Another frustrating problem, this time- Brakes - PeachParts Mercedes-Benz Forum

ROLLGUY · #1 05-10-2017, 04:42 PM

My buddy of mine brought over his girlfriend's TD with a brake problem the other day. The car stopped fine, but when stopped with pressure on the pedal, it would slowly go all the way to the floor. We replaced the master cylinder, bled the brakes, and it seemed to work just fine. The car had great brakes, and everything worked as it should. After driving the car for a couple hundred miles, the same exact thing started happening again. We know it is not a caliper, as all four were recently replaced. There is no leaking fluid anywhere, and the fluid in the reservoir is right where it should be. I don't believe it could be the brake booster, but stranger things could happen. From what I know about brake systems, it sounds like a master cylinder. Could this new one be bad as well? Anyone have any ideas?

kerry · #2 05-10-2017, 05:20 PM

Yes, it could be.

jay_bob · #3 05-10-2017, 07:06 PM

I concur. Sinking pedal with no leaks or air in system sure sounds like bad MC to me too.

Worst feeling in the world when you push that pedal, it keeps sinking, and the car isn't slowing down.

BillGrissom · #4 05-10-2017, 07:43 PM

I know if Rollguy is asking, it must be a tough one. But, I concur that the new MC is likely bad. I had a rebuilt one once (for 1960's Chrysler) that failed soon. The bore was clean inside, but they apparently had machined it and left a sharp edge at the ports which cut the rubber piston seal. If there are no leaks anywhere, and no hoses are swelling, it is most likely leaking past the piston seal. The only other thing I can think is a restriction in the tubes or hoses that let one or more calipers slowly extend, but that seems more likely with brake shoes which have more travel. TD's are all 4-wheel disks, I think. Unlikely the booster could do this, though it could give a similar feel of sinking.

engatwork · #5 05-10-2017, 07:45 PM

Quote:

I know if Rollguy is asking, it must be a tough one.

understatement of the day

I had a BMW suv in not too long ago that kicked my butt and finally figured out that there is some kind of obscure, bs way of bleeding brakes. Did it that way and issue resolved.

Mxfrank · #6 05-10-2017, 09:58 PM

Does it return to normal after the brakes cool overnight?

ROLLGUY · #7 05-11-2017, 10:30 AM

Quote:

Originally Posted by Mxfrank

Does it return to normal after the brakes cool overnight?

No. The pedal goes to the floor every time pressure is held on it. Two other shops were consulted, and both said it was a bad MC. It just seems odd to me that the MC was replaced because of this exact same problem, and the new replacement MC acted the same! That makes me think it is some other problem, but the consensus is the MC. That is the frustrating part. I will be replacing the MC with yet another new one today......Rich

tyl604 · #8 05-11-2017, 10:35 AM

If you are not getting a leak on any of the four wheels or a connection, do not see how it could be anything other than MC. Did it bench bleed OK?

ROLLGUY · #9 05-11-2017, 10:41 AM

Quote:

Originally Posted by tyl604

If you are not getting a leak on any of the four wheels or a connection, do not see how it could be anything other than MC. Did it bench bleed OK?

Yes, and the brakes work great. It is just that the pedal goes to the floor after stopping.

Diesel911 · #10 05-11-2017, 11:05 AM

I bought a Cardone rebuilt Master Cylinder and when I fist put it on it had a little sinking. But, needed to keep the Car on the road at the time as it was the only one.

I bleed the lines at least 4 times trying to fix that. So it was never right to begin with.

In less then 2 years my foot went to the floor when I braked. At the time there was a good deal on FENCO (made in Taiwan) brand new MCs (I could not find the receipt to get an exchange on the life time warranty) and I bought one and have had no issues since. (Note that FENCO also sells rebuilt ones which I have no experience with).

I found the receipt and exchanged the MC but I am just keeping it as a backup.

On a rebuilt Master Cylinder you ought to get a minimum of 5 years of normal functioning use out of them.

One of the issues in the rebuilding industry is as time passes the really good rebuildable cores start to disappear and they end up using less suitable cores.

Since both Mastercylinders and Calipers get pitted in use inside of the bores it is likely too expansive to resleeve the bores so they either toss them or use them anyway.

The other issue is that if they are sent overseas for rebuilding the person doing the rebuilds is likely paid by the how many he does. That means he is going to be in a hurry and less picky about assembling them

Diesel911 · #11 05-11-2017, 11:23 AM

If the vehicle is a W123 the diagram of the Master Cylinder and the actual ones I have show a pin going through the front Piston assebly. If you cannot pull out the pin you cannot inspect the bore of the front piston.

I know that on the original I removed there was no pin end sticking out to grab.

DeliveryValve · #12 05-11-2017, 12:23 PM

Quote:

Originally Posted by ROLLGUY

No. The pedal goes to the floor every time pressure is held on it. Two other shops were consulted, and both said it was a bad MC. It just seems odd to me that the MC was replaced because of this exact same problem, and the new replacement MC acted the same! That makes me think it is some other problem, but the consensus is the MC. That is the frustrating part. I will be replacing the MC with yet another new one today......Rich

You didn't mention if the master cylinder is rebuilt, new or used. If rebuilt, as D'911 has stated, it could rebuilding flaw. I suspect you have a 50/50 chance of getting a bad master cylinder with rebuilt.

The other thing you have to consider since it was not an immediate problem, is the contamination of the brake fluid. If there is any petroleum inside lines, it will swell and destroy the seals. Also, if there is grit in the fluid, you can have a seal issues too later down line.

I would replace the master cylinder with a brand new $80-$90 dollar ATE unit and flush the heck out of the lines and calipers with denatured alcohol just to be on the safe side. And while you are flushing the calipers, inspect them thoroughly also leaks in the pistons and body seams.

.

tyl604 · #13 05-11-2017, 01:18 PM

In my view it should last 20 years.

ROLLGUY · #14 05-12-2017, 10:49 AM

We replaced the MC yesterday with a new ATE unit. After bench bleeding it, we bled the brakes like always. The pedal felt real good, and the car was test driven. The brakes worked excellent. The car was parked for a couple hours while we worked on other things. My friend drove the car away, and called me five minutes later with "the pedal goes all the way to the floor". I said "you have got to be kidding me!". He was not kidding. I can't believe that three MC's in a row could be bad. On the other hand, I can't believe this problem is anything other than the MC. I don't believe it is possible that the booster could be the problem. The past few days I have worked on this car and another wagon (http://www.peachparts.com/shopforum/diesel-discussion/385909-300td-b2-piston-replaced-still-no-forward-movement.html), and there has been no progress on either. After much expense for parts and labor, we are no further ahead than before. It is very frustrating.

yuke · #15 05-12-2017, 03:25 PM

I had a sinking pedal on my sd a while back put a rebuilt from rock auto on and pressure bled problem has not re occurred. I too would say it has to be the mc.A while back someone stated to test the vaccum booster would be pump the brakes with the engine off if the pedal goes to the floor then start the car if the pedal rises to normal height you have a leaking booster. Not sure if this information is correct but easy to try. Or maybe there is air trapped somewhere and it only shows up after a little time. Hope you figure it out soon I'm sure you will.