W123 Dragging brake question - PeachParts Mercedes-Benz Forum

Whiskeydan · #1 11-03-2005, 11:17 PM

Having problems with the brakes dragging on the '80 300D thats been sitting for a yr or so.
I replaced all the hoses, found one rear caliper stuck so, went ahead and replaced both rears with good used ones.
Front calipers are now dragging. I released the line at the master cylinder and it released the dragging fronts.
So, I think MC is shot, not returning all the way. I replace it with a localy available rebuilt. Same problem! Fronts still drag, release when I open the line at the new mc.
Whats next? I'm suspect of the rebuilt mc.
Thoughts?

t walgamuth · #2 11-04-2005, 08:19 AM

thing to do is replace your front brake lines the rubber ones. sometimes thay will act as one way valves after the insides begin to deteriorate.

tom w

Whiskeydan · #3 11-04-2005, 09:50 AM

Quote:

Originally Posted by t walgamuth

thing to do is replace your front brake lines the rubber ones. sometimes thay will act as one way valves after the insides begin to deteriorate.

tom w

That was my first replacement.

I did find one of the rears swollen so, I replaced all four. When I started bleeding I noticed the fronts dragging. So I cracked the line loose at the MC. They then released.
That indicated, to me, the pressure is held at the MC. I replaced the MC and have the same thing.

My fear is the price of brake fluid will rise soon.

I will say this... its got to be clean after all the fluid I've passed thru there.
I'm thinking a bad, new mc.

Ralph69220d · #4 11-04-2005, 10:39 AM

I'm thinkin though the front caliper fluid is clean with mutiple bleeds, there well might be corrosion broke free that is interfering with caliper's cylinder seal; that is stuck between piston and body. Perhaps with the MC line closed, force the pad(s) open to their maximal travel, release the brake pedal, apply bleed pressure, then repeat a couple of time's. On a probability basis it's of lower probability that your new MC is faulty. Since the rear caliper's were shot and new caliper and line's fixed it, that the fron't have a high probability of suffering the same fate due to sitting. I have a 70 dodge d300 one-ton with 54k original miles. It sat for ten years, five years, a few year's at a time, only being started and idled a couple of time's a year. The one rear cylinder that was totally shot i rebuilt, but the pitting was too bad to hone sufficiently, but it hold's pressure til I get back to fix it (I don't drive it as is). I do know that brake bleeding only get's that combination of gunk/corrosion that is easily soluble within the piston cylinder. The D300 dodge used a brake pressure equalization system whereby the rear left wheel was pressure equalized with the front right wheel (and right rear with left front were seperately pressure equalized). Anyway, rebuilding the rear left wheel cylinder, bad as the cyliner was, it was function, perfectly correlated in time with locking problem in the right front cyliner. My guess is that in the bleeding process performed before I drove it and was still ignorant of there being problem caused the righ front piston to move to relaxed brake positions it hadn't seen in a long time, thus scrapin up corrosion, deteriorated rubber cup seal so that the rubber cup could not do adequate compensation to valley's caused by pitting and hills caused by debris, eventually sticking that right front piston in a dragging position. Unless someone more knowledgeable than myself can figure it out better (like tom w), I'd replace caliper's and pads on the front also. Second thing, are you using DOT3, DOT4, DOT4/5 brake fluid. A fairly sharp mechanic once told me if a vehicle is designed to run DOT3, to never go up in rating as the rubber cups (in my old shoe system brake system) are not designed to run with these newer fluids and can rile up the corrosion. I can't see your post as I'm writing this, but if this is a vehicle in the modern ABS area, those pistons really get banged. Sorry for the top of my head replie's here, but it also occurs that you may have developed some adjacent "land's" versus "groove's" right near the maxiimum extension to shoe caliper contact (meaning that you were developing a drag problem before the car was stored for a year). I'd like to hear your thoughts on my comments. Tom W, what do you think. Also what is both of your opinion(s) on the special stresses that ABS, any fluid pressurization equalization method's might cause (of any).
So, the first trouble shoot is to see if the maximum relaxed distance (forcing the front pads out with wood equal's the distance equal to the relaxed distance betweed pad and rotor you see in the rear. Check again, the front pad(s) not forced to the maximum distance from the rotor.
My guess is the front two caliper's are both bad.

I see I had multiple typos and said some things in an unclear and/or misleading way relative to my intent. Don't have a lot of time rite now to fix them, but if anything unclear still ring's a bell, please let me know. As my background I should say that I am new to MB and MB old cars (69 220d), so I rely heavily on my old MOPAR experience, which was extensive, from the 50's and 60's; though as part of my USMC term in the 70's I did do a two week course on Detroit Diesel's (two-stroke) and the Continental V-12, the engine powering the M-60A1-3,, used in the main battle tank (replaced by the newer gas-turbine powered Abram's they now use.

Whiskeydan · #5 11-04-2005, 11:10 AM

Ralph, Front pads are fresh, thick. I did cycle the front calipers thru their extreme piston travel to make sure they did not suffer the same problems as the rear. Actually, I only had one piston on one rear binding. I robbed known good calipers from another car so, I felt to have equal wear and age on the rear calipers, I'd change both sides.
The puzzler is the fact that there is pressure in the line at the MC. This rules out, in my mind, a binding front caliper piston and I would think the rubber hose as well.
I did suffer from the "check valve swollen hose syndrome " at one of the rears. The soft innards of the hose would expand when you mashed the pedal, swelled shut as the pedal was released holding the pressure at the caliper.
Let me re-state that this IS a rebuilt MC from a local parts supplier. Not new. I have another 300D that has a new mc that I may substitute to verify this one is bad. I am able to change 'em in around 15min now.

Thanks for your help.

signalredcoupe · #6 11-04-2005, 11:35 AM

Whiskeydan,
When your front calipers get stuck, & you say you have to open the lines at the master cylinder, instead of doing that, loosen the two nuts that hold the master cylinder to the booster & move the mc forward away from the booster a little & see if that relieves the pressure. Don't know if Benz has any adjustment, but I had an old Lincoln that drove me nuts till I found that somebody got a little over anxious about pedal free travel & adjusted the nut on the booster pin too far. Or you may have some corrosion in there causing the same result. Paul central FL
'84 300CD

Whiskeydan · #7 11-04-2005, 11:36 AM

Quote:

Originally Posted by signalredcoupe

Whiskeydan,
When your front calipers get stuck, & you say you have to open the lines at the master cylinder, instead of doing that, loosen the two nuts that hold the master cylinder to the booster & move the mc forward away from the booster a little & see if that relieves the pressure. Don't know if Benz has any adjustment, but I had an old Lincoln that drove me nuts till I found that somebody got a little over anxious about pedal free travel & adjusted the nut on the booster pin too far. Or you may have some corrosion in there causing the same result. Paul central FL
'84 300CD

Thanks! Easy check. I like those.

Ralph69220d · #8 11-04-2005, 06:49 PM

You're probably right about maintaining pressure at mc being inconsistent with piston being stuck. Yet, intuitively, is this necessarily the case, I ask myself? Is your MC a design with two or more lines output (front/rear or abs scenarios)? What's happening with the opposite caliper on the car?

t walgamuth · #9 11-04-2005, 07:10 PM

what it is. what type of brake fluid are you using? sometimes i think silicone will cause problems with mb brakes.

as to the abs, i doubt that that would be causing this. they seldom are activated in my experience. so any damage from them would seem to be minimal.

the adjustment on the travel that one poster above mentioned sounded as if it might be of merit.

sound pretty sure that the calipers are good. the prob has persisted after changing the mc so the mc seems unlikely.

i dont know... seems like a puzzler.

i will keep thinking.

good luck

tom w

Whiskeydan · #10 11-04-2005, 11:23 PM

Just before dark tonight I loosed the mc from the booster. At around 2-3mm seperation (MC/booster) the front calipers were free. It was too dark to see if there is a travel adjustment.
I'll re-examine this weekend.
Is there a proceedure for this if there is a pedal free play adjustment?
I did not find any mention in the MB chassis service manual.
If no adjustment, then what?

t walgamuth · #11 11-04-2005, 11:59 PM

there is. every car i have ever seen has it. someone prob just got overly concerned about the free play on the pedal.

tom w

Whiskeydan · #12 11-05-2005, 03:30 PM

I removed the master cylinder and I see NO free play or travel adjustment on the rod or the pedal end of things.

I found the pushrod protruding approximately 2-3mm past the edge of the booster. I used a straight edge held across the hole.

To rule out the MC I swapped one from my other 300d which is an '82. While I had it off I measured the rod on the '82 . It measures about flush with the edge of the booster.
The known good MC has the same dragging front brake issue when mounted on the '80 300d.
I am at a complete loss here. Grind the rod off?

t walgamuth · #13 11-05-2005, 03:40 PM

maybe shim the mc. it is a mystery how it came to this. maybe something is bent in your car like the firewall or something on the pedal assembly.

tom w

Whiskeydan · #14 11-05-2005, 04:52 PM

I tried to gently tap the rod back just on the off chance it would pop back to the correct position. No such luck.
I don't see any evidence of anything bent or damage. It is a mystery how this came to be.
I may make a shim for a temporary fix until I can research this a bit more.
I'll need to maintain the seal for the booster though.
Or, I even thought of drilling the MC about 3mm deeper. Either way, it will be a "patch".

I think I'll adjust the valves. I could use a 'break'.