Rebuilding W123 Master Cylinder (seal kit?) - PeachParts Mercedes-Benz Forum

johnathan1 · #1 04-09-2010, 08:17 PM

Yesterday, while in LA traffic, I went to stop and the brake pedal suddenly went all the way to the floor, and I had to pump it several times to get any feel back in the pedal. After that happened, the car still seemed to stop ok, but the brakes would engage with the pedal MUCH lower than it ever has before.

I'm assuming the master cylinder is failing? Are these difficult to rebuild? I've heard of other members replacing the internal rubber seals, and they make it sound like it's a pretty easy job to do. Can anyone comment on how to do this, and where I might find the seal kit?

barry123400 · #2 04-09-2010, 08:25 PM

I used to kit mine. The kits where withdrawn from the market by many vendors because of liability issues I was told.

Too bad if the bore checked really good you were in business again very cheaply. Or just refreshing an older master cylinder was easy.

You may find someone still selling them by looking around. Although I have not seen the kits for quite awhile myself.

leathermang · #3 04-09-2010, 11:09 PM

Rebuilding anything having to do with the brake system by someone who has to ask how to do it is not a smart thing...
Buy a new or rebuilt one from a good source and have fun bleeding and flushing the brake fluid ....

JimSmith · #4 04-09-2010, 11:52 PM

leathermang has the general approach nailed. If a safety system key component needs work, be cautious.

I would be more inquisitive about the actual problem before beginning to change out parts. How is the brake fluid level in the two compartments in the little tank on top of the master cylinder? Does the brake feel the same, just a longer travel? Any other cases of the pedal hitting the floor?

In my experience when a seal fails in the master cylinder, it does not heal. The pedal goes to the floor at a steady rate - not like there is suddenly no solid fluid connection between the master cylinder and the brake calipers, every time you apply the brakes, and you empty the reservoir of fluid. The idea that the brake pedal stiffens up after hitting the floor suddenly makes me wonder if there is not either a leak somewhere else, or a way for air to get into the master cylinder.

Jim

TnBob · #5 04-10-2010, 12:00 AM

Might very well be worth the time and money to stop at an indy and have them look at it.

Diesel911 · #6 04-10-2010, 12:21 AM

The Master Cylinder on my Car had a Pin (#27 in the pic) that is inserted down through the front resivior hole that held the front portion of the Master Cylinder Piston captive. I could not figure a easy way to pull it out.

Next was the Master Cylinder kits I found were above $47+ shipping.

Then there was if the condition of the Bores was OK or not.

Section 42-315 in the FSM covers: Disassembly, Inspection and Assembly of Stepped Tandom Main Cylinder.

None of the Master Cylinders pictured in the FSM have the mentioned Pin so there is no instructions on removing it.

barry123400 · #7 04-10-2010, 07:04 AM

Quote:

Originally Posted by Diesel911

The Master Cylinder on my Car had a Pin (#27 in the pic) that is inserted down through the front resivior hole that held the front portion of the Master Cylinder Piston captive. I could not figure a easy way to pull it out.

Next was the Master Cylinder kits I found were above $47+ shipping.

Then there was if the condition of the Bores was OK or not.

Section 42-315 in the FSM covers: Disassembly, Inspection and Assembly of Stepped Tandom Main Cylinder.

None of the Master Cylinders pictured in the FSM have the mentioned Pin so there is no instructions on removing it.

I was buying them for 10.00 each including the installation kit. Then they dissapeared. At 47.00 may as well buy a rebuilt master anyways.

leathermang · #8 04-10-2010, 09:08 AM

Quote:

Originally Posted by barry123400

At 47.00 may as well buy a rebuilt master anyways.

BINGO !!!!!

300dOwner · #9 04-10-2010, 09:26 AM

May well be the master cyl but please be sure to check the calipers also...imo that's where you will find a leak, especially as your problem seemed to resolve itself. Ideally have someone stomp on the brakes while you're looking at each cylinder...report back, it'd be nice to know...

Diesel911 · #10 04-11-2010, 01:38 PM

Quote:

Originally Posted by barry123400

I was buying them for 10.00 each including the installation kit. Then they dissapeared. At 47.00 may as well buy a rebuilt master anyways.

That is what I did. I bought a Cardone Mastercylinder from what was at the time a Kragens Autoparts.

However, after it was installed the Pedal would still somtimes slowly creep down some. I lived with it as at the time it was the only Car I had runing. About 1-1/2 years after installing the Pedal went to the Floor.
So it was never a good rebuilt to begin with.

I saved the paper work on it but had no idea were it was. I opted to buy a new Fenco (Taiwan) Master Cylinder that at the time AutoZone sold for $64 (a few months later the price shot up). That has been perfect.

I later found my paper work and went back and exchanged the no good rebuilt for another rebuilt just out or principle. I hope I do no need to use it.

The issue is that rebuilt stuff has a high rate of defects. If I decide to get a rebuilt I already have a mindset that it might need to be returned.

leathermang · #11 04-11-2010, 03:12 PM

Quote:

Originally Posted by Diesel911

The issue is that rebuilt stuff has a high rate of defects. If I decide to get a rebuilt I already have a mindset that it might need to be returned.

The other issue in this thread had to do with someone who had never rebuilt a master cylinder...and probably not a wheel cylinder either.... trying it WITHOUT SUPERVISION the first time... too much at stake...not enough difference in cost to justify taking the chance...
I was talking to my mechanic yesterday... I do not deal with cars with computers ... and he was saying that even done correctly things like water pump replacements typically last only a third to a half as long as the original which came on the car....

Diesel911 · #12 04-11-2010, 05:16 PM

Quote:

Originally Posted by leathermang

The other issue in this thread had to do with someone who had never rebuilt a master cylinder...and probably not a wheel cylinder either.... trying it WITHOUT SUPERVISION the first time... too much at stake...not enough difference in cost to justify taking the chance...
I was talking to my mechanic yesterday... I do not deal with cars with computers ... and he was saying that even done correctly things like water pump replacements typically last only a third to a half as long as the original which came on the car....

I have put on Rebuilt Master Cylinders on other Cars and they used to last 4-5 years before there was another problem.
I think what is happening is the rebuilders are not doing anything about the bores when they are pitted. They are just putting the new kit in and hopeing they work.

johnathan1 · #13 04-11-2010, 08:28 PM

Well, my problem didn't really resolve itself...my brakes still WORK fine, but the pedal now feels spongey (was always firm before) and there is more travel than before.

So is a cheap new cylinder better than a cheap rebuilt?

Diesel911 · #14 04-12-2010, 12:40 AM

[QUOTE=johnathan1;2446089]Well, my problem didn't really resolve itself...my brakes still WORK fine, but the pedal now feels spongey (was always firm before) and there is more travel than before.

So is a cheap new cylinder better than a cheap rebuilt?[/QUOTE]

Except in the case of the new Fenco I bought the price went up to around $88 about 2 months after I bought it.

Sometimes Master Cylinders have problems when the feed holes become restricted or plugged or some have little valves on the pistons that get gunked up. This can cause intermittent problems.

The same gunk can also temp fill in pits in the bottom of the Master Cylinder so they will seal sometimes and sometimes not.

But, from the symptoms you describe I would at least start shopping for a new or rebuilt one so you already have a line on a good price. I say this because in the end it is not going to get better and unless you have an exterior leak a properly Filled Mastery Cylinder should never cause your Pedal to go to the Floor.

leathermang · #15 04-12-2010, 01:24 PM

The chances are you have not done a proper bleeding of your system...
no use looking to replace any parts until you have done that .