Brake Caliper Sticky, pad wear

[Schurkey]

Found excessive pad wear on the Left Rear of my '03 Trailblazer LTZ. This seems to be due to the piston being stiff to move in the caliper bore--not completely seized, but doesn't move freely like it should.

Far as I know, the calipers are essentially the same for the whole production run.

The aluminum calipers are prone to corrosion. The corrosion behind the square-cut seal pushes the seal tightly against the piston, making for difficult piston movement. In addition, the piston itself tends to build-up contamination in the area of the square-cut seal, which adds to the friction.

Digging the corrosion out of the seal grooves, and polishing the piston returns the system to the low-friction easy movement it's supposed to have.

Ten photos showing the problem...and the solution.

Photo 1. Pinch RUBBER brake hose with a needle-nose Vice-Grip, just tightly enough to seal it so fluid doesn't leak out. DO NOT pinch aftermarket "steel braided" brake hoses. Caliper disconnected from brake hose. (SAVE THE COPPER WASHERS, and replace them in the same orientation as they were removed.) Calipers removed from vehicle. Piston blown out of the caliper using compressed air. Be sure to use a piece of wood or plastic (I used a giveaway "Paint stir-stick" to pad the caliper when removing the piston. It comes out of the bore with enough force to break your fingers if you tried to "catch" it. If the piston is truly seized, you might have to thread a grease zerk into the caliper, and then pump it out with the ~2000 psi of a grease gun. Note the "rings" of contamination build-up on the piston.

Photo 2. Caliper seals removed. The seals used on the Trailblazer calipers can be peeled out with your fingers, or hooked with a pick and pulled out. (DON'T puncture or damage them!)

Photo 3. Caliper bore corroded at seal grooves.

Photo 4. Caliper seal grooves cleaned with a pick, and then loose corrosion removed with a wire brush. I did not touch the bore below the square-cut seal groove.

Photo 5. "Crust" on square-cut seal scraped off with a fingernail. Also inspect and clean the dust seal. Inspect carefully--the seal must not be cut or cracked. Any defect whatsoever and the seal(s) must be replaced.

Photo 6. Chromed-steel caliper piston polished on a wire wheel. If the chrome had real defects, the piston would have to be replaced. This one had some staining, but no chrome flaking. Seals cleaned and inspected. Bleeder screw has anti-seize on the threads, but not on the tip. Ready for lubrication of the seals with brake fluid, and reassembly.

Photo 7. Use compressed air (carefully) to "inflate" the dust seal over the bottom of the piston. Lube the piston sides with a THIN coating of brake fluid. Not enough air, the seal won't inflate over the piston, too much air and you won't be able to push the piston into the dust seal. As soon as the dust seal covers the bottom of the piston, you're done with the air pressure.

Photo 8. Piston slides past the square-cut seal and to the bottom of the bore with finger pressure. FAR less friction/sticking than before cleaning the corrosion and deposits.

Photo 9. Inspect the hex-head caliper bracket pins for free movement. If the rubber seals are good, the pins will glide inside the bracket. If they're stiff or binding, they need to come apart for cleaning and lube with caliper grease.

Photo 10. The only part I had to buy for this operation aside from brake pads--the bleeder screw was almost seized in the caliper. I needed to "persuade" it with the oxy-acetylene torch to get it to unscrew. Threads in the caliper were still good, but threads on the old bleeder screw weren't trustworthy. It would not be unusual to have to replace the two copper washers, but in this case I didn't need to.

Torque specs for the various fasteners are listed in the Service Manual. On MY vehicles, I reduce the torque spec by about 20% because I lube the bolt threads (except for the banjo bolt on the brake hose) with anti-seize. Bleed the caliper after re-assembly. You'd want to bleed out all the old fluid from the master cylinder to the caliper, so the caliper has nice, fresh fluid in it. Good time to bleed (or remove and clean) all the other calipers, too. (Brake fluid flush) Let's face it--if one caliper is crusty and sticking, the rest probably are just as bad. You'll need to pump the pedal a few times to push the piston back out so the pads are in proper contact with the rotor. Verify fluid level. Inspect the caliper for leaks after standing on the brake pedal for 60 seconds.

[Schurkey]

So, having fixed a sticky caliper in the rear, I've moved to the front.

The front calipers have twin pistons. This makes them approximately twice as involved as the rears, at best. If one piston sticks...they're more than twice as difficult to rebuild.

Photo 1 and 2. "Treasure Yard" caliper I bought to practice on. I have no idea of it's history. This caliper floated freely on the bracket, but I pulled the pins and re-greased them anyway. Aside from superficial corrosion all over the outside of the caliper, which is ugly but unimportant, I had high hopes.

Photo 3. The problem with overhauling multi-piston calipers is when one piston is stuck. The other pops out, leaving a gaping hole that won't seal pressure. In this case, the stuck piston wouldn't move with 140 psi behind it.

Photo 4. With no way to seal the caliper, the only choice is to poke the other piston back in the bore, secure it with a C-clamp or block of wood, and get the high-pressure equipment. You might blow half-a-tube of grease, but ~2000 psi WILL push out that stuck piston. In this case, I finger-threaded a NPT grease zerk into the metric-threaded hole in the caliper. This resulted in zero thread damage, and it held enough pressure to eject the stuck piston. When this piston popped free, it sounded like a single kernel of popcorn exploding.

Photo 5. Minor deposits on the plastic pistons. Plastic (Phenolic?) pistons are used because they're cheap, and they don't transfer the heat from the pads back into the brake fluid. They do have a history of sticking in calipers. Ford and Chrysler both tried phenolic caliper pistons decades ago, and it didn't end well. The scuzz on my pistons wiped off nicely with a bit of brake cleaner. These pistons have a thin steel cap on the pad-end, so that the pad can't abrade the plastic.

Photo 6. (Waiting for a photo re-shoot.. Sorry.)

Photo 7. Cleaned-up caliper. Very little corrosion in the square-cut seal groove. Unlike the rear calipers, the dust seal on the front caliper isn't going to be removable without destroying it. I left them in place, but did clean them up inside and out.

Photo 8. As always, don't forget to clean out the bleeder screw, and assure that the threads are in usable condition. I apply anti-seize to the threads but not below the threads.

Photo 9. Ready for lube with brake fluid, and re-assembly. Getting the pistons back in the bore is complicated. One piston has to be LIGHTLY held in place over the dust boot with a C-clamp to seal the air leak, while the other gets the dust boot inflated with compressed air like normal. With one piston back in the bore, clamp it with the C-clamp while the other piston gets the dust boot inflated.

[ctesla]

great write-up and pics.. thanks..
and i see we have a similar weather pattern..
hope you enjoy your spring too.. we should see snow gone about the same time;
mid-june..

[Blue Bowtie]

Nice primer on aluminum caliper rebuilding. I think the only thing that detroys them faster than salt is powder coat.