Rear brakes sticking: 1984 300D - PeachParts Mercedes-Benz Forum

littlerobot · #1 05-22-2010, 07:55 PM

While driving on the interstate yesterday I noticed I couldn't go over 40 mph. After a few minutes the car overheated and I pulled over. Both back wheels were spewing smoke that smelled of fried brake pad. Called Triple A and had it towed home.

Today I popped off the wheels, replaced both calipers and brake hoses, and put in fresh brake pads. The whole process was semi-long, but not that bad. Feeling like a handy kind of guy, I took the car around the block to test it. After 15 minutes of driving I pulled into the driveway and got out, only to smell that good old hot brake pad smell and smoke from the same wheel. The pistons are deploying, but not releasing. After all this work I'm thinking master cylinder. I'd cleaned any residual brake pad crud off the Disc, so I know it isn't residual smoke from the old pads. This smoke is from the new pads.

Any advice? Am I on the right path, and is changing the master cylinder a P.I.T.A?

vstech · #2 05-22-2010, 08:02 PM

it could just be the rotors are fluid soaked, also, the wheels have residue on them, and the pads could be outgassing, some new pads do this.
also, did you lube the sides of the pads? you are supposed to put a ULTRA THIN layer of caliper grease on the edge of the pads where they slide in the caliper.

layback40 · #3 05-22-2010, 08:15 PM

when you finished your test run were the brakes binding? Could you rotate the wheels?
It could be that they are just bedding in.
Take it for a run without using the brakes much & see how hot they get.

littlerobot · #4 05-22-2010, 08:23 PM

I'm pretty sure the brakes were binding after my test run. More on the rear driver's side than the rear passenger-side. Does anyone know if the process for releasing the pressure on the pistons that push the pads against the rotor is an electrical process or an mechanical? Could this simply be a blown fuse that controls some electrical system on the master cylinder, or could this be some electrical failure at the cylinder, or should I be thinking more mechanical?

littlerobot · #5 05-22-2010, 08:24 PM

Also, I felt the rims after my test run and the back rims were super-hot compared to the front rims. Unnaturally hot. Pretty sure the pistons are sticking.

layback40 · #6 05-22-2010, 08:39 PM

Nothing electrical. Just the retraction of the master cyl piston. Can you move/release the calipers with a screw driver against the pad backing plates?
If they are new or re-manufactured calipers they should be ok. Is there pressure in the brake line after you stopped?
Dont use junk yard brake components! Its not worth the risk!!

littlerobot · #7 05-22-2010, 09:30 PM

Thanks Layback,

Unfortunately, they are junkyard calipers, although they looked very new. But I'm struck by the problem just being in the back, not the front brakes. If the master cylinder forces the pistons to compress the pads, then wouldn't all the brakes stick equally? There are some electrical connections on the master cylinder, which is where you put the brake fluid in, right? Could one connection be for the front and the other for the back???

I'm sure I can move the pistons back with a screwdriver or something, as I had to when I put the new pads in.

Still confused.....

vstech · #8 05-22-2010, 09:35 PM

ahh, junkyard parts, got it.
did you put new flex hoses on at least?
the wires on the MC are for low fluid warning sensors, nothing to do with retraction.
clean everything really well, get all gunk, rust, etc off the sides of the calipers, and have someone push on the brake, then release, and see if you can spin the wheels by hand.
if not, reach around, and release the bleeder, see if the wheel spins then.
if so, you have bad hoses.

littlerobot · #9 05-22-2010, 09:41 PM

Also, the thought occurs that they all may be sticking, it's just that the back pads are sticking more? Although there is no smoke from the front, maybe that is temporary.

What are the symptoms of a bad master cylinder, too much brake or too little?

vstech · #10 05-22-2010, 09:45 PM

brake master cylinder problems usually present themselves as no pressure, pedal falls to floor.

layback40 · #11 05-22-2010, 09:46 PM

X2 with vstech, try what he suggests. The junk yard calipers are probably seized.

littlerobot · #12 05-22-2010, 10:41 PM

Junkyard calipers may indeed be seized, but why would the same one on the same side fail twice in a row. I'm willing to change the calipers to new ones, but what is the next step if the brand-new calipers present the same problems?

littlerobot · #13 05-22-2010, 10:42 PM

I did replace the hoses, with brand new ones when I put on the junkyard calipers.

littlerobot · #14 05-22-2010, 10:45 PM

Oh yeah, how do I release the bleeder? Is that where the rubber brake hose connects with the metal brake line? Takes two wrenches to loosen: a 14 mm for the rubber hose and a 10 or 11mm for the metal brake line?

Thanks vstech.

If so, I'd imagine brake fluid would squirt out there as I manually push the calipers back?

Also, both back brakes seized at the same time, heating up and eventually burning the pads off. Is it possible for both calipers to seize at once?

vstech · #15 05-22-2010, 10:54 PM

bleeder is the fitting that you bleed air out of the calipers.
it's certainly possible for both, if not all 4 calipers to seize! water in the brake fluid causes rust pretty evenly.
that's why the flush of fluid is so important every 2 years.