High-temperature brake grease should be lightly coated on the hub the rotor sits on so it can slide freely when you torque the lug nuts. Rust and debris can flake off and get stuck between the rotor and hub. That can let it wobble not enough to feel but enough to continually push the piston back into the caliper as the wheel rotates. That means you have to push the brake pedal further than normal before the pads contact the rotor.
Machined rotors make better contact with the new linings but not nearly as much contact as old linings that have worn to match worn rotors with large grooves. Machined rotors have very small even grooves, like a record, and the pads only contact a very tiny percentage of them until those high spots wear down. That's where the test-drive comes in. Mechanics will get up to about 35 - 45 mph, then perform a rather hard stop. That starts those high spots wearing down. They'll do three or four more like that with a couple of minutes in between for the linings to cool down.
In the worst case, which happened to me once, the customer gets antsy and doesn't have time to wait so we give them their car without doing that test-drive. Most of the time there's no problem, and we just tell them to take it easy for the next 100 miles. It's when they immediately do only city driving that the brakes overheat. The lack of friction between the rotors and new linings means you have to push the brake pedal harder than normal to slow down, and that heats them up faster, leading to one type of brake fade. The heat causes the coefficient of friction to decrease so you have to push even harder on the pedal, and that just aggravates the overheating. Eventually there is almost no stopping power although the brake pedal will still feel normal. The cure for that brake fade is to just let them cool down for an hour, then drive the vehicle like normal.
One of the biggest causes of a mushy brake pedal is simply due to mis-perception. This used to be real common on older GM vehicles. While standing still, it is real easy to push the brake pedal too far and too easily, but what you have pushed would equate to the wheels locked up and tires skidding on the highway. We don't feel the expected pedal resistance. You can observe this easily by coming to a normal stop up to a stop sign, then you can push the pedal a lot further, maybe even all the way to the floor. Again, that's mainly an older GM thing.
One of the biggest causes of a low pedal is pushing the brake pedal more than half way to the floor. GM front-wheel-drive vehicles have their own built-in problem, but the master cylinder can be damaged on any brand of car. Crud and corrosion build up in the lower halves of the bores where the lip seals don't normally travel. Pushing the pedal over half way runs those seals over that debris and that can rip them. That often happens when a rubber flex hose pops a leak and the unsuspecting driver pushes the pedal to the floor, and it happens to do-it-yourselfers when they pedal-bleed the hydraulics with a helper, or when they stroke the pedal to run the retracted pistons out of the calipers to get the new pads to contact the rotors. Almost all front-wheel-drive vehicles use a "split-diagonal" hydraulic system with one front brake and the opposite rear brake on the same circuit. Typically when one circuit isn't working on a GM product the pedal will feel perfectly normal but you'll see a small twitch in the steering wheel when you press the brake pedal. Chrysler has the suspension geometry perfected to offset a brake pull so perfectly that you won't see any steering wheel movement, but you'll have other clues in the form of a low pedal and the red warning light will turn on.
If you haven't driven the van yet, I would do that carefully before looking for a problem that may not exist. If the red warning light doesn't turn on, both hydraulic circuits have equal pressure and you should observe the pedal becoming higher and more firm. If the warning light does turn on, the first suspect would be the master cylinder.
Thursday, December 20th, 2012 AT 7:33 PM