The low pedal suggests air in the hydraulic system or mushy rubber parts due to fluid contamination.
Some things to check when the piston won't go back into the caliper are a stuck piston, constricted hose, and contaminated fluid.
If a ring of rust or dirt around the piston will prevent it from sliding easily under the square-cut seal. The hint is the piston will move out when you press the brake pedal. A large c-clamp will usually press the piston back in, but that much pressure should never be necessary.
A constricted hose will make it necessary to push harder than normal on the pedal to apply that brake. The clue is the piston will move freely when you open the bleeder screw. If the front rubber brake hose has a metal bracket near the middle, look for rust buildup inside where it's crimped around the hose. Fluid will go to the caliper thanks to the very high pressure from the brake pedal, but the fluid won't go back so it keeps the brake applied, hence the smoking brake. You won't be able to force the piston back by hand either unless you open the bleeder screw. The remedy is to simply use a large pliers to open the crimp a little on the bracket.
Fluid contamination has similar symptoms as the constricted hose, but a big clue is found by looking at the rubber seal under the fluid reservoir cap. If it blows up and is mushy, the fluid is contaminated with petroleum product. Engine oil can enter by doing a simple oil change, then checking the fluids and using oily fingers to push the seal back into the brake fluid reservoir cap. The same thing used to happen by people repacking wheel bearings during a brake job, then handling the brake fluid cap or other hydraulic parts with greasy fingers. Power steering fluid and transmission fluid are other common contaminants. Brake fluid can even be contaminated by wiping out a funnel used for engine oil or transmission fluid, then using it for brake fluid.
The ONLY proper repair for contaminated fluid is to replace every rubber part and every part with rubber parts inside at the same time, and flushing and drying the steel lines. Any rubber part not replaced will leach the contaminant out which will transfer the contamination to any new parts.
Along with the mushy seal under the cap, the lip seals in the master cylinder will grow and expand past the return ports. Fluid will be trapped and keep the brake applied. As friction causes the linings to heat up, the heat transfers to the fluid which expands. Since the fluid is trapped, pressure builds which applies the brake even harder, and the vicious cycle continues. Opening the bleeder screw will release the brake and, unlike the constricted hose, it will also release the mate on that hydraulic circuit. On older rear wheel drive cars, the two front brakes were on the same circuit, but most front wheel drive vehicles use the split-diagonal system. The left front and right rear brakes are on the same hydraulic circuit. Rear drum brakes don't usually stay applied due to fluid contamination because the front disc brake is affected more and first, so the system is repaired before the rear brakes are affected.
Thursday, February 25th, 2010 AT 10:58 PM