Overtorquing lug nuts warps discs

Dear Mike: When I brake, the pedal bounces up and down. The bouncing slows and then stops as the car comes to a complete halt, so I think it’s safe to say that it’s a bad brake disc.

I’ve jacked up the car and inspected the brakes on all four corners, and they all look fine. I just had these discs machined last spring. Without simply replacing them, or having all of them machined, how can I tell which one is the culprit?

A: It takes only a few thousandths of an inch out of true to make a brake pedal feel like the seesaw at recess. You can use calipers or a micrometer to measure thickness variations. Unfortunately these variations are usually the result of a disc that’s either warped or is mounted so that it wobbles.

Overtorquing the lug nuts with an air wrench is probably the biggest cause of warped discs. Removing a disc and then reinstalling it with corrosion or foreign matter between the hub and the disc is another.

To check for disc wobble, you’ll need a dial indicator. Set the indicator’s probe against the disc surface, zero it out and spin the disc. Any more than one or two thousandths of an inch of runout is unacceptable. Check both the inner and outer swept surfaces of the disc, and check radially at a couple of points.

Brake discs should always be machined or replaced as pairs, not individually. Use a wire brush to clean the surfaces of the hub and the new or machined disc where they mate, then smear on a thin coating of anti-seize compound. You will probably need to use a couple of lug nuts to hold the disc against the hub while you check for runout again.

Still too much runout? Spin the disc on the hub 90 degrees and see if it improves. This may reduce any runout caused by combined hub and disc runout, which will cancel one another out. Keep this up until you get less than one-thousandth of an inch of runout. Then use a proper torque wrench to tighten the lug nuts.

Some car manufacturers are so fussy about runout that they specify that the discs be machined while mounted on the hubs.

Dear Mike: I have a 1995 Chevy C/K 1500 pickup with a disc-brake problem. Two years ago the left front disc brake began to stick after I applied the brakes. It’s not a firm lockup, but it heats up the wheel and causes a noticeable drag. You can smell the pads burning or rubbing. It eventually releases after the wheel cools down, when it will freely rotate until the next brake application and subsequent sticking.

I thought that the caliper-piston assembly was probably pitted, due to the salt air where I live and because of infrequent use, so I replaced the entire caliper assembly with a new one, replaced the inner and outer bearings, got new brake pads, greased the caliper sliders and bled the system. The wheel spindle looked OK, not scored.

Amazingly, after I’d replaced all those parts, the brake still sticks! Could my master-cylinder assembly have a problem, at least where that one brake line is concerned? The other three wheels are fine and do not have the same problem.

A: I assume that the new caliper came with all-new hardware, that the sliding pins, return springs and other assorted hardware are working as designed and that the piston is at least trying to push back a few thousandths of an inch, the way it’s supposed to. That small clearance is to let the disc spin freely. Any small friction in the sliding parts of the caliper, or in the inner pad’s path, can cause drag.

Try to push back the piston with a screwdriver. If it’s hard to move, open the bleeder bolt and try again. If it moves easier, your problem is pressure in the line holding the pads down. If not, something is dragging mechanically in the caliper/pin/pad assembly.

That leaves the rest of the hydraulic system. The proportioning valve, the master cylinder or even a pinched brake hose could be retaining pressure in the line, and you’ll have to chase down which it is. I’d check the rubber line to that caliper, because, if it were anything else, it should affect both front calipers, not only a single one.

If it’s not the hose, check the brake pedal to see if it’s sticking or if the pushrod from the pedal to the master cylinder is misadjusted.

Every once in awhile, I get a bad caliper right out of the box.