How To Choose Brake Fluid And Bleed Brakes
Keeping Your Mustang's Braking System In Proper Working Order Calls For Regular Preventative Maintenance
All the horsepower and good looks in the world are meaningless without safe brakes. To have good brakes, you must stay on top of regular preventative maintenance in every respect because they need tuning up periodically just like engines. While engines need spark plugs, fuel filters, and oil changes, brake systems require fresh pads, turned rotors and drums, replaced seals, and the periodic change of fluid. Every so often, we need to reline brakes and machine rotors and drums, which gets brakes back on top of their game. But hydraulic brakes should have more than just good pads and machined surfaces. They need clean, uncontaminated fluid to do the best job.
Brake fluid contaminants include air, moisture, solvents, dirt, rust, rubber, and a host of other elements. Hydraulic brake fluid must be able to withstand high temperatures during hard braking and still be able to stop the car-and it can't freeze when the mercury plummets. It's also a lubricant, which means it has to protect moving parts. Clean fluid is necessary for safe operation. Flush and bleed brake fluid every two years regardless of mileage.
Where brake fluid differs from engine oil, transmission fluid, gear lube, or antifreeze is its status as a hygroscopic fluid. It absorbs moisture, even through steel lines, reinforced hoses, and capped master cylinders. Take the cap off your Mustang's master cylinder, and brake fluid takes on air and moisture, which can make unpleasant things happen under pressure.
Slamming hard on the brakes results in a lot of pressure and heat, which can boil contaminants in the fluid. Those moisture pockets become steam pockets, which generate air pockets, making your brake pedal pulsate during hard braking. Don't get this confused with a warped rotor or irregular drum, which also causes pedal chatter in rhythm with wheel rotation. With excess moisture in the brake fluid, hard braking will generate little pulses, or hammering, in the pedal, resulting in poor braking performance. The more moisture there is in the brake fluid, the lower the fluid's boiling point.
As little as 3 percent moisture in your Mustang's braking system can reduce its effectiveness by 30 percent. This is when your Mustang's braking system becomes dangerous because it won't stop you in a panic situation. In fact, it will struggle to stop under normal conditions.
Moisture in brake fluid also aggravates corrosion inside brake lines no matter how protected they are with galvanizing or protective coating. This makes high-pressure lines rust through, causing brake failure.
Brake failure typically happens without warning when a seal fails or a line bursts from fluid contamination, rendering your Mustang's braking system inoperative, especially if it's a '65-'66 with a single master cylinder. Sometimes brakes fail gradually as pads wear and fluid becomes more contaminated.
Also remember that brake fluid absorbs moisture in the bottle. Keep some on hand for brake servicing, but properly dispose of it every two years to keep your inventory fresh. It's a good idea to write the purchase date on the bottle to keep track.
The different types of brake fluid offer their own advantages and disadvantages. If you still have DOT 2 on the shelf, dispose of it because it was designed for older drum brake systems and has a much lower boiling point than DOT 3 or DOT 4. Any DOT 2 fluid still on the shelf has absorbed more moisture than you'd want in your braking system anyway.
Although DOT 3 and 4 are similar, we discourage mixing the two. If you have DOT 3, stick with it unless you're putting in all-new brake fluid. In that case, use DOT 4. The advantage of DOT 4 is its boiling point and synthetic makeup, making it less prone to moisture absorption. It must still be flushed and changed every two years.
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.
Open the bleeder and have a helper slowly depress the brake pedal about halfway. You will get a lot of air bubbles until there's solid fluid flow. Be patient and wait it out. Keep the master cylinder filled with fluid and continue the bleeding process. Again, slow pedal depression (halfway) until all the air bubbles are gone. Do this with each brake until there's solid fluid flow.
Negative Pressure Method
When you don't have a brake-pedal helper, it's nice to know there are alternatives that enable you to bleed brakes all by yourself. Mark Jeffrey of Trans Am Racing showed us a homemade negative pressure brake bleeder he created using easily obtained parts. He demonstrated his self-bleeder with exceptional results by removing all contaminated fluid and air from our '67 Mustang's braking system.
The Eastwood Company offers a couple of brake-bleeding kits. Most popular is the single-handed brake bleeder known as the MITYVAC. Instead of relying on master cylinder pressure, it draws brake fluid from the caliper or wheel cylinder, which removes air and contaminants into the small MITYVAC chamber. With each stroke, it removes 1 ci of brake fluid and air, enabling you to bleed brakes by yourself.
While cruising through Eastwood's catalog, we discovered another approach to brake bleeding, one that involves applying pressure at the master cylinder without pumping the brakes. The Motive Products Power Bleeder from The Eastwood Company involves pressure application and administering brake fluid at the master cylinder. Pump up the pressure and crack the bleeders. It's like having someone's foot on the brake pedal.
Bench bleeding a master cylinder fills the bore with fluid and removes all the air. Here's how to bench bleed with the master cylinder on your Mustang. Whenever you bleed brakes, always make sure your helper doesn't press the pedal too far. Push the pedal all the way to the floor and you can shove the master cylinder piston too far, wedging it deep in the bore and rendering it useless. The pedal needs to be depressed just enough to move fluid and remove air without jamming the piston.
Also be aware of power brake booster preload. Excess preload moves the master cylinder piston into the bore, cutting off fluid supply. Make sure the piston is at rest before beginning brake bleeding.