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Mechanics stumped by brake problem

Hello, everyone!

We’re on our third mechanic with no solution to our problem. My 2007 Chevy Equinox (93,000 miles) has performed flawlessly since I bought it new. Over the last 2 years, the brake light has come on a couple of times. Each time the brakes were reviewed by different mechanics who found nothing wrong other than the brake fluid being 1/4 cup low. Two bottle or less of 3 dot fluid were used to top of the brake fluid over 2 years. Two weeks ago we purchased 2 new tires and a REAR brake job from SHOP ONE, a tire/brake dealer. The brake job was only because the car was nearing 100,000 miles and it seemed like a good idea. Immediately after the job, the brakes were extremely tight, barely giving to foot pressure. At freeway speeds one tire wobbled terribly, which SHOP ONE explained as the tire counterweight having fallen. SHOP ONE replaced the counterweight and pronounced the car fixed. Within 2 days, while on the freeway, the car started throbbing again, the brake stiffness was worse and the acceleration began to fail.

We towed the car to SHOP TWO, a very capable mechanic we’ve known for awhile. Computer codes show no problem with transmission or ABS or anything else. He said the problem was likely the master cylinder and booster, and took the following actions: Replaced FRONT brakes, master cylinder (OEM), booster (OEM), and rotors. The removed booster had more than a quart of liquid in it. He drove the car back and forth to his home to check performance with no problems.

When I drove the car back to my office the next day, the brakes seized up again, and I had to floor the accelerator to keep the car from dying. The car was towed back to SHOP TWO. The mechanic was able to replicate the problem but can’t figure out what’s causing it. There are and have been no signs of leaks. After seizing up. the car can be driven after 10 minutes of cool down.

The car is now at SHOP THREE, the dealer. Their computer diagnosis shows nothing wrong, but they recommend replacing the new master cylinder and booster with ANOTHER NEW master cylinder and rotor ($1400) but of course can’t guarantee that will fix problem.

SO, we need recommendations that don’t involve just replacing new parts willy-nilly to see what happens! Thanks!

The brake master cylinder should be disconnected from the vacuum booster and the booster ram shortened to allow free play so that the master cylinder piston can fully retract. Insuring free play is part of installing a master cylinder.

I think you had a master cylinder that was leaking into the brake booster for a while, hence all the brake fluid in the booster. I think at some point someone (at an oil change service perhaps?) saw the fluid was low and accidentally topped it off with power steering fluid instead of brake fluid. This eventually damaged all the seals in the system, causing them to swell up, keeping the brakes dragging after application. You’ve already replaced the master, and you say the front brakes but you don’t mention the calipers.

I think you need to replace the calipers, brake hoses, pads, and rotors again as they are surely damaged from the heat, very thoroughly flush the brake system and ABS using the scan tool operation to do so, and hope for the best. Hopefully the ABS hydraulic unit won’t need replacing.

Just a swag over the internet, I think you need one or both rear calipers replaced. The problem is not with the master cylinder or booster, never was.

Contaminated brake fluid has always resulted in the master cylinder cap seal swelling to the point that it would not fit in the cap, @asemaster. I wonder if @JMatcartalk ever noticed that problem. That would explain the premature failure of the master cylinder.