Disc brake renewal
Call it a litmus test, if you will. Renewing a tired set of disc brakes is guaranteed to break you – pun intended – or bring you to a new level of understanding and appreciation of your car’s braking system. In any event, if your garage doesn’t have certain tools, you’re going to have to farm some things out. Don’t worry; you’ll still be part of the FIT (fix-it-yourself) auto fraternity. Besides, getting the job done correctly is more important than trying to do a job and failing, when you’re talking about stopping your car. A blown engine is something you can laugh about later. Running into another car, because your brakes weren’t probably prepared is another matter.
Ever since Elmer Ambrose of Cleveland, Ohio designed spot-type disc brakes for his electric car way back in 1898, disc brakes have been the preferred brake system for automobiles. Disc brakes are superior to drum brakes because discs can handle higher braking temperatures and dissipate heat more quickly.
They also do not trap water as drum brakes can; when drum brakes become wet, they can suffer a decrease in braking – what is generally called “brake fade.” That can happen when driving through deep puddles. However, if you take a car with four-wheel disc brakes through standing water, you’ll have no worries about the effectiveness of the brakes, if you find yourself needing them, shortly thereafter.
Disc brakes generally consist of a metal disc called a rotor, which is connected to the wheel. A caliper rests on the edge of that disc, and holds two friction pads on either side of the disc. Applying the brake pedal causes brake fluid to push down a piston within the caliper. That, in turn, pinches the brake pads against the disc and slows the wheel.
Disc brakes do not have return springs, like those in drum brakes, to disengage the brakes. Instead, a seal around the piston bends slightly, and then retracts to pull the piston away from the disc, when the pedal is released.
The most common caliper design utilizes a single-hydraulically-actuated piston, within a single cylinder. Disc brakes such as this were first configured on a compact sports car called the Crosley Hot Shot in 1950. Dunlop developed the first, fully-functioning modern disc brakes in 1953 for the Jaguar C-type racing cars that helped that marque win the 24 Hours of LeMans several times; those brakes were then fitted to Jaguar production cars.
Then at the 1962 New York Auto Show in April, Studebaker showed its Avanti sports coupe; fitted with servo-assisted Dunlop 11-1/2 inch disc brakes on the front wheels. Kelsey-Hayes disc brakes with four-piston actuation came to be used on several American cars in 1964. And by 1967, Volvo brought out its 144-series sedan with four-wheel disc brakes, equipped with two brake circuits: if one failed, 80 percent of braking effort was still available.
American cars came to offer “front disc clips” to help provide better stopping for those high-performance offerings. By the 1970s, many American cars offered four-wheel disc brakes, as safety became a paramount concern of auto manufacturers. Today, disc brakes on high-performance cars can use up to 12 pistons.
Another peculiarity of disc brakes is they rely upon a small amount of “wobble,” also called “run-out” by brake shops, which is normally present in the disc. When the brakes are released, the amount of run-out of the disc or “rotor” simply pushes the pads away from the disc.
But what about rebuilding disc brakes? Well, other than the calipers, which are best left to a shop that specializes in rebuilding calipers, disc brake work is as straightforward as any other type of brake system repair.
When addressing disc brake problems, it is important to address the core problems, not just the symptoms. Tremendous heat builds up where the calipers and rotors intersect. This sometimes will cause the brake fluid to boil, and brake pressure is lost. The diagnosis is sometimes made that the calipers are faulty, when in fact; the problem lies with the hoses.
Rubber brake hoses wear out with time and miles. When that happens, the rubber lines will act as one-way valves. Line pressure is unable to feed back normally, and the effect is similar to driving around with the brakes applied. A good way to dispense with that problem is to fit one’s car with braided steel brake lines.
Removal of caliber and (original) rubber brake hoses
Beginning brake repairs requires dismantling each wheel. You’ll need to remove the brake pads and draw back the brake pistons, using a tool made for that purpose. If you find the brake pads at an angle, that is indicative of abnormal wear. You should replace those pads, in that case.
Be sure to take a screw out of the bracket that holds the brake hoses; this is a necessary step to taking the hoses off of the car. The hoses should come out easily, after that.
The calipers come off before the final removal of the hoses. Breaking loose brake calipers is aided with the application of fluid to clean out the area to be worked on. WD-40 would work fine to do that, as would Wurth “Rust-off.”
Once the calipers are off, the final removal of the brake hoses is easily accomplished with a wrench. Removing the caliper in the rear of the car requires loosening two bolts.
In the case of the car used to write this article – a 1972 Volvo 142E sedan – there were two lines in the back of the car. They were connected to an equalizer valve and then connected to the calipers, via that same valve. (There were four brake lines up front.) Your car or truck likely has a similar or identical set-up.
Even if you farm this portion of the job out, it’s a good idea to know what goes into it (especially if, on the other hand, you do the work yourself).
The front calipers, on the car used to do this article, used four-piston calipers. The rear brakes used just two pistons. But as on any disc brake rebuild, it is vitally important to clean out the rust inside the calipers; since the coefficient of friction has gone so high.
Heat in the disc brake pads is the key issue, when the pistons have been unable to retract in the bores (due to fluid not having been returned, due to the leaky rubber hoses).
Volvos were simply hydraulically actuated, back in the day. The disc brakes used on American performance cars, such as the Chevrolet Camaro or Ford Mustang, were actuated hydraulically and mechanically, simultaneously.
The unfortunate truth of the matter is that calibers have to be rebuilt, for ultimate safety, according to John Slipper, owner of Mobile Brake Services in Seattle, Washington. “You can’t run a caliper from a wrecking yard,” asserted Slipper. “The difference is between hydraulic fluid, such as is used in brakes, and gear fluid. Brake fluids draw moisture. The minute they are parked, they start deteriorating.”
Brake boots hide a multitude of rust and wear. Absolute cleanliness is imperative when working on brake calibers, since they use hydraulics. If you remove the pistons, you need to mark them. That way, you can ensure that they will go right back into where they were before.
Remove the dust cover retaining rings and the dust covers themselves. Clean the exposed area with compressed air. Then, remove the inner sealing rings with a small screwdriver. Be careful when doing this, as to not scratch the bores or damage the grooves.
Unscrew the bleed nipples and hose connections. However, under no circumstances separate the two halves. That is a factory job, requiring specialized equipment.
With the boots off, one can see the rust and wear inside the piston bores. The best way to clean disc brake calibers is inside a blasting cabinet. Give the various parts and connecting paths a thorough cleaning with ethylated spirits. That’s about the only fluid that won’t deteriorate the rubber seals.
Once the calipers have been rebuilt install new pads and the two inlets for the brake hoses. Reassemble with the same processes – in reverse. But do coat the various parts with brake fluid before assembling them.
Fit the pistons with the large ends inside the housings. Then, fit the inner sealing rings into their grooves. The outer end of the piston is perpendicular to its axis and should incline about 20 degrees along a line, determined by the markings made earlier (on a template). New pistons – if needed – will require comparing new with old, as well as marking the new, as was the old one.
Note: If you aren’t up to this portion of the job, due to not having the correct equipment or skill set; go to a professional such as John Slipper, give that person your old calipers (for which you will get a small “core fee” discount, since he or she can rebuilt them) and simply buy a set of rebuilt calipers. There’s no shame in that, just knowledge of knowing your limitations. Slipper charges $59 to $79 for a rebuilt caliper.)
Installation of new calipers and hoses
Now you need to remove the old brake disc, and then install the new disc. Follow this procedure on all four-wheel/brake assemblies.
Spin the rotor to ensure it rotates freely. If not, a hammer and punch can be employed to give it more room. The backing plate up against the A-arm will limit how much room you can give the rotor, up front.
New, flexible steel brake hoses can then be installed. Check for clearance of the caliper; the rebuilt calipers may require shims to ensure proper distance.
Installing stainless steel, flexible brake hoses in the rear will simply require loosening, and then tightening, the same connections worked on before. The new brake lines will extend from the equalizer valve to the rear disc brake assembly. (Two brake lines go to an equalizer valve, and then connect to the calipers, via that valve.)
After installing the new discs (rotors), use a dial gauge and magnetic base to check oscillation – fluctuation from the original settings – is highly recommended. To do that, position a dial gauge in the external braking surface, about 5 mm from the end of the external diameter. You’d want to do this after driving the car a bit – maybe a mile or two --- to get a better sense of how much adjustment is needed, if any.
Disc oscillations shouldn’t be greater than 0.10 mm after driving the car a few thousand miles.
It’s imperative to make certain the brake fluid to every line is absolutely clean. The best way to ensure this is to bleed the brakes first. Hook up a reservoir to bleed out the brake fluid. The bleeder screw is on the back of each caliper.
One end of the bleeder hose is connected to the air vent on the brake assembly. Someone then needs to get into the driver’s seat and depress the brake pedal. The (other) person at the brake then opens the air nipple and allows brake fluid to flow out, free of air bubbles. The key thing you’re doing with all this is eliminating air from the system, while adding brake fluid.
Different cars have different sequences for doing this. Your best bet to find the correct sequence for your particular vehicle is to check a shop manual, oftentimes available at the reference desk of your city’s or town’s main library. (If you’re really fortunate a branch library might have one.)
The preceding activity also takes the brake pistons close to the new pads, without reaching full extension; thus pedal movement can return to normal.
The recommended brake fluid is DOT 4, whose boiling point is as high as you can go, without using synthetic brake fluid. Old brake fluid must be caught and properly recycled. If you do this at your home shop, telephone your local municipality or county government to find out where the appropriate recycling center is. An auto repair facility will likely charge you about $15 to $30 to dispose of such fluids.
If the library doesn’t have what you need, Books4Cars specializes in hard-to-find auto repair manuals.
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