Brake Master Cylinder regular maintenance item W123??

[unkl300d]

I was wondering how many of you W123 diesel owners replace your brake master cylinder on a periodic basis for the sake of basic maintenance?

(even though you don't feel soft brakes etc.)

I don't find rebuild kits for the non turbo 300D and mine has about 170K miles over some 3 decades but brakes feel ok.

[Marshall Welch]

Good question. In the case of my car, a 1982 300D-Turbo, the original brake master cylinder failed in 2006, with 69,000 miles on the car. I had it replaced by an independent shop.

The pricing between what a shop has to pay their supplier vs purchasing the part online can be substantial. I recently picked up a replacement Ate master cylinder to have on hand, as I'm now prepared to do it myself, when signs of failure begin to appear.

At present, I don't have any plans to replace the master cylinder. I do wonder a little if the rubber/seals in the replacement cylinder will degrade with it sitting on the shelf, but my guess is that 2-3 years will not make much difference. Maybe I'll replace it after it has been on the car for 20 years, but we'll see.

[gregp1962]

I'm not aware of these failing very often. I don't think it's something most people do unless there is an actual failure.

[Sugar Bear]

Not a bad idea to replace a 38 year old or older safety item just because. You're probably well aware one of the best things to do is flush the brake system with new fluid from a sealed container every two years with a pressure bleeder. I've seen masters get damaged while using the "pump the pedal" method because the seals get pushed into a part of the cylinder bore where they don't normally travel which is no longer well polished.

[unkl300d]

Yes, in my particular case I do have the brake fluid replaced every two years. I ususally use the mighty vac to remove fluid from each caliper.

I agree that the brake pedal method is rough and if the pedal is pushed too far down to the floor , the brake booster can be harmed.

I would like to hear others' experiences as well.

Thanks!!

[280EZRider]

Change the MC when it needs it, not as periodic maintenance. I prefer to bleed the system once a year - winters and springs here are very moist. If you must push to bleed, place a short 2x4 under the pedal to prevent damaging the MC seals.

[unkl300d]

280EZrider, most prescient of you.

I was going to mention the 2X4 trick but declined.

Fortunately my climate is temperate. No snow, no excess rain/moisture.

Indoor garage parked.

[imgolden]

I used to think no, but, with boosters being so hard to find these days I'm more inclned to spend the $70 for a decent MC every 20 years or so...

[Sugar Bear]

Hmm...Good thought about saving the booster by changing the MC before it leaks at the back in to tthe booster.

[unkl300d]

Sugar Bear has a point underscored by one of "Uncle Ken's" latest videos in which he makes the same point.

He shows how leaks from the main bushing/washer at the end of the MC drip down into the brake booster and deteriorate the diaphragm and corrode the metal body's bottom.

Under such a scenario, the only way to detect a slow leak is to observe low brake fluid levels in the reservoir or remove the MC and try to look down into the booster (if the leak is not already obvious at the MC end after removal.

[gregp1962]

MY pedal is rather hard. Is there a way to check the boosters ability to hold a vacuum?

[unkl300d]

I would first check to make sure that your vacuum pump is working.

You can unfasten the brake booster vacuum hose from the brake booster while engine is running and test with finger for a suction sensation.

Quote from a member' post

03-23-2018, 09:32 PM
vstech
DD MOD, HVAC,MCP,Mac,GMAC Join Date: Mar 2006
Location: Mount Holly, NC
Posts: 26,571



Ok, the brakes on these cars are extremely simple, and extremely high quality.

If the pedal is mushy, you have air in the system.

If the pedal drifts to the floor, you have a bad master cylinder.

If the pedal is hard to press with the engine running, you have bad vacuum supply, or a bad booster.

That’s it.

If a wheel is dragging, you could have caliper issues, flex hose issues, or rarely master cylinder issues.

[BillGrissom]

I have used DOT 5 silicone brake fluid (not 5.1 which is glycol) in all but one of my vehicles for years. One even has ABS and the bottles spec "not for ABS", but the ABS works fine (pedal chatters if I brake hard on a paint strip on wet road). My guess is they simply never qualified the ABS pulse rate or such w/ silicone fluid so lawyer-language?

With silicone, you never get internal corrosion since it doesn't absorb moisture. I began that after corrosion issues w/ glycol fluid (DOT 3, 4, 5.1), one in my 1982 Chevy S-10 w/ original factory fill at ~6 years (eastern U.S.). I swapped the MC in my 1965 Chrysler to install a safer 2-pot MC and found no rust inside the cast-iron MC, despite some outer rust. That was after ~15 years using silicone. Many hobbyist geniuses spread the idea that not absorbing water is bad since drops of water will then sit at the bottom against steel parts to cause rust. If true, the same worry in your power steering system. Don't let water drip into either. But, when surrounded by silicone fluid, no oxygen could get to the water drop to cause corrosion anyway. Silicone is slightly more compressible, but insignificant compared to other compliances in the system (rubber hoses, ...). It is benign to rubber seals and won't destroy paint. So, if it does leak into the vacuum booster, should be no worries.

I don't think a non-turbo MC differs from a turbo, and even between gas and diesel in W123 cars, but check P-P and rock listings. The worst thing with my 300D MC is the rubber test caps at the top crack and even fall off. That allows easy entry for moisture, or even water if you garden-spray your engine (bag it first). I put some better caps on one of my cars after the black rubber got bad, poly-something from tube fitting caps. The purpose of the test cap is to push it and verify the "Brake Warning" dash lamp comes on for that simulated "low fluid in reservoir". Most cars don't have such, but M-B engineers can't filter their crazy ideas and budget must have been unlimited (really need the Aux Water Pump? Ripped mine out).

[unkl300d]

Hi Bill, interesting anecdote about silicone brake fluid.

In the past I have researched it and it seems that the brake line system and MC has to be voided 100% of the basic DOT 4 stuff in order to avoid captured moisture,even if it is a small amount.
Claims of possible inefficient braking if silicone fluid is floating ontop of some old brake fluid residual, plus the small amount of possible moisture residue. Especially in the calipers.

Come to think about it, back around 1982,I had a MIDAS brake shop replace my brake fluid with silicone brake fluid on my 1967 MGB. No idea what procedure they used but they did use a machine to remove old fluid. Never had problems afterwards but sold it around 3 years later.

I would not mind that switch over on my 300D.

[t walgamuth]

To the OP....I seldom change any part that is working correctlly. I will flush the fluid though if it gets dark.