Rear brakes sticking: 1984 300D - Page 2 - PeachParts Mercedes-Benz Forum

littlerobot · #16 05-22-2010, 11:15 PM

Does it require a wrench to operate the bleeder? Is the bleeder on the caliper?

vstech · #17 05-22-2010, 11:19 PM

wait, how did you install the calipers and not know where the bleeders are?
you have to bleed the air out of each caliper when the lines are opened... did you put the calipers on, then bleed out the new rubber lines you bought? no.
you are supposed to mount the calipers, install the rubber lines to them, then attach the lines to the hard line mount on the frame of the vehicle, then open the bleeders on the top of each caliper. then have someone press slowly about 3/4 pedal travel and hold, while you tighten the bleeders.
should be either a 8mm or a 10mm box end wrench. and it's best to put a short chunk of clear hose on the nipples of the bleeders to control where the fluid goes.

littlerobot · #18 05-23-2010, 12:14 AM

The calipers are only held on by two bolts, and I'm pretty new at it and only had the Haynes manual to help me (not much help). But I succeeded in installing them, but have failed to bleed them properly, as it was my first time. Tomorrow entails a successful bleed to make sure all the air is out of the calipers.

If I loosen the bleed valve, that will release the calipers grip on the rotor?

You've been a great help Vstech. I appreciate it. What is the next step if the calipers are replaced with new ones, I bleed the system properly, and if they still stick, then what?

vstech · #19 05-23-2010, 12:19 AM

ok, since you didn't know what to do, I'll ask another question...
once you find the bleeders, are they on the top or the bottom of the calipers on your car now.
they must be on top.
if they are on the bottom, you have the calipers on the wrong wheels, swap them.

layback40 · #20 05-23-2010, 12:20 AM

It will release any pressure that may be in the caliper. If they are seized it will not do much apart from letting some good fluid in as any contaminated flows out. You should remove the calipers & have some one pump the brakes to move the pistons a little & then you can push them back in. Doing this a few times may loosen them up.
Maybe best if you do a search on bleeding brakes before you do any more.

t walgamuth · #21 05-23-2010, 06:22 AM

If you have changed the calipers and hoses, I think your problem lies in the master cyl and or booster. We have seen this problem here before. Did you search for it? I cannot remember the solution though I suspect it might be in the booser, though for the life of me I cannot imagine how it could apply rear brakes without the front.

The fronts are working, right?

vstech · #22 05-23-2010, 10:54 AM

all symptoms are void, he has not bled the lines yet.

t walgamuth · #23 05-23-2010, 12:12 PM

I don't see how air in the lines can cause the calipers to sieze.

vstech · #24 05-23-2010, 12:16 PM

sure, the caliper piston retraction is based on a solid column of fluid. air will not allow correct retraction!
that aside, water or other trash in the calipers make issues hard to follow.

t walgamuth · #25 05-23-2010, 12:54 PM

Calipers don't retract.

Chas H · #26 05-23-2010, 01:01 PM

Quote:

Originally Posted by vstech

sure, the caliper piston retraction is based on a solid column of fluid. air will not allow correct retraction!
that aside, water or other trash in the calipers make issues hard to follow.

The piston seal causes the piston to retract slightly when brake pressure is released. As the piston retracts it pushes the brake fluid or air behind it back towards the master cylinder. The presence of air in the system will not prevent the piston from retracting. There are a number of causes that WILL prevent to piston from retracting, but air isn't one of them.

littlerobot · #27 05-23-2010, 05:54 PM

After bleeding the system properly and taking a test drive, things seem alright for now. I felt the rims after the drive and they weren't hot. HOWEVER, my gut feeling is that this "sticking" of the back brakes could be intermittent, and return at any time. I'll keep you posted on the outcome. If this week goes by smoothly, then I'll consider myself lucky, if not, it's time to move on to either the master cylinder or vac booster.

littlerobot · #28 05-23-2010, 08:41 PM

The problem is back. While on a long drive later today I noticed the brake pedal getting tighter and tighter. At first I pushed it halway down to stop. By a few miles later, I only had to barely push down on the pedal. I looked in my rear view to see smoke pouring out of both rear wheels. This issue is reflected in the cars running temp. It usually runs at 80 c, but as the brakes lock down, it rises to 90, then 100. From the cars point of view, I'm riding the brake and gas pedal at the same time.

1. I am now SURE this isn't a caliper, brake pad, or air in the brake lines problem. This is a bigger problem not related to anything in the rear end of the car at all.

2. If the car sits for awhile, the pressure gradually subsides. This means that the liquid pressure that pushes the pistons is allowed to go one direction, but the pressure of that fluid is not releasing properly. I'm letting off the brake pedal, but the calipers are still pressing the pads against the rotor.

Any suggestions? Should I disable the brake booster to see what happens, or go ahead and assume the master cylinder is a dead man?

Chas H · #29 05-23-2010, 08:57 PM

Quote:

Originally Posted by littlerobot

Any suggestions? Should I disable the brake booster to see what happens, or go ahead and assume the master cylinder is a dead man?

You could disable the booster, but the brakes are gonna need a whole lot more pedal pressure to stop. Be careful.

littlerobot · #30 05-23-2010, 09:02 PM

Right, I'll either have too much brake pressure or too little, although if I can rule out the booster, then all that's left that could be wrong is the good old master cylinder. From research on other threads, the rebuilt MC's are crap. How difficult is a MC replacement?