Mustang MonthlyHow To Wheels Tires
How To Choose Brake Fluid And Bleed Brakes
Keeping Your Mustang's Braking System In Proper Working Order Calls For Regular Preventative Maintenance
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.
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.
|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|
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.
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.