Should you ever add brake fluid to the master cylinder reservoir on a car with disc brakes?” asked Dann Ingebritson, technical instructor, Affinia Under Vehicle Group. “The answer is no. Instead you should replace the brake pads on that car.”
This tech tip opened the third day of the four-day Affinia-Raybestos Brake System Diagnosis & Repair Course, i.e., Foundation Brakes. This course is half classroom-based and half hands-on. The course is held at Affinia-Raybestos
headquarters in McHenry, Ill., and is also conducted at locations across the nation. Class sizes run a maximum of 12 because of all the hands-on work.
The Foundation Brake class covers hydraulic braking theory, brake system dynamics, diagnostics, troubleshooting, repair techniques, preventive maintenance practices, rotor resurfacing techniques, bench and on-car lathe operation, brake (friction) pad materials and noise and dust solutions.
This class is not one bit product-oriented, and it is not a veiled sales job. It is pure tech training, and you get your hands dirty. You install Raybestos pads, rotors and calipers in this class, and in other classes Raybestos master cylinders, hoses, lines and valves. However, brand to brand comparisons are never made, and brand names are seldom mentioned. Instead, the brake tech school is all about brake jobs, accurate diagnosis, fixing it right the first time and preventing comebacks—not about what brand of pad or rotor is better than another. Police Fleet Manager attended the course and recommends it, without reservation, to all new police fleet managers, new maintenance techs, new shop superintendents and as an update for veteran maintenance techs and maintenance supervisors.
Affinia-Raybestos conducts 400 training clinics a year, teaching more than 12,000 techs a year. This article is the last of a multi-part series. For Part 1, “Rotors Don’t Warp,” see the March-April 2010 issue of Police Fleet Manager. For Part 2, “Rotor Turning and Slotted Rotors,” see the May-June 2010 issue. Both are available online at www.hendonpub.com; click Resources, then Article Archives.
Which Brake Fluid?
Pop Quiz: What brake fluid should you put in the police vehicle? The answer is just as simple as deciding what engine oil you should use in the police vehicle. Read the embossed cap on the master cylinder fluid reservoir. Use whatever the cap says to use, and nothing else.
Three popular types of brake fluid exist: DOT 3, DOT 4 and DOT 5. DOT 3 is the standard fluid with a boiling point (dry) around 400 F. DOT 4 has a higher (dry) boiling point; it is around 445 F. Both DOT 3 and DOT 4 are glycol-based with a fluid color between clear and amber.
DOT 5 is a silicone-based brake fluid and is purple in color. Silicone fluid does not absorb water, like the glycol fluids do. And DOT 5 has a much higher dry boiling point: 545 F. DOT 5 has advantages but one clear and disqualifying disadvantage. Antilock Braking Systems (ABS) do not like silicone brake fluid, and every police vehicle uses ABS.
With a higher boiling point, why not put high temp DOT 4 in everything? After all, police brakes get hot. It is because DOT 4 absorbs water faster than DOT 3, which means it must be flushed and changed more frequently.
Brake Fluid Flush
And yes, the brake fluid really does need to be changed. Glycol brake fluids, like DOT 3 and DOT 4, are hygroscopic, which means they absorb moisture even inside an otherwise “sealed” brake system. (The term is hygroscopic, not hydroscopic. Hydroscopic refers to a device used to view objects far below the surface of the water.)
At any rate, DOT 3 and DOT 4 absorb water from condensation inside the braking system. The more the brake fluid is contaminated by the water, the lower the boiling point of the brake fluid. When pursuit-hot brakes cause the brake fluid to boil, liquid turns to a vapor or gas. Fluid is non-compressible, but vapor compresses easily. The result is a spongy brake pedal, or even the loss of brake pressure, and all this under just the emergency conditions that the full brakes are needed.
While not in any PM maintenance schedule, police platform engineers with all three police automakers recommend that the brake fluid be completely flushed every couple of years. That advice matches the declining-over-time boiling temperature of DOT 3 brake fluid. Because DOT 3 absorbs less water than DOT 4, every couple of years is best case. The hotter the brakes get, the more the brake fluid absorbs water, and the sooner it must be flushed.
When new, DOT 3 has a boiling point of 401 F. After one year, DOT 3 has absorbed about 2 percent water and the boiling point has lowered to 325 F. After two years, DOT 3 has absorbed about 3.5 percent water and the boiling point has lowered again to 292 F. That is important!
The average operating temperature of brake fluid is around 300 degrees F. Get the brakes on a two-year old police vehicle hot, use the brakes hard, and the brake fluid will boil, period. DOT 3 will continue to absorb water, to about 5 percent in the third year and 6.5 percent in the fourth year. The boiling point will continue to drop to 275 F, then to 250 F. The brake pedal will continue to get more spongy after even less aggressive brake use.
Using test strips and other tools, it is possible to measure the amount of water in the brake fluid, or at least, the moisture in the brake fluid in the master cylinder reservoir. The problem is that no clear consensus exists on age (years) of fluid or mileage that correlates to percentage moisture. That is why the fluid change interval is not in any PM schedule. The Affinia-Raybestos average of 3.5 percent and sub-300 F boiling point in just two years is a good starting point.
Police vehicles get new front pads and front rotors far too frequently to use pad replacement as a signal to flush brake fluid. However, a caliper change, master cylinder change or ABS (computer, hydraulics or electric motor) change should be a clear flag to completely flush the brake system.
Moisture in the brake fluid lowers the boiling point and causes poor brake performance at the most critical time. Getting this moisture out of the brake system is one reason for a complete fluid flush every two or so years.
The other reason is copper contamination. All brake lines are brazed tubes. The movement of brake fluid inside the brake lines erodes the copper brazing. Brake fluid can run up to 200 ppm of copper. ABS, traction control and stability control units all have problems with copper contamination, i.e., the internal galvanic corrosion from two dissimilar metals. It may sound overly technical, and it is certainly not as obvious to the driver, but getting this copper out of the fluid is nearly as important as getting the moisture out.
Brake Pad Wear
Don’t add brake fluid to the master cylinder reservoir. Instead, replace brake pads. Adding brake fluid on a low master cylinder is “drum” brake thinking. A low fluid level on a disc brake car means the pistons are pushed all the way out in the caliper, i.e., the pads are worn out. In fact, a clever brake tech can look at the master cylinder fluid reservoir level and tell what brakes the vehicle needs—just the front pads or both the front and rear!
The square cut seal (SCS) is one of the simplest, most ingenious parts of the brake system. In the old days, lateral runout from the rotor would “push” the brake pad (and piston) back into the caliper. With today’s almost zero runout rotors, the SCS does the job of “pulling” the piston back into the caliper.
The “square” cut seal has a tight fit around the caliper piston. It lodges in an “angle” cut machined groove in the caliper cylinder bore. With no pedal pressure, the rubber seal holds its square shape. When the brakes are applied, hydraulic fluid pushes the piston (and the brake pad) into the rotor, which slightly deforms the SCS.
When the pedal pressure is released, the SCS around the piston only retracts the piston by 0.002 inches. As the pads wear, the piston protrudes further out than the caliper, and the fluid level in the master cylinder drops. As a general rule, the minimum master cylinder reservoir fluid level equals the minimum pad thickness based on all four wheels.
Pop Quiz: When does the lining wear sensor touch the rotor and make that screech noise? When the pad is about the same thickness as the backing plate, which is between 1/32nd and 2/32nd inches, which is the minimum lining thickness. If the rotor scraping metal clip is bent and is not going to give an audible warning, the thickness of the pad equaling the backing plate is the visual warning during the Preventive Maintenance (PM) inspection.
Isolation Diagnostic Test
Is the soft pedal problem in the master cylinder or one of the brakes? If it is one of the brakes, which one? In an isolation test, the rubber brake hoses are clamped off one at a time to check for what causes the soft pedal. The tool used is a pair of plastic pliers that work like a hose clamp.
You are not going to damage the flexible brake line no matter how hard you clamp down on it; the pliers don’t fully close. The hoses will not be permanently deformed even after four hours under a full clamp. These clamp pliers are one of the best diagnostic tools.
Consider this Raybestos fact: Most master cylinder complaints are not the fault of the master cylinder. Isolate each wheel circuit with the clamp pliers. Press the brake pedal. If the pedal remains high with the clamps on, the problem is downstream of the clamps. If the pedal still sinks, the problem is upstream of the clamps. It may be the master cylinder or perhaps the ABS unit.
Road test hint: Be sure to remove all of the clamp pliers from the flexible brake lines!
The Dirtiest Fluid
Where is the dirtiest and most contaminated fluid in the entire brake system? It is behind the piston in each front caliper.
RAYBESTOS BRAKE TIP: Clamp the flexible brake hose near each caliper and open the bleeder screw, then push the piston back into the caliper.
You can simply force the piston back without opening the bleeder screw. However, you will push the dirtiest fluid in the system into the ABS unit, the most debris-intolerant part of the brake system. Moisture is everywhere in the brake fluid, but the most dirt and debris is at the caliper. Drain this. Don’t push it backward into the ABS and master cylinder.
Some techs are concerned about damaging the flexible hoses with the clamp pliers, but don’t think a thing about dangling the heavy caliper from the same hose! Use some sort of hanger to suspend the caliper, not the brake line hose. And be careful not to drop the caliper only to use the flexible brake hose as a bungee cord. Most flexible brake hoses have a white line visible at the base of the metal hose fitting. If you see a second white line, that means the hose has been pulled out a bit from the clamped-on fitting. Don’t drive the car. Replace this flexible hose.
Always Bench Bleed
When you install a master cylinder, bench bleed it before you put it on the car. You don’t want to push air into the ABS unit because it is very difficult to bleed the ABS. Simply put the plugs in the new master cylinder and route the hoses back into the master cylinder reservoir.
A couple of “don’ts” with the bench bleed. Don’t use a screwdriver as a plunger because it may scratch or gouge the piston bore of the master cylinder. It is also difficult to control the depth of the plunge with a screwdriver. And that leads to the second “don’t.”
Don’t push the master cylinder piston too far. The piston cylinder wall is not fully machined all the way, just the 1-inch to 1.5-inch travel of the piston in normal use. The proper tool for the job has a depth control to prevent the piston seal from being pushed past the fine honed area into the rough cut area. A damaged seal will ruin the master cylinder.
Don’t pump the pedal all the way to the floor when you bleed the brakes. In fact, place your left foot under the pedal to limit the pedal travel. A great deal of debris and corrosion are in the master cylinder at the end of the normal piston stroke. If you pump the brakes past the normal stroke, you push all this contamination directly into the ABS unit. This is the most sensitive, closest tolerance and most complex electro-hydro-mechanical part on the vehicle.
Pop Quiz: Once you have the caliper off the car and the mounting bracket off the caliper, how do you tell which caliper goes where? Which goes on the left and which goes on the right? Look for the bleed hole. You bleed to remove air. Air rises. The bleed hole goes up.