I guess I would remove the two front wheels, then watch what's happening when a helper pushes the brake pedal. Be sure the calipers' slides are free of rust and dirt and are coated with high-temperature brake grease. If one caliper doesn't apply, look if there are four steel lines coming out of the master cylinder. That's normally a front-wheel-drive car thing, but if you have that, you have a split-diagonal hydraulic system. There is a valve that trips and blocks fluid flow to one front wheel and the opposite rear one. That valve trips from pushing the brake pedal more than half way to the floor after installing new pads. Pushing the pedal that far must never be done on a GM vehicle or on any other brand that is more than about a year old. Besides that valve on GMs, crud and corrosion build up in the bottoms of the bores where the pistons don't normally travel. Pressing the brake pedal more than half way down runs the lip seals over that corrosion and can tear them. If both front brakes are applying, you don't have to worry about whether that system is used or if that valve tripped.
Also check the rubber bladder seal under the reservoir cap. If it is blown up and mushy, the brake fluid is contaminated with a petroleum product. A slight residue of engine oil or wheel bearing grease on your fingers when you reset that seal into the cap is enough to cause problems. Fixing that is real expensive, especially if the vehicle has anti-lock brakes.
If the low pedal only occurs while driving, and not while on the hoist, check for a sloppy wheel bearing. That will allow the rotor to wobble and push the piston back into the caliper. On the next brake application, the pedal will have to travel further to push the piston back out. If it stays there, the second pedal application will feel normal.
Use a dial indicator to measure rotor runout. If a warp was machined in accidentally on the brake lathe, you may not feel that while driving and braking but it could be enough to push a piston back into the caliper. You're looking for sideways runout, not thickness variation.
Monday, January 23rd, 2012 AT 5:40 PM