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Brakes Auto Service Tech Tips
These Tech Tips are provided courtesy of autoclassroom.com, an online training resource for automotive instructors, students, and technicians.
Two quick checks
If the vehicle is hoisted on a frame contact lift, spin each tire. This test is used to check that the brakes are not dragging. All four wheels should be able to be turned by hand assuming that the parking brake is off and the transmission is in neutral. Also, when spinning the tire, look over the top of the tire to check to see if it is round. An improperly mounted tire or a tire that is out-of-round can be seen by watching to see if the outside of the tire moves up and down as it is being rotated.
The one drip per second test
Excessive brake wear is often caused by brake cylinder linkage or brake light switches keeping the brake pedal from fully releasing. If the brake pedal is not fully released, the primary piston sealing cup blocks the compensating port from the brake fluid reservoir. To test if this is the problem, loosen both lines from the master cylinder. Brake fluid should drip out of both lines about one drip per second. This is why this test is also called the "Master Cylinder Drip Test". If the master cylinder does not drip, the brake pedal may not be allowing the master cylinder to fully release. Have an assistant pull up on the brake pedal. If the dripping starts, the problem is due to a misadjusted brake light or speed (cruise) control switch or pedal stop. If the master cylinder still does not drip, loosen the master cylinder from the power booster. If the master cylinder now starts to drip, the push rod adjustment is too long. If the master cylinder still does not drip, the problem is in the master cylinder itself. Check for brake fluid contamination. If mineral oil such as engine oil, power steering fluid, or automatic transmission fluid (ATF) has been used in the system, the rubber sealing cups swell and can block off the compensating port. If contamination is discovered, every brake component that contains rubber must be replaced.
"Competitively priced" brakes
The term "competitively priced" means lower cost. Most brake manufacturers offer "premium" as well as lower price linings to remain competitive with other manufacturers' brake lining material produced overseas or in foreign countries. Organic asbestos brake lining is inexpensive to manufacture. In fact, according to warehouse distributors and importers, the box often costs more than the brake lining inside! Professional brake service technicians should only install brake linings and pads that will give braking performance equal to that of the original factory brakes. This means that "competitive" asbestos linings should never be substituted for semi-metallic or NAO original linings or pads. For best results always purchase quality brake parts from a known brand name manufacturer.
Too much is bad
Some vehicle owners or inexperienced service people may fill the master cylinder to the top. Master cylinders should only be filled to the "MAXIMUM" level line or about 1/4" (6 mm) from the top to allow room for expansion when the brake fluid gets hot during normal operation. If the master cylinder is filled to the top, the expanding brake fluid has no place to expand and the pressure increases. This increased pressure can cause the brakes to "self apply," shortening brake friction material life and increasing fuel consumption. Overheated brakes can result and the brake fluid may boil causing a total loss of braking.
The brake pedal depressor trick
The master cylinder can be used to block the flow of brake fluid. Whenever any hydraulic brake component is removed, brake fluid tends to leak out because the master cylinder is usually higher than most other hydraulic components such as wheel cylinders and calipers. To prevent brake fluid loss that can easily empty the master cylinder reservoir, simply depress the brake pedal slightly or prop up a stick or other pedal depressor to keep the brake pedal down. When the brake pedal is depressed, the piston sealing cups move forward, blocking off the reservoir from the rest of the braking system. The master cylinder stays full and the brake fluid stops dripping out of brake lines that have been disconnected.
Hint: Try this: Put a straw into a glass of water. Use a finger to seal the top of the straw and then remove the straw from the glass of water. The water remains in the straw because air cannot get into the top of the straw. This is why the brake pedal depressor trick works to prevent the loss of brake fluid from the system even if the brake line is totally disconnected.
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