Is it safe to use 2 person brake bleed on w124?

Hello, I was wondering if it is safe to use the traditional two person brake bleed technique on a w124 with 109k miles. I have heard warnings that beeding a system by pumping the brake pedal all the way to the floor can rapidly damage your brake master cylinder by pushing the seal and piston into corroded areas of the cylinder. Is this true??

Very true! Pushing the brake pedal any further than normal braking distance can (and probably will) damage the master cylinder. I have heard that placing a block of wood under the pedal to limit it to the correct movement is the safest bet. This is also insurance since your buddy's fine motor skills will be impaired by all the brewskies -- aren't beer and home car repair synonymous?

The key here is to flush the system frequently to prevent there from being a corroded area. Brake fluid(unless you use silicone fluid) has a high alcohol content, and absorbs water out of the air. Keep the reservoir and brake fluid containers sealed.

If you maintain nice, clean fluid in the system, you should have no worries.


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Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in
Mercedes automobiles

Thanks for the advice. One more question. People on the board seem to recommend using MB Dot4 fluid only and defenitely not silicon. Is some other brake fluid better in order to improve brake performance plus how much should one buy to change the fluid.

[Aaron]

Just stick with DOT 4. If you use a silicon fluid, all the rubber in the system will have to be changed and you can't really go back to DOT 4 again without major hassle. Just keep the DOT 4 changed regularly and you'll have no problems--even during light track use.


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Rgds,
Aaron Greenberg
MB technician
Precision Motorcars, Cincinnati, Ohio
'67 250SE Cabriolet
'77 450SL
'79 6.9 (SOLD)
'80 300SD
'85 380SE
'89 420SEL
'93 300E 2.8

How much MB Dot4 should I purchase?

Ken:

I don't have my Owner's Manual with me at work here, but look into the section "Maintenance" in yours, and you should find the correct amount of brake fluid which is in your system. Haynes might also have that documented.

At the worst, it is unusual that you would have to go through more than two cans of brake fluid, assuming the 12 oz. cans; you might consider going with the ATE Gold and ATE Blue Brake Fluids, since that way you can alternate them between fluid changes, and you will see immediately when the "old" color comes out of the caliper. The next change, use the other color, and repeat the process.

Many German-car dealers sell them, and Zims in Texas advertises them for US$ 8.95 for a pint, which is a lot more than 12 oz. Telephone number is 1.800.356.2964...Andras

KenT:

If by "brake performance" you mean "feel" of the brake pedal when braking, you might want to consider performance brake pads (Mintex) and/or stainless steel braided brake lines. The "squishy" feel one experiences with stock lines is the rubber hose expanding under pressure. The braided lines prevent such expansion which translates into a "firmer" feel when braking.

Hope this helps.

Brian