Help please- brake problem
 

I have a 1987 420 sel which had been producing a humming noise while driving after warming up. I thought it sounded like a tire going bad at first so I checked them all and found them good. Now the car makes the noise much worse at most any speed- after warming up. It will temporarily stop when applying the brakes. I have raised the wheels off the ground and rotated them and checked for any sign of abrasion or wheel bearing play and found none. The last time I drove the car I noticed a slight burning odor coming from the outside and, in looking around the car touched the rear wheels which were BOTH hot enough to fry an egg after about 10 miles of driving. At this point I also noticed (while babying it home) that the brake pedal had become somewhat "spongy". After cooling off, the noise stops and the pedal returns to normal. What's going on??? :confused:


BTW- The car steers and brakes normally. It does not pull to one side, and it does not feel like anything is dragging while driving at any point.

You've probably got either or both of two problems: Sticking rear calipers that aren't fully releasing, or a sticky parking brake that's not fully releasing. Since the front brakes are apparently ok, you'll not notice a problem with steering. As you found out, the heat can be major; you'll need to check calipers, pads and almost certainly rotors, which may well be warped. The master cylinder and hydraulics are usually split for safety purposes, and I don't know how the split is set up on your car--it might be one front and the opposite rear per circuit, or front on one circuit and rear on the other. If you've got one circuit operating both rear brakes, that's another point at which there could be a problem.
On my 560SL, it was the parking brake linkage; hadn't been used by the PO in heaven knows how long, and was a corroded mess that didn't release fully after I used it. However, there was a godawful squeal underway that told me help was needed pronto. The shop wound up replacing most of the linkage, as the parts were corroded beyond usability; the service brakes were fine and needed no attention. No problems since.

I think you're exactly right. Thanks. Now that I think about it, the last few times I used the parking brake it didn't release fully. The brake light stayed on until I picked the pedal up with my foot while holding the release. I didn't think much of it because I rarely drive the car or use the parking brake. Where should I begin checking and or lubricating the brake?

john420,

I think you should have the e-brakes checked and the grease in the wheel bearings changed. I had a buddy with a 1968 230 that had the e-brake light in the instrument panel burned out. He drove the car about 90 miles with the brake on and then the left rear wheel siezed. At highway speed this was pretty exciting. It broke loose before he went out of control, and he kept driving. By the time he got home the left rear wheel bearing was spinning as a single chunk inside the bore in the semi-trailing arm. The whole mess was ruined and he needed a new semi-trailing arm. The high heat will destroy the bearing lubrication and that will ruin the bearing. Good luck, and I hope this helps. Jim