Rebuilding W123 Master Cylinder (seal kit?)
 

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?

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 The kits where withdrawn from the market by many vendors because of liability issues I was told.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

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 ....

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

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

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.

BINGO !!!!!

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...

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.

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.

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=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.

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 .

Not done a proper bleeding? I have never done any work on the system...so why would it need to be bled? Pedal feel, travel and firmness has always been perfect.

I'll start shopping around for a NEW unit...thanks for the help guys! :)

Any recommendations on brake fluid?

If his system was working and now it isn't, something other than bleeding is wrong. He either has a leak in the master cylinder or caliper as a bad hose would go to the floor. Simply replacing parts without knowing which part is bad won't fix the problem.

Rebuilding is a matter of RTM and having the proper tools. It used to be common back when these cars were new. Results varied as do the results of today's reman & cheap imports. Calipers and master cylinder can be rebuilt IF he can find kits and a hone.

Regardless of whether he rebuilds or replaces, he is going to have to diagnose. Leaky calipers are usually apparent as soon as the boot is pulled back from the piston. Masters are usually apparent by pulling them away from the firewall or booster and looking under the seal. Sometimes masters have internal leaks that don't show up visually but never give a solid pedal no matter how much they are bled.

Any deteriorated hoses or brake hardware should also be changed when brakes are repaired. Brake disks have minimum thickness specs and can be turned id thick enough. Replacing pads will be a judgment call based on how much pad is left and whether they are contaminated with brake fluid.

Buying local will help if parts have to be returned. Balance this against quality of Phil's parts. I suggest including him in the decision process. Do it once & be done for another 30,000 mi & then replace pads.

If your pedal is soft do a proper bleeding...
while you are down there you just might see something that is causing the problem...
even moisture in the fluid after a while can cause a soft pedal...
So bleeding a system with reports of the pedal being soft is the logical first thing to do...
If you SEE leaks ... then certainly look at those items causing that...

i have an 87 300d , my rears brakes failed last week when they got hot and one tire was in fire, i managed to change calipers, hoses, pads and rotors but now when i put them all together and have the lines flushes and bleed the rears tiree got again hot, i replaced the master cylinder with an used one from a junkyard and now i have spongy pedal after firestone perform me a new bleeding and flushing for the brakes.
You might think that could be some air in the master or this replacement master is shot too?
I'm feeling desperate now and any help will be more than welcome

You did not say if the used Master Cylinder solved the overheating problem?
If the Air has been bled out sounds like the used Master Cylinder is not good either.

The answer is to get a good rebuilt or a New Master Cylinder (I bought a new one FENCO made in Taiwan has been working OK for several years now).
If using a Master Cylinder Rebuild kit is cheaper you can do that but you need to inspect the Bore of the Master Cylinder for pitting to evaluate if it can be reused.
Also on my Master Cylinder there is a Pin that needs to be removed in order to take the guts out of the Master Cylinder.

In the pic below item #27; the Pin needs to be pulled up and out of the Master Cylinder in order to get the Piston out.