brake master cylinder??
 

1981 300D

Hi Guys,

My car has been losing brake fluid for the last few weeks. The calippers are dry and the lines seem to be ok. This morning I noticed some weeping between the master cylinder and the booster.

Do you guys think I need to replace the master cylinder or could it be just the seal that goes between the master cylinder and the booster? Is it a good idea to rebuild the cylinder with a repair kit?

The brakes are working just fine.

Thank you.

master cylinder
 

The seal between the master cylinder and the booster is a vacuum seal....there should be no brake fluid there. You have a master cylinder leaking internally. I would just replace the master cylinder. I dont know about rebuilding it.....master cylinders are only about $60.

good luck

Mark

Before you replace a perfectly good M/C,be sure the leak isn't coming from the seals where the reservour plugs into the M/C. This occurs alot.

Peter

How can I determine that the reservoir seals are the cause of the leak? The fact that the brakes are working fine is a clue? Or can you have good brakes while there is an internal leak in the master cylinder?

If it looks like this they are leaking. Not too much trouble to replace. All the work is getting the fluid back into the system.

I just replaced my brake master cylinder because I found brake fluid where the MC and the power booster connect - like your's. The fluid was seeping down the booster housing. If it was the fluid resevoir to MC seals you would expect to find not only the rubber grommets in the condition that engatwork's pictures shows, but also brake fluid on the MC around the grommets and probably drops coming off the bottom of the MC onto the sheet metal below the MC.

A MC leak from the seal around the activating piston (at the point where the MC connects to the booster) may also deposit fluid into the booster and show some seeping on the outside of the booster (like your's). Sounds like most of your fluid may be going into the booster because it sounds like you've added a lot of fluid without any puddles. If the MC is leaking a lot into the booster, then I suggest taking the booster off and draing the fluid out then cleaning the remaining fluid out of the booster. Inspect the booster's rubber diaphragm for damage caused by the brake fluid.

My last two MC's leaked where this seal is because of rust (the inside of both MC's beyond this seal were perfect - no rust, no pitting, etc.). The first began leaking not too long after I got the car (1988-89) and lasted 13-14 years before leaking again. This time I put some silicone grease onto this seal in the hopes that the rust that caused the last two to leak will not occur again (if you try this, use only silicone grease and put it on the seal, do not put grease in the MC because the seal will push excess grease into the MC and into the brake fluid perhaps causing a clog and regular petroleum based grease will cause damage to the seal - it will swell).

So far everything looks good, but I replaced the MC only 10 days ago. I believe the rust may begin because I wash the area of the MC/booster from time to time and a bit of water got in there. To prevent water from entering this area I smeared a bit of silicone across the junction of the MC and booster and left an area on the bottom unsealed so if the MC leaks the fluid will come out there and not into the booster - and I will know there is a problem. However, the outside area on the MC where the leaking seal is located did not have any rust indicating that water was not contacting this area because it would rust. So, I'm a bit perplexed why the interior of the MC where the seal is located was rusted causing the fluid leak, but the inside of the MC just beyond the seal was perfect (no rust, no pitting, etc.) and the outside area where this seal is located did not have any rust.

Also, before installing both rebuilt MC's I took them apart to inspect everything and there was no rust anywhere, including where the seal that eventually leaked is located. I change the brake fluid every year to prevent rust/corrosion, so I'm a bit pissed that rust killed this MC. Also, when I removed the MC the o-ring seal for vacuum was good (you should have heard the inflow of air when I removed the MC) and the grooves for the o-ring did not have any rust in either the MC or booster. However, the area just outside of the o-ring grooves were rusted on both the MC and the booster housing.

Go figure!:rolleyes:!

Bleed the MC before installing it (if you replace the MC). I suggest the use of the Mighty Vac vacuum tool to bleed the brakes (about $30 from many auto parts stores - O'Reilly, NAPA, Pep Boys, etc.).

Good Luck!
Tom

Kefer,

A simple way to test for leaks in the hydraulic system is to press the brake pedal down hard (two feet will help) for about 5 minutes or so. Your pedal could go to the floor or sink a little, and if it does, you will see hydraulic fluid leaking out somewhere. That is where your problem will be.

On other hand, if you don't see leaks, and the pedal sinks, you have air in the lines. You will need to bleed the brakes.

If you determine that your reservoir is the problem, you probably can order new seals. By the way, a rebuilt master cylinder may not come with a reservoir, and you will have to reuse your old one. It might pay to replace the reservoir seals if your master cylinder is fine.

The seals I'm referring to are at the base of the white reservoir pictured. A new M/C comes with them installed.I suggest you buy two new seals,suck as much fluid out of the white thing as possible,and pop it off with a prybar.That reservoir just pops out of those seals,but be careful of the paint.If you get any fluid on the paint, wipe it off right away and wipe it several times with a wet rag.Pop two new seals into the M/C. Pop the reservoir into the seals,and fill the reservoir. You're done;there is no need to bleed the M/C. Just wait to see if you quit losing fluid.

Good luck,
Peter

I have a 300SEL6.3 that actually had a porus master cylinder casting. The brake fluid came out thru the walls of the master cylinder. I took a while to find the leak and the only fix was a new master cylinder.

P E H

PEH:

That is strange, but that is the way mechanical things are sometimes. Was the fluid "sweating" from the casting?

I found a wiring harness that had a "layer short." The individual wires had good insulation and no grounds/breaks, but the harness as a whole was shorted - caused me fits until I determined what the problem was. Replacing the harness fixed the problem. I've also had vibrations cause weird things to happen to electrical components.

Tom

Tcane,

Yes, it was sort of sweating from the casting. I had to keep adding brake fluid and couldn't find the leak in any of the calipers or lines.

I finally found the leak by the second person stepping forcefully on the brake pedal method and me looking for leaks. Luckily I started at the master cylinder first and there it was, oozing thru the cast iron casting.

After I replaced the master cylinder the leak stopped so I knew it was the original casting that was the problem. It was still hard to believe that a casting could leak like that.

I got the dealer to buy the new master cylinder because I bought the car in NYC and there is a law that all safety parts must be replaced if defective. I made him a deal that I would do the labor if he bought the parts. Instead of the paying me for the parts, he sent me the parts because he could get them for less. He sent genuine MB parts. I also get a new muffler and a few other small things I can't remember. That was in 1972.

Has anyone else seen this problem?

P E H

PEH

After 30 years as a dealer trained MB mechanic,I have never seen it.I guess that's why the word anomaly is in the dictionary.I have seen plenty of other strange things though.

Regards,
Peter