The other day I noticed that my brake pedal was losing pressure when I was stopped and idling. Today things changed drastically and I would have to pump the pedal at least 3 times to get adequate pedal pressure. The problem is intermittent, sometimes the brakes work just fine.
I'm not loosing any brake fluid, I've only had the car for 7 months, but the vacuum booster looks kind of new. I'm suspecting that the master cylinder is starting to fail. How do you bleed the master cylinder before installation? Do I need to bleed all the calipers after installation? Thanks

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'81 300SD

It sounds like a brake booster failure as well.

See page 9/17 - Brake booster diagnosis.

http://www.autoshop101.com/forms/brake05.pdf

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'86 560SL - 110K
'84 300D - 280K
'81 240D, 4speed - 320K
'74 240D, 4speed - 130K - just sold
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2000 DiscoveryII - 90K
1997 Saab 900S - 190K

More description please. Brake hard or soft? Brake goes to floor? Hard brake pedal is booster/vac (checkvalves in vac pump). If its going to the floor its most likely master cylinder.

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http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

It could be just air in the lines too. I'd get someone to help you bleed the brakes or use an auto bleeder by yourself.

That's where I would start first then if you bleed and there is no air then you know to go to the next step which is probably the master cylinder.

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86 Mercedes 300SDL - 400k+ miles

Brake pedal is soft, and comes close to hitting the floor. Thanks for jumping on this thread so quick. This 240D is my daily driver.

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'81 300SD

Sounds to me like you have air in the lines especially if you can pump it up and they grab. You are probably getting air in the calipers when you press the peddle down. I would bleed all four wheels before doing anything else and see if that fixes your issue.

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86 Mercedes 300SDL - 400k+ miles

If you are going to bleed all 4 then go ahead and look at the rubber brakelines and see if they need replacing first.

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http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

Hey guys,

I got a hold of a master cylinder this afternoon and installed it, and that took care of it. Bleeding was a *****.
I was thinking that if it was a brake booster leak, then the car would have had trouble shutting down, correct? Thanks

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'81 300SD

When I install a mc I crack loose the two primary brake lines and use them as bleeders. I put a rag under them to prevent the brake fluid from leaking on my painted inner fender.

People talk about bench bleeding but I never had much luck with that.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I've bench bled (mmmm that a "official" shop word? ) a few masters. It really does speed things up -- see this step by step http://autorepair.about.com/od/fixityourself/ss/MC_bench-bleed.htm

pretty straightforward stuff...

Bleeding at the calipers I use that universal tool that is absolutely necessary for us 123'rs a mightvac with a scrap piece of tubing going from the bleeder valve to the collection bottle (brake fluid will harden the tubing, hence the "scrap piece). Keep the brake fluid topped off and mightyvac the air out at the caliper. One person job -- no pedal pumping