i've had an ongoing problem with my 1987 560 SL with the resr brakes locking up ...... here's what i've done so far ...... put new calipers on both rear wheels, put in new brake lines on the rear, bled the system more times than i can count, and yet they still lock up ...... i've NEVER seen this in any car i've ever owned, and it's really beating me up now ........ any ideas about the cause and cure for this.....

What does "locking up" mean?

They stick and drag?

Or that they cause the rear wheels to lock up during braking.

means thier not releasing ...... bled the master cylinder again today ..... car stops perfectly, everything seems in order, but it takes quite a bit of power to get the beast underway ....... when they finally release, you can feel the difference big time ..... i keep thinking maybe there is air somewhere in the system ...... this car was a garage queen when i bougt it, so i kinda expected a few problems from the car sitting in an east coast garage, but this one sure has me scratching my head .... it's not like i'm a rookie at this chuck, i ran the d+d benz service in d c for about 12 years, but never encountered anything like this ..... our work was all diesels, but you know brakes are brakes, and i've done brake work on my customers SL's before ...... any idea's ???????

Just because the calipers are new doesn't mean that they are good. Maybe they've been sitting for a long time and have corroded? Did they retract without much effort when installed? Are the rotors new and the calipers aren't "hyperextending" because they are thin? Pads are new? Possibly dumb questions but I thought I'd ask.

seems as though the last time i bled the master cylinder i hit on something ...... took the beast out and could not get the rear to drag, seize, not release, or any of that good stuff ....... maybe i had air in the system ????.... all i know is i did everything i could to get the brakes to stick and they would'nt ....... would have never figured on the problem being at the master cylinder .... in any case, i got all new stuff on the rear of the beast ..... i was'nt really paying enough attention to the master cylinder i think, i was looking mostly at the rear ...... i was ready to put on a new master cylinder, but if this is the fix, i guess it will be a pretty cheap lesson ....... 2 cans of brake fluid is a whole lot cheaper than a master cylinder ....

bleeding the master cylinder seems to have solved this problem ....... knowing what i know about cars, i should have known better than to overlook this procedure ........ but its good for old dogs to re-learn new tricks sometimes .... we have a couple of top benz guys in here, would one of you guys tell me if the brakes not releasing properly could have been at the master cylinder all along ????

I've never seen this, but that doesn't mean that much. Usually, the master won't give you enough pressure if it's leaking inside. I really doubt that air in the lines or calipers would ever cause the rear brakes to lock up.
In most cases you would find a blocked or plugged brake hose or a stuck piston or maybe brake pads that are too tight - this is very common.
With all new parts his shouldn't happen but I found that the small parking brakes can grab and cause problems as well.
There's a relief valve in the master that's supposed to let all the brake pressure drop after you let off the brake pedal. This valve will keep a small amount of pressure in the system so that when you hit the brakes the pressure build up will be instant. Any failure of the internal parts in the master should be dealt with harshly. I won't screw around with anything that looks like it's not working right and I'd never try and rebuild a master.

It's always new parts when it comes to brakes.

i agree dan, i never mess around when it comes to brakes ........ the rat bastard stuck again last night, so i'm gonna put a new master cylinder on it......... if i keep putting new stuff on this beast, i might get lucky .... backed off the parking brake some time ago to take that outta the loop ........ i have never in all my years of messing with cars been beaten so badly by a set of brakes ..... you know my old lady's real happy with me .... she thinks i know how to fix these beast's .... if you got a trick up your sleeve, it would sure help my love life here.......

Yeah.........try VIAGRA.

Undo the bleed screws at the back when it locks up. If a lot of fluid shoots out it's a pressure problem. If very little comes out it's a mechanical problem.

thanks dan .........so if i put viagra in the brake fluid, the brake pads will get harder and release better ???????? hell of a fix, but i'll try anything ........

Welllllllll.......... maybe.

Problem is, you may not be able to stear the car ~) NO BULL EITHER!

Hi Davidw and Dan c,
So what was the outcome on this? Did a new master cylinder fix the issue? I have an '89 560SL and the rear brakes are dragging, I think. I'm just getting started here so doing some reading...seems like I should only use non-synthetic Dot 4 fluid (from previous posts), any recommended brand? (mercedes book says call mercedes, which I think translates into "only buy our brand")...

As a bit of background, I drive the car about 5 miles before I can feel what seems to be a badly out of balance tire in the back...shakes the whole car. If I continue to drive, it stops about 5 more miles down the road and when I get out and feel the hubs, I have two that are hot to the touch...one in front and one on the other side in back. Thought my emergency brake was the culprit when I inspected the hot rear brake assy, I could smell burnt brake pad, it was obviously the emergency brake, pad was cracked and dark color. Turns out the cables were rusted and seized. I removed everything from the emergency brake system, old cables (that was fun), and brake pads. Still had the bumping after 5 miles and hot hubs..so I'm embarking into the main brake system.

I have parts for rebuilding the calipers and replacing the rubber hoses, (figure if I lick it with the fluid change, I will still need to refurbish them later...maybe a rainy day. The only thing I don't see in the forums or even in the manual is how you bleed the master cylinder. Although it appears from this post it wasn't the issue for you.

So before getting in too far, figured I would ask you a few questions.
What did you used for brake fluid?
How do you bleed the master cyclinder?
and
Did a new master cylinder do the trick or was it a combination of all the above?