it off with fluid. The system either has a leak somewhere or the brake pads are worn, causing the pistons to be more extended and retain more fluid.

DOT Specs

Most do-it-yourselfers are familiar with DOT 3 brake fluid, the grade most commonly recommended for passenger vehicles. The Department of Transportation (DOT) numbers refer to the fluids' minimum boiling points, rated both as "dry" (no moisture in the system) and "wet" (3% moisture). The wet rating is more meaningful for vehicles that don't receive regular fluid flushes.

The two brake-fluid styles: poly glycol ether (commonly known as "glycol") and silicone-based. Glycol-based brake fluid is available in DOT 3, 4, and 5 ratings. It's hydroscopic and compresses less than silicone fluid, which gives a firmer pedal feel. Glycol's main drawback is that it makes paint bubble if it comes in contact with it.

The DOT 5 grade is reserved for silicone-based fluid. Unlike standard glycol fluid, it doesn't eat paint (which is why DOT 5 is popular with the show/collector-car crowd) or absorb water. Theoretically, this makes it a "lifetime" fluid. However, if moisture gets into the silicone fluid, droplets can form and boil, which produces steam and introduces air into the system. Silicone fluid is also more expensive and more compressible than glycol, so it gives a mushier pedal feel. Never mix the two styles of fluid-the system must be completely flushed if changing from silicone to glycol or vice versa.

Two-Person Flushing

Now that we've hammered home the importance of bleeding and flushing brake fluid, here's how to do it properly. The traditional "two-man" method involves one person stepping on the brake pedal (whom we like to call The Pumper) while the other (The Dumper) opens the bleeder screws at the brakes. Verbal communication is critical so that the pedal is never up while the bleeder is open, which draws air into the system.

Bleeding patterns vary for different cars-consult the owner's manual and/or service manual for bleeding order as well as DOT-grade fluid recommendation and any ABS-specific procedures. Rear-wheel drive vehicles are bled beginning at the wheel that's farthest from the master cylinder, then gradually working in: typically right rear, then left rear, then right front and finally left front. Front-wheel drives are sometimes bled in a diagonal pattern.

For the actual 2-person blood-letting, The Pumper begins by slowly and steadily applying pressure to the brake pedal while The Dumper opens the bleeder. This forces fluid through the system. The Pumper alerts The Dumper to close the bleeder before the end of the pedal stroke to avoid damaging the master cylinder. The cycle continues until no more air bubbles are visible in the tubing and bottle. Remember to top off the master cylinder after every few pumps and dumps. Keep the lids on both the master and brake-fluid bottle to minimize sloshing and the opportunity for air to invade the fluid.

Once all air is bled, it makes sense to keep bleeding until fresh fluid emerges from the bleeder. When all wheels have been bled (and also the proportioning/combination valve in some vehicles), top off the master cylinder a final time, replace its lid and discard any remaining "new" brake fluid properly along with the old fluid-otherwise, moisture will accumulate inside the unsealed bottle during storage. Pump the pedal a few times to make sure that it's at least as firm-if not firmer-than in its pre-bleed state. Finally, road-test the brakes in an out-of-the-way place to verify they're functioning properly before driving in real-world conditions.

One-Person Methods

The automotive aftermarket includes a few products designed to make brake bleeding a 1-person operation. The three most common: vacuum pumps, pressure pumps and one-way bleeder screws.

Hand-operated vacuum pumps are the most popular for do-it-yourselfers. Affordable, these units suck fluid out through the bleeder, eliminating pedal pumping. However, air can be drawn in through the bleeders' threads if they don't seal tightly, giving the appearance of air in the system. Follow the instructions that come with the pump to learn its operation and the proper wheel-bleeding order for your car.

Pressure pumps are more expensive than vacuum units and are geared toward the professional mechanic. These units attach to the master cylinder and pressurize the system-then you simply open each bleeder. Some pressure pumps also function as a vacuum. They purge air from the brake system and also "bench bleed" master cylinders before they're installed.

One scenario in which air can enter the system while using a 1-person pump is when dirty bleeders' threads don't seal. If using one of these units, consider first removing each bleeder, then cleaning it and its wheel cylinder/caliper hole with brake cleaner before reinstalling.

One-way bleeder screws are a fairly recent boon for the lone brake bleeder. When opened, their check valve allows fluid out but springs shut before air can enter. Best of all, these bleeders are affordable and apply pressure.

When all's said and done, brake bleeding/flushing is one of the least-enjoyable ways to spend up to an hour. While you're thinking of all those things you'd rather do, though, remember that bleeding brakes saves human bloodshed.


ATE (Super Blue, Super Gold),

Bendix/Allied Signal,

Classic Tube,

Earl's Performance Products,


Phoenix Systems,

Russell Performance Products,

Stainless Steel Brakes Co.,

Super Blue,

bleeding your brakes
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