The root cause of premature front disc pad wear on some General Motors Corp. cars may be sticking rear calipers. According to brake service professionals, the problem also is likely to ruin the inboard rear disc pad. The condition normally occurs on 1988 and newer W Body automobiles, which include the Buick Regal, Chevrolet Lumina, Oldsmobile Cutlass and Pontiac Grand Prix.
Rear caliper slides on the W-Body cars have become notorious for corroding and causing the caliper to stick. When that happens, the outboard brake pad often won't even touch the outer surface of the rotor.
So it's common to find a severely worn inboard pad, an almost unused outboard pad and a rear rotor on which the outer braking surface is heavily rusted from non-use, said Doug Harmon, a brake specialist with Raybestos Products Co.
The car stops almost exclusively on the front brakes when sticking caliper sleeves hamper rear brake operation. So the first and most obvious clue to rear caliper trouble is severe and premature front disc pad wear.
Although this problem is easy to identify, it often eludes and frustrates technicians. The reason is that so many shops still haven't adopted or aren't enforcing a policy of inspecting the entire brake system during every brake job, Mr. Harmon said.
Caliper slide service
The W-Body rear caliper slides on a set of sleeves or bushings protected by small, bellows-shaped boots. Some technicians note the wear pattern of the rear pads, grasp the caliper with both hands and manually check its movement.
But Ned Lineback, an instructor at Wagner Brake Products, urged techs to visually inspect each sleeve-even though it may mean damaging the boot covering the sleeve.
If the sleeves look well-lubricated and slide freely in the caliper housing, just replace the sleeve boots and proceed with the inspection or brake job.
Any evidence of corrosion or sticking justifies removing the sleeves and cleaning them and the sleeve bore inside the caliper housing. Thoroughly clean all rust from the outside diameter of each sleeve, the inside diameter of the caliper housing sleeve bore and the groove into which the sleeve boot fits.
Some brake specialists mentioned the W-Body caliper service tool kit (No. J-39495), from Kent-Moore Division of SPX Corp., which contains a hone for cleaning out rusted caliper housing sleeve boots. The kit includes a brush designed to clean sleeve boot grooves inside the sleeve bores.
It also provides a sleeve driver for an air hammer or air chisel in order to apply extra persuasion to badly rusted sleeves.
But Jeff Smith, service manager at Smitty's Alignment Service, Doylestown, Ohio, said many W-Body caliper sleeves are so corroded repeated hammering with the Kent-Moore driver and an air chisel is needed to loosen them.
Some techs automatically upgrade the W-Body's OE steel caliper sleeves to stainless steel sleeves offered in aftermarket repair kits. Obviously, stainless steel parts don't corrode and the kits create a new caliper sliding surface via a sleeve-within the-sleeve construction.
Other brake specialists said reusing the OE caliper sleeves presented no problems provided they were cleaned thoroughly and lubricated with pure silicone grease.
According to Mr. Harmon, GM-as well as most brake parts firms-recommend using silicone lubricant on caliper slides that contact rubber. Techs shouldn't use petroleum-based lubricants on rubber brake parts because these tend to make rubber parts swell. The swelling, in turn, leads to sticking calipers later on, he said.
According to Mr. Harmon, GM and major aftermarket parts suppliers offer pure silicone lubricants. Whenever a tech can't find a pure silicone lubricant from his regular suppliers, he suggested sourcing Dow-Corning Molykote 111 silicone lubricant from an industrial supplier or a bearing supplier.
Driveability and electrical specialists also use Molykote 111 or equivalent lubricants to prevent spark plug boots from seizing on the spark plug porcelain and to prevent corrosion inside battery terminals.
Mr. Smith warned technicians to be wary of post-brake job caliper seal leaks on rear W-Body calipers. He explained that after retracting the caliper piston in the normal manner and taking routine service precautions, the caliper seal began leaking soon after the job was done.
Furthermore, the leaks have not caused the customary low, spongy brake pedal symptom. Rather, the customers have called complaining about a brake fluid odor in the rear of the car. Subsequent inspections showed brake fluid leaking out onto the rear brake pads.
As a result, Mr. Smith now recommends remanufactured rear calipers to W-Body car owners.