W108 master cylinder rebuild

[RDM]

I'm having trouble getting the master cylinder apart. I have taken out the snap ring and the threaded washer behind it. I have also taken out the stop bolt with the ~10mm head. Still, it is clear that something holds it all together. The rod moves in easily, but will not come out beyond where it did when the snap ring and washer were there.

What do I need to do?

Thanks

[Benz Dr.]

Originally Posted by RDM

I'm having trouble getting the master cylinder apart. I have taken out the snap ring and the threaded washer behind it. I have also taken out the stop bolt with the ~10mm head. Still, it is clear that something holds it all together. The rod moves in easily, but will not come out beyond where it did when the snap ring and washer were there.

What do I need to do?

Thanks

Buy a new one. I never rebuild a MC for any reason.

[RDM]

Originally Posted by Benz Dr.

Buy a new one. I never rebuild a MC for any reason.

Why is that?

[Benz Dr.]

Originally Posted by RDM

Why is that?

For me, itwould be liability. For you it would be safety.

[Manoubm8]

Not trying to restart a dead thread but, I tried this earlier, and really, there is nothing else holding it in. Of course, considering this is 2021 now, I would have expected you to have already gotten a new master cylinder. I just would like to put this here for anyone else who wishes to attempt it.

I got the piston ("rod") to come out by hitting it ridiculously hard (pushing it into the body) with a set of channel locks. Of course, I'd recommend a rubber mallet or if that fails, a deadblow/engineer's hammer and a rubber pad. It popped out after a few smacks and flew across the room!

Just be very sure to take photos. I forgot exactly how it went back together, so I chose not to take any chances and just bought a new one. But during my test reassembly before having tampered with the seals, it fit perfectly and the piston sounded and moved the same as before, so I do not believe the internals were damaged by this process.

[RDM]

Thanks for your reply. I had forgotten about this thread.

For me, it was copious amounts of solvent / lubricant (WD40) and repeatedly compressing the spring. Eventually it came apart.

It took repeated bleeding, but my new-but-the-hardlines braking system works very well.

[Idle]

I have run into this and the answer is pretty simple: The interior bore of the MC is eaten up by pitting caused by water in the brake fluid.

Not everyone changes their brake fluid every few years, and the fluid from way back when was hydrophobic, which I think is the correct term. Anyway, it attracts water vapor from the surrounding air.

This will not mix with the fluid so it just starts to eat away at the metal used in these MCs. When I could finally get the rod and rubber seals out the inside of the MC looked like it was full of holes due to the pitting.

If it is not too bad I could use a hone to clean things up. But no amount of honing is going to replace metal that is no longer there.

When I get a car that is 40 years old it has had a few owners, and those owners let thing slip that I never would. Hey, we have all been there. I tell my friends who are considering a Mercedes a neglected Mercedes will be the most expensive car they have ever owned and a well cared for one with be the cheapest.

[aluminum]

Mercedes Like Dot 4 Fluid
I Have rebuilt a Lot of them and it's Easy to see when they should not be rebuilt most often when you hone them out .
Most often the Rubber is what is worn out the most but the entire Cylinder will wear mostly by Contaminates affecting the cylinder .
I found the wrong Push Rod more the expected and that can throw you off fast . If long Pressure will build up . You see such things in Old Car's with lot's repairs through the years
Although I would Not personally hesitate to rebuild cylinders I would not recommend just anyone to rebuild such things simple because it requires Experience to see what's Worn / Bad Etc
Brakes Save Lives always have Excellent Brakes don't take them down to the end replace them , When I see them 2/3rds worn I replace them.

[ejboyd5]

Originally Posted by Idle

. . . the fluid from way back when was hydrophobic, which I think is the correct term. Anyway, it attracts water vapor from the surrounding air.

Suggest the proper term is "hydrophilic."

[Tomguy]

Hygroscopic. Brake fluid still is highly hygroscopic. That's why routine flushes are so important. It not only cuts down on corrosion, but it prevents the fluid from boiling at much lower temps, which can cause a deadly loss of braking.

[vwnate1]

Unless they're the stepped bore typ many can be rebuilt, as mentioned the sealing surface is the critical part .

Long ago I learned that most over size the bore by not understanding proper honing technique .

The hone should never remain stationary ~ it needs to be moving in and out the entire time it is rotating .

I often use a bit of leather taped to an old drill bit to polish the bore rather than hone it ~ this will clean up the crud and clearly show you how pitted the metal is .

You can also use the leather to polish the bore so the piston's seals seal better .

Never, EVER dry hone nor polish brake cylinders .

Remember : even trace amounts of chemicals will mix into the brake fluid and attack the rubber seals throughout the entire hydraulic system ~ I've had several jobs over the years come in because folks did foolishness like rub each new rubber seal with penetrating oil (?!? WHY ?!?) before final assembly or the Cal. Tech Professor who grabbed a can of "brake fluid" off the shelf in his garage and topped off his brake reservoir then was surprised when the entire system failed in a week ~ as soon as I sniffed the can I knew ~ this idiot had saved some paint thinner in an old brake fluid can ....

[Rook]

I didn’t rebuild the mc on my Mercedes but I couldn’t get the piston out on an old Spitfire until I held it like a horn to my lips, covering all the side holes, & blew it out.

[vwnate1]

Yes, some times I have to use a grease gun and Zerk fitting to blow out a seized piston, brake or engine, either way out they come sans damage .