Before we get into the specifics of brakes and their problems, you may want to look at some of the basic rules for troubleshooting in general, as written in the beginning of every Haynes manual: Troubleshooting 101
Much like our earlier examination of clutch issues, there are two general failure modes for brakes: They aren't engaging well enough, or they aren't disengaging. Then there are minor issues such as brake squealing, brake shudder/pulsing, burning smells, leaking fluids, pulling to one side, etc.
Brakes not working well
Even today, your brakes can fail completely, but there are failsafes built into the system to prevent that from happening in most cases. Every car since 1968 has had redundant hydraulic systems, not to mention the parking/emergency brake, to prevent complete brake failure. The master cylinder actually contains two pistons and seals, which in simpler systems controlled the front and back brakes separately, or the diagonally opposite wheels, so at worse you would only lose half your brakes.
Soft Pedal- The brake pedal is easy to push but there is less braking.
- Low brake fluid
- Air in brake fluid
- Bad master cylinder
- Bad caliper/wheel cylinder
- Blown brake line
- Bad residual valve
- Fluid leak in system
- Boiled brake fluid
- Bad wheel bearing
The first thing to check with any brake issue is the fluid level and condition in the master cylinder reservoir. If there is a leak somewhere the fluid level may be so low as to make even the redundantsystem ineffective. If there is fluid, but it is below the full indication, that may indicate that the friction material on the pads or shoes,or the rotors or drums themselves, may be worn beyond effectiveness.
If there is plenty of fluid in evidence, check if it is hot; repeated hard use, towing, descending a steep hill, or a dragging calipercan boil the fluid. DOT 3 brake fluid should not boil until it gets above 400 degrees Fahrenheit, but old brake fluid that has absorbed water can boil at as little as 284 degrees, which is why it needs to be changed every other year.
Brake hydraulics are a sealed system, so there should not be air in the lines, but if after service you have a sponge pedal that is the most likely cause. A bad master cylinder can introduce air into the lines, as can a failure at a caliper or wheel cylinder. The residual valve (often in the master cylinder or proportioning valve) keeps a small amount of pressure in the brake lines so the pedal doesn't have to travel so far until initial engagement; a failure here can be difficult to pinpoint.
Another uncommon cause of a soft pedal and long travel, is a bad wheel bearing, which is unrelated to the brakes except for being in the vicinity of the rotors. The failing bearing allows the hub to move around, effectively pushing the brake pads back into the caliper as far as it can.
Hard Pedal- It takes much more pressure to get the same amount of pedal travel and braking force.
- Bad power booster
- Vacuum hose leak
- Bad check valve
- Collapsed vacuum hose
- Seized caliper
- Extremely worn brake pads
Most instances of a hard pedal come down to a failure in the power boost system, whether vacuum based or hydro-boost (used mostly on diesels, we won't be covering it here). The power booster uses engine vacuum to assist your leg when applying the brakes, and is nearly universal in modern cars. If it the booster, check valve, or vacuum hose is bad, it will likely also cause a vacuum leak and rough engine idle as well, but an engine with a bad valve or head gasket can also cause low vacuum which can effect the brake boost.
Another cause of a hard brake pedal can be a brake caliper that is stuck or seized. Typically a seized caliper or pads will cause a burning smell, a pull to one side, and dragging at one wheel, but it is possible for it to stick without those, and effectively lessening your braking by nearly half (if it is a front wheel, which provide 75% of the braking). Wearing your pads down to the metal baking plates will also result in a very "wooden" brake pedal as well, as there is not as much friction when it is metal on metal, and they have to clamp harder to slow you down; the horrible grinding noise is usually an indication before the hard pedal.
