Hot Brakes
 

Friday of last week, i was on my way home from work in my 190e 2.3, when i started to notice that the car was accelerating hard. I pulled over and checked and let it set for a minute (because it has happened before and that works)

Then i started again, and it was still driving hard, since i wasn't far from home, i went on home.

when i got there i took a glance at my brake and i could see my rotor (at 11PM) they were a bright red, and need less to say the wheels were hot to the touch.

my dad looked at it the next morning and the brakes weren't locked up, i could push the car, also, i looked at the rear brakes and noticed that the rear rotors were rusted as if they hadn't been working.

i have since replaced the master cylinder the rotors (but i didn't put new sensors on, since they were broke off) on the front and the front brake pads, the tires seemed to turn freely, rubbing a little but that is normal with disc brake, right?

I took the car out for a little drive pulling over and testing it to see is the brakes were getting hot, after about four miles, i pulled over again and the wheels were starting to get slightly warm expecially around the plastic center cap.

so i headed on home, and took the right front tire off, and it was warm, but not glowing warm. The brakes were also making a smell.

i was wondering if:
1. the sensor wires are affecting this
2. is the new pads and rotors are getting warm as part of the breaking in process.
3. how hot are the rotors supposed to get.

jaybird366,

How long have you owned the car, and when was the last time the brake lines were flushed?

Based on your description of the rear brakes I would assume the brake fluid has been allowed to age to the point where it has absorbed quite a bit of water. This leads to corrosion that had likely led to the ruin of the calipers.

The rear brakes sound like they are frozen, and the front brakes sound like the caliper to piston seals are shot. The lack of evidence that the rear brake pads are contacting the discs suggests the pistons are frozen in the calipers and do not move, while the failure of the front pads to withdraw slightly when you take your foot off the brake is a sure sign the seals are shot. In either case the best way to solve the problem definitively is to replace the damaged caliper. This is especially recommended for the front ones. Brakes tend to be pretty rugged, and I have unlocked frozen pistons with brute force before, with success. But I am not sure I would do it again.

I do not recommend you buy rebuilt calipers as the caliper rebuilding process requires a certain precision that is not always met (honing to clean up the bore of the caliper makes the inside diameter larger while any cleaning up of the piston makes it smaller and those two effects combine to ruin the seal preload), and the same problem can arise again (the "sticking" brake).

Good luck, Jim

From a personal experience I had some years ago, I would doubt your brakes have soaked enough water to give you this kind of problem, I think they would have failed first with the water boiling off and your foot pedal going to the floor with a smell of burning to follow. Brake rotors should never glow red hot, unless you are poling a Formula 1 race car around a circuit ! I guess here you have a case of a piston sticking causing a permanently dragging caliper and pad. Alternatively, have a look at the calipers with the wheel removed and see if they are free to slide in their mounts inboard to outboard, if the design is of a single piston type, they can often seize . . Good luck

I had a similar experience several years back with the back brakes on my 240D. They were glowing red hot. I cracked open the line for the rear brakes at the base of the master cylinder, it released pressure and I drove it a zillion miles 'til brake time without ever having any further problem like this.

The problem was either the master cylinder or possibly the proportioning valve. You replaced the master cylinder and I think that cured the problem.

The warmth of the rotor could be just new pads that are wearing in. I would ensure that the entire system is flushed with fresh fluid and see what happens. You may very well have it fixed now.

Best of luck,

I had a similar problem with the front brakes on my 190E. I thought it was the pistons and after rebuilding them it did not go away. It ended being the master cylinder not releiving pressure in the lines.

Does the brake pedal seem unusually firm at times? Does cracking open the brake line at the master cylinder help (at least temporarily) as Larry suggests? If so, then I believe the problem oints to the master cylinder. I think you can get a new one for a little more than $100.

After i replaced the master cylinder the back brake worked, i think i'm going to try to flush the lines.

If you replaced the master cylinder, then bleeding the brakes properly would have flushed the lines. Did you bleed them properly, however?