Jim Smart
August 1, 2008
This is how you bleed brakes with a helper. Begin with the right rear, then left rear, right front, and left front. The hose must be a tight fit around each bleeder with the end submerged at all times. Have your helper do a slow pedal depression (halfway) and watch closely. Flush until there is solid fluid flow from the hose. If you're starting with an empty system, be prepared for a lot of air bubbles.

DOT 5 silicone brake fluid stands alone and must never be mixed with DOT 3 or 4. Because silicone brake fluid is less prone to moisture absorption, it's popular for collector cars that are seldom driven. The downside is how it interacts with moisture. Instead of moisture in the fluid like we see with DOT 3 and 4, it tends to settle at the lowest point in the system-usually calipers and wheel cylinders-causing corrosion if the vehicle sits a lot. In hard braking with high pressures and heat, this causes the moisture to boil, causing ineffective braking.

Silicone fluid is also more compressible than mineral-based brake fluid, which makes the brake pedal feel spongy. Silicone fluid foams when it's poured, putting air in the fluid and adding to the spongy pedal feel. When that happens, you don't have the braking efficiency of mineral-based DOT 3 and 4. Silicone also has a slimy feel (similar to vinyl protectants). An upside of silicone fluid is it won't damage paint.

Your braking system components determine how the pedal will feel, as well as how effective the brakes are. Master cylinder bore size is everything to pedal hardness. A larger bore yields a softer pedal. By the same token, a smaller bore provides a harder pedal.

When air bubbles stop and there's solid fluid flow, have your helper pump up the brakes so there's a hard pedal. While the helper maintains pedal pressure, open the bleeder and watch flow. Close the bleeder while the pedal is still depressed. Then pump up the brakes again. Do this at least three more times to ensure all the air is removed. Even tiny bubbles are unacceptable.

Brake hoses and lines also affect how the pedal feels. Reinforced rubber hoses provide a softer pedal than braided aftermarket hoses because reinforced hoses stretch more than braided ones. Although you can't see it with the naked eye, rubber reinforced hoses expand with pressure; braided hoses expand a lot less. This leads us to brake lines. They must have double-wall construction with double-flares at all connections.

Brake Fluid Facts
The Department of Transportation specifications-DOT 3, DOT 4, and DOT 5-indicate a brake fluid's wet and dry boiling points. Wet in both cases means allowable maximum 3-percent moisture content. Moisture content beyond 3 percent is unsafe by DOT standards. DOT specifications are under the best circumstances, so don't forget to flush and bleed your braking system.

Fluid Type Wet Dry Specifics
DOT 2   374 Degrees F Not Recommended
DOT 3 401 Degrees F 284 Degrees F Glycol Base
DOT 4 446 Degrees F 311 Degrees F Glycol Base
DOT 5 500 Degrees F 356 Degrees F Silicone
With Eastwood's MITYVAC brake self-bleeder, you never have to rely on anyone's help again. Pump and hold approximately 25 inches of vacuum, open the bleeder, and draw fluid to remove contamination and air in less than a minute. It retails for $34.99.

Bleeding Methods
There are three basic ways to flush and bleed a hydraulic braking system-foot brake and jar, negative pressure, and boosted pressure. The objective with each procedure is to get all air and contaminants out of the system.

Foot-Brake Method
The foot-brake method is the most common bleeding technique using a hose and jar. The brake pedal is slowly pressed, brake bleeders are opened one at a time, and air is expelled from the system. Some undertake this method with reckless abandon, but flushing and bleeding should be approached carefully to avoid pushing the master cylinder piston too far, meaning it must be disassembled and repositioned. Be careful and don't push the brake pedal too far.

Begin by ensuring the master cylinder is completely full of fluid. Always secure the lid before pedal depression. Pump the pedal slowly with all the bleeders closed to build initial pressure. If you're starting with an empty system, pressure won't build until fluid is all the way to each brake. So begin brake bleeding and follow this pattern: right rear, left rear, right front, left front. You need a jar at the brake that's 1/4 full of fluid. You also need a clear plastic or rubber hose that's a tight fit at the bleeder. The other end of the hose must be submerged in the jar.