Brake Master Cylinder
 

So I was wondering if anyone had any insite as to what the correct (or if all) cost might be on the brake master cylinder on a 1974 240D? Put new brake lines and rotors/pads on it last week and drove it all weekened, weeknights and then this past weekend... saturday afternoon the master cylinder went out

My concnern before purchasing a new one is that the price variance between auto shops is pretty siginificant and I wanted to verify that I could in fact use the least expensive one -

Autozone - $36 (with $22 core)
Napa - $98
Advance Auto - $189
Pelican Parts - States there are two: 1 - $150; 2 - $315 (something like that)
Pep Boys - States there are two: 1 - $360; 2 - $618

I'm going to call German Auto Parts on lunch to see what they say... But would the one from Autozone be correct?
It's this one
Master Cylinder (Brake System) | 1974 Mercedes Benz 240D 4 Cylinders 2.4L Diesel | AutoZone.com

Also, let me add just to ensure it's the master cylinder instead of the booster. Initial stopping pressure is still there, but once I start slowing the brake pedal 'sinks' to the floor. While sitting at a stop I can pump the brakes to rebuild some pressure, but that doesn't last and the pedal starts to sink again.
Sounds like the master cyilnder to me.. just wanted to be clear as to the issue and repairs/parts

This is not a recommendation where to buy but just My experience.

I bought a Rebuilt Master Cylinder From Kragens Auto Parts (no longer around sold to another company). It was not as good as it should have been right out of the Box. I had a tiny bit of Pedal sinking. I kept it as I needed to get the Car on the Road.

About 1-1/2 years later the Pedal went to the floor. From past Experience a good Rebuilt Master Cylinder should last about 5 Years. So that one failed too soon.

I could not find the receipt for an exchange under the Life Time Warranty and I bought a brand New FENCO Master Cylinder (made in Taiwan) and have had no issues with it for about 3 Years.
A few Months after installing the FENCO I found the Receipt and exchanged the Rebuilt Master Cylinder for another free Rebuilt Master Cylinder. I hope I never have to find out if it is a good one or not.

A lot of Members from the various Mercedes Forums have had problems with Rebuilt Master Cylinders Right right after installation.

With a New Master Cylinder you face the Price issue. In My case at the time the New FENCO one was quite a bit less than the other New ones available but the price gone up on it.

So I think a Person can take a chance on a Rebuilt one (save the Receipt) and you may have to try a few till you get a good one or pay the higher price for a new one and be done with the job.

Also new Brake Hoses are not that expensive.

Awesome guys, thanks! I checked the hoses again last night, they're all good - no leaks anywhere... but pedal is still 'soft' and 'sinks' I'm about to order one and hopefully get it before the weekend to put it in! Thanks, won't go with the cheapest, I'll go ahead and get the one that Forced linked to - thanks a bunch you two

Answer
 

The correct master cylinder for your year and chassis is MB# 0024302801
Pelican Parts - Automotive Parts and Accessories - Porsche & BMW

Here is some BRAKE related data you need to know.

For best durability: Mercedes Benz Factory Service Recommendation is to flush the brake fluid every two years.

Some Facts About Brake Hoses:
#1. The industry standard average safety life of a brake hose is six years. FYI data SAE: J1401, J1703, J1705, J1873, J1406, J1288, J1403, J1833, J1402.
#2. Brake hoses deteriorate from the inside as well as the outside.
#3. Moisture is absorbed into brake fluid systems through brake hoses.
#4. Contaminants in brake fluid act abrasively on the inner wall of brake hoses.
#5. The brake hose reinforcing fabric deteriorates through expansion and moisture.
#6. High operating temperatures contribute to the deterioration of hoses.
#7. Brake hoses swell with age and restrict flow.
#8. Generally all brake hoses on a vehicle deteriorate at the same rate, So all hoses should be replaced if one is found to be faulty!

Brake hose failure mode:

#A. Rupture (burst hose) = age, exercise (too many miles) or impact are the typical cause.

*B. Partial internal collapse = the inner ply becomes damaged/detached acting as a partial restriction and/or one way valve.

#C. Full internal Blockage = the inner ply becomes damaged/detached acting as a plug = brake pedal still feels good but there is no brake application.

whunter@pelicanparts.com

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Don't forget to Bench Bleed the Master Cylinder before you connect the Barke Lines to it.