Troubleshooting brake failure and brake fluid leaks
Copyright 2011 by Morris Rosenthal All Rights Reserved
I've also posted a series of flowcharts for
troubleshooting laptops and PC
troubleshooting.Cars are dangerous and
can kill people who work on them - proceed at your own risk. Diamond symbols
linked to decision text.
Diagnosing braking problems and noises with shoes, rotors and calipers
Do the brakes fail to stop the car? If you have to step on the brake pedal harder than you want to in order for the brakes to operate normally, the brakes do stop the car. What we're interested in here is when you have to push on the pedal with both feet and the car slowly rolls to a stop rather than locking up the wheels, or when the pedal goes to the floor, won't pump up, and you have to yank the emergency brake or turn the engine off in gear in order to slow it down. I drove around ten miles through city street in a car with no hydraulic brakes when I was a stupid teenager, though fortunately it was a standard transmission and I was just smart enough to ignore the cars behind me and time all the lights.
Does the brake pedal go to the floor? The absolute first thing to check when you still have pedal but the car won't stop is that something really dumb isn't going on, like you took your shoes off to drive and they are jammed under the brake pedal! But assuming nobody would need to go on the Internet to figure that out, there is always the possibility that the pedal linkage is bind up under the dashboard, so make sure the brake pedal is really going through its full travel and not hanging up 3/4 of the way down. Other problems that can lead to minimal braking are glazed brake parts, frozen pistons in the calipers (though the steering should pull in only one caliper is frozen), brake lines that are pinched or obstructed, or a problem with in the master cylinder or power booster unit. If you're fond of hitting the brakes when the car isn't running, you may exhaust the vacuum to the power booster and have the brake pedal go lower and lower until you start the car. A bad seal in the master cylinder can leave the pedal with some feel of resistance but fail to apply enough pressure to the caliper or wheel cylinder pistons to stop the car.
Is the brake fluid level OK? If there isn't enough brake fluid in the master cylinder to resist the piston, the pedal will go to the floor and the brakes won't function. The only "good" reason to be short brake fluid is if you just did a brake job didn't tighten a bleeder screw enough or somehow forgot to make up the fluid you lost after bleeding the brakes, which you might blame on your pedal pushing assistant. Make up the brake fluid in both reservoirs of the master cylinder to the fill line, and poke the rubber seal on the lid back into shape so the reservoir doesn't overflow as soon as you put it back on. Bleed the brakes following the order given by the manual, and make up the level after each wheel. Your brake fluid level does fall normally with the years as your disc pads and rotors wear down, leaving the caliper pistons further and further extended in the cylinder. If you have rear brake shoes and drums, their wearing will make the pedal softer and softer, but the return springs may keep the reservoir level up.
But the usual reason for the fluid level to drop through the bottom, especially in cases of outright brake failure, is a leak in the hydraulic system. Start be inspecting the wheels and tires on the ground to see if there's obvious brake fluid leaking out. Check around the master cylinder are for a leak, and make sure the master cylinder lid was secured, or it will simply overflow when the brakes are applied. Before you move the car, check carefully for the location of any suspicious fluid spots on the ground, because if the failure is a pinhole leak in a rusted steel brake line, it may be difficult to locate later without refilling the master cylinder and having somebody hit the brakes to squirt out brake fluid (normally in your eye, if you're under the car looking).
Does a brake warning light come on the instrument panel? Check to make sure the parking brake isn't on, and then take the time to find the mechanical switch for the parking brake and make sure it isn't stuck. If the brake warning light is in fact an error condition being reported by the antilock braking system (ABS) or power booster, you'll need the shop manual or a good aftermarket manual to find out if any information can be coaxed out of the idiot light by following some procedure. But in the case that the brake pedal goes to the floor, the brake fluid in the master cylinder level is fine and the brake warning light isn't complaining, you must have a problem in the master cylinder or power booster. Before you jump to the conclusion that there's a blown piston seal inside the master cylinder, read up on the power assist unit, and see if the pedal to the floor behavior can be caused by a vacuum failure or other booster problem.
I(s this a parking emergency) brake failure? When you pull the emergency brake lever between the seats (or under the dashboard) or push down the parking brake pedal, it should stiffen up quickly and ratchet audibly, whether or not it provides much braking force. If the parking brake won't hold the car from rolling backwards on a hill in neutral, it's not working properly. Some heavier vehicles with all disc brakes are notorious for having crummy parking brakes kludged onto the disc systems, but if they don't hold the car from going forward from a dead stop in gear, good luck passing inspection.
Is a rear wheel locked? Sometimes the emergency brake works too good, which is to say, it locks up a rear wheel and doesn't let it go. This is more likely to happen when a car sits for a while and the return springs take a set or the cable rusts up in the its sheath. Drive forward a couple feet so you can see which wheel is locked (it will leave a drag mark), then shut down, get out of the car and try bouncing that corner of the car up and down vigorously. If you hear a snap or a clunk and the brake releases, that was probably the cable letting go from its bound or slightly rusted state. Spraying some penetrating oil up into the metal sheath of the cable may help prevent it from happening again for a while, just don't get any inside the brake drum of on the pads. If that doesn't work, flexing the cable sheaths (within reason, you can't crimp them) and further applying and releasing the brake may help. You can often drive off a locked parking brake, but if it doesn't free up within a couple hundred yards, your risking overheating and causing real damage. If it happens once after leaving a car for the winter, it's no big deal, but if it happens every time you use the parking brake, it's time to replace the cables and springs.
Does the parking brake lever or pedal ratchet or move without force? If the parking brake moves through its range without any force, whether it make the ratcheting noise or not, it's usually a cable problem. The cable may be badly stretched or broken, the adjuster may have broken, or the spreader or equalizer that sends cable to both back wheels may have pulled off the undercarriage. You shouldn't try to repair a broken cable, just get a replacement and install it. The only trick is locating the brake, so which cable you need. Some systems run a single cable all the way from the emergency brake to one wheel, and attach a second cable to the spreader at the back of the car. Other systems use three cables, one from the parking brake back to the spreader, and one from each side of the spreader to each back wheel. If the parking brake offers very weak resistance, it's most like a stretched cable which you might ba able to correct with the adjuster on the spreader, or rear shoes with frozen self adjusters in the drums.
If the parking brake seems to operate normally in terms of the amount of force to apply it but doesn't lock the wheels, it's possible that the cable is badly constricted in the protective sheath by corrosion, but it's more likely that the brakes are simply worn. If you have drums on the back, check the shoes for wear and check for any fluid leaks or glazing. Four wheel disc brakes accomplish the emergency brake function in two basic ways. The elegant solution is a more complicated caliper that includes a mechanical linkage, such as a cam or threading, that can be used to force out the piston and close the pads on the rotor by pulling on the cable. If you have discs with minidrums on the backs, they should be good for the life of the car unless you accidentally drive a distance with the emergency brake on and glaze the parts.
Does one or more wheels drag too much? If you notice that your car can't roll forward from a full stop in neutral on gentle inclines, or even slows down and stops, your brakes are dragging too much. Put the car up on jack stands and spin each wheel to see if they are all about the same or if one wheel stands out as dragging to the point of being hard to spin, not even turning a full revolution without stopping when you give it a strong tug. If the problem is with a back wheel, a dragging parking brake is the first suspect. Make sure that it's off, that it's the cables to the back wheels aren't taut, and back off on the adjuster to loosen up the cable to the spreader if they are. Drum brakes generally drag because they are over-adjusted. Removing the drum when the brakes are over-adjusted is a drag in and of itself, you need to remove the little rubber access plug on the back of the wheel plate, push the auto adjuster out of the way, and back off the adjuster gear by hand with a screwdriver or brake spoon.
For disc brakes, the main suspect is a stuck piston, either one that is mechanically binding in the cylinder, or that is under continual hydraulic pressure. Try loosening the bleeder screw for that wheel and see if the piston backs off. If it does, you had (have) a problem with the hydraulic system, probably at the master cylinder or some balancing valve. If the wheel drags in jerky fashion, moving easily and then locking, you probably have a warped rotor or some other rotor deformity. On rear disc brakes with the parking brake function built into the caliper, they may be adjusted too tight, but I've never had to deal with it myself.
Do you need to pump up brake pedal? If the pedal is soft and you need to pump it and release it several times before it firms up, it's usually a sign of air bubbles in the brake lines. The real question is why there's air in the brake lines, whether there's a slow leak somewhere, a loose bleeder screw, a leaking seal on the master cylinder. If the problem creeps up on you very slowly and you never checked the master cylinder, it's possible that the brake fluid level has been falling slowly as the parts wear, and the lid seal isn't perfect. If it's not strictly a post-turning issues, skip down two paragraphs for bleeding.
Does the problem only happen after turning? If you only have to pump up the brakes after turning, it's a pretty good sign that something is forcing the piston back into the caliper as the front wheels turn. This means something is forcing the pads of their rest position, barely dragging on the rotor or disc. The most likely culprit is a warped rotor, but anything that cause the rotor to wobble is also suspect. If the rotor was ever removed, it's probably held in place my the wheel lugs at this point, so it could be a sign one or more lugs is loose, especially on a four bolt wheel where one missing lug can lead to wobble. It's also possible that the wheel and rotor are bolted up tight by the wheel bearing is worn or separating.
The fix for air in the brake lines is to bleed the brakes, normally starting as far as possible from the master cylinder (rear-passenger side on most vehicles), followed by driver-side rear, passenger front and finally driver side front. Some vehicles may allow you to bleed right at the master cylinder instead. Brake are bled with the help of an assistant who sits in the car and presses and holds the brake pedal when told. You crack the bleeder screw open while there's pressure on the brake pedal and close it again before the brake pedal is let up, otherwise air would be sucked right back into the system.
If you aren't stingy about brake fluid use and you keep topping off the master cylinder and replacing the lid, you can do a good job bleeding the brakes without any extra hoses or syringes. The technically correct way to do it is with a transparent tube fit over the end of the bleeder screw, so you can see the air bubbles coming out (and see when the stop). But even without the tube, you can usually tell what's going on by whether there's a clean stream of brake fluid as you assistant steps on the brakes, or if it gurgles and spurts.
Are the brakes making noises? If you have noises coming from the wheels, it may be the brakes or it may be something else. Squealing and screeching noises are usually the brakes, ticking , grinding and clunking noises may be the brakes, may be noises from the wheel bearings, CV joints or suspension. Brake noises do not imply bad braking performance, and in some cases, they are intentional warning noises caused by the brake pad manufacturers putting slits or a different material in the pads at a certain depth to worn of wear.
Are the brakes squealing? The high pitched squealing noise from brakes when you stop the car or slow down has two possible sources. The first is that disc brake pads are simply vibrating when the brake piston pushes them in, and this can be corrected with brake grease on the back of the pads (where the piston contacts) or anti-vibration clips. The other cause is broken brake pads or foreign substances stuck in the pad and scoring the rotors. Inspect the rotors for grooves.
Do you hear clunks whenever the brakes are applied? Clunks are generally easy to diagnose because the are generated when something heavy moves enough of a distance to go "clunk". In the case of brakes, the only part that's like to make a clunking noise is a caliper that's moving too freely on the wheel assembly, and the only reason for this (unless the steel is worn away) would be a loose pin. It's common to secure an automotive caliper with a single pin that's threaded at the end and which sits in a rubber sleeve. If the pin backs out so that the bottom of the caliper is just along for the ride, you'll get a nasty clunk as it rides up when you hit the brakes. Eventually, it will mess up either the caliper or the wheel assembly, so fix it immediately.
Do the brakes scrape or grind? You'll recognize the scraping sound if you hear it, grinding noises are more in the ear of the beholder. The scraping noise may be intentional, like the squeals mentioned earlier to warn you the pads are nearing the end of their design life. But scrapes and grinds are more often cause by a broken pad, a grooved rotor or drum, or a foreign piece of metal or rust jammed in between the braking surfaces. Grinding noises may also be from wheel bearings or CV joints, and if they are, they need to be replaced, not greased.
Do you get rattles from the brakes when driving on rough pavement? Rattling noises are almost always due to disc brake pads with missing spring clips (anti-noise clips) that are intended to keep them from rattling against the wheel assembly guides when the brake isn't applied. If the rattling noise goes away when you apply the brakes, however lightly, you know that it's the pads rattling. If you can't find the slips or figure out how to install them (don't laugh, it's tricky on some calipers), you may be able to use an after-market product, check your local parts store.
If you're getting ticks or short chirps from the brakes as you drive, it's likely a high spot on the rotors or drum, or warping. Heat can warp metal brake parts in the course of a few minutes, like applying the brakes steadily down a long hill on the highway. While it probably won't get any worse under normal driving conditions, it will wear out you pads faster, and the brakes may lock quicker or pulse when you jam them on.
Do the brakes pull the steering wheel? If you step on the brakes and the steering wheel pulls to one side or the other, that's the front brakes pulling it. The problem is either too much braking on the side that's pulling or too little breaking on the opposite side. Since too little breaking is easier to troubleshoot than too much, follow the instructions for bleeding that side, and check the brake parts for excessive wear compared to the pulling side. You should never replace the disc brake pads on one front wheel and not the other, they should always be done at the same time even if one side appears to have plenty of meat left. If bleeding doesn't do it, there may be a problem with the caliper piston getting cocked in the cylinder, and if it's near the end of it's throw, replacing the pads on both sides, or even replacing worn rotors, may force the piston back into a good section of cylinder where the extension is less and it may live happily for years. It's also possible that you have a crimped brake line, or a problem with the master cylinder.
It's a good time to note that torque steer is sometimes confused with pulling brakes, though it's really the opposite thing. Torque steer is cause when a front wheel drive differential powers sends power unevenly to the front wheels when you are going straight and accelerating. So if the steering wheel pulls when your foot isn't on the brakes and only when accelerating, you want to check all the issues that can cause torque steer, like unbalanced axles (could have been bent by impact with something under the car), stuff stuck to the axle, or any problem with the differential, like low transmission fluid, etc. And if the car pulls to one side ALL the time, whether or not you are accelerating or braking, it could well be that one of your front calipers is seized or partially seized. I've had this happen with break line failure when the line fails on the inside. When you hit the brakes, the car briefly pulls to the side that's operating properly since it gets the fill hydraulic pressure, but the stuck soon catches up as hydraulic fluid gets past the blockage. When you release the brakes, the steering pulls to the blocked side because the blockage maintains pressure until the fluid can leak past the blockage. Since there's no real pressure to push the fluid back from the caliper, other than imperfections in the rotor, the caliper can remain very tight, and your brakes will soon overheat and likely start smoking and smelling on that side.
Is braking action jerky or pulsing? If you have anti-lock brakes and you step on the brakes hard, a pulsing pedal may just be the brakes in anti-lock mode doing their job. If you have anti-lock brakes and you aren't stepping hard on th pedal, it could be a sign that the ABS computer or sensors have gone haywire. Otherwise, it's normally a sign of abnormal brake wearing, rotor or drum warp. One way to check if the back brake part are involved is to pull on the emergency brake very slowly and see if it results if you can feel pulsing in the handle, which would be due to a deformity in the rear brakes.
Is it hard braking and bring the car to a halt? If you have to exert a lot of pedal pressure to get the car to stop, that's hard braking. It's normally due to normal brake pad wear or glazing, but as with most braking issues, it could also be a problem with the hydraulics, leaking seals in a piston cylinder in a caliper or wheel cylinder. Can easily be a master cylinder or power boost problem as well, but if you aren't getting any warning lights, it makes sense to replace worn parts first.
If you are getting a warning light for brakes, make sure the emergency brake isn't set. If the emergency is released and you have a brake warning light with no apparent braking problems, it could be a sensor in an ABS system or an error condition reported by the power booster, but it could also be a failed switch on the emergency brake. Try to obtain the shop manuals or a decent after market manual for the car model so you can find the switch for the emergency brake light and see if it's failed closed.