Disappearing brake fluid - PeachParts Mercedes-Benz Forum

lrg · #1 11-10-2003, 12:59 PM

My 123 has a brake fluid leak somewhere that I can't seem to find. The brakes themselves work perfectly but I lose an ounce or so of brake fluid a week. A visual inspection of the calipers showed no sign of leaks. Yesterday I hooked up my EZ Bleed and pressurized the fluid resevoir to about 30psi but still saw no signs of leaking (nor did it seem to produce any fluid loss either). I'm reluctant to take the pressure up much higher. At my last brake job the outboard piston on my left front caliper was a bit stiff when I tried to press it back in for the new pads so I thought that might have a slight leak but upon close examination it seemed dry and tight. I've heard stories of master cylinders leaking into the boosters but am not sure how to check for that. I did look up under the brake pedal for signs of a leak on the inside of the firewall but it too was dry. All the steel brake lines appear to be in good shape. This is a longtime dry weather car so rusted lines is definitely not the problem. Anyone got any good ideas?

speedy300Dturbo · #2 11-10-2003, 01:40 PM

The seals in the master cylinder are probably going bad, thus leaking brake fluid into the brake booster. Best way to check is to remove the master cylinder and see if there's fluid in there! Maybe use a MityVac to siphon it out?

lrg · #3 11-10-2003, 02:50 PM

I tend to agree. Is it possible to check for brake fluid in the booster without removing the master cylinder? I was thinking maybe I could remove the vacuum line and carefully slip a small diameter tube into the booster to see if I could suck out any fluid. According to the workshop manual CD the booster should be OK as long as I don't have more than 100cc of fluid in there. If I have anywhere near that amount it ought to be pretty easy to find. What do you think? I'd really like to verify that's the problem before I have to drain the fluid out and take the master off.

lrg · #4 11-10-2003, 06:59 PM

I got ambitious and tried it. I used a soft plastic tube and gently stuffed it down to the bottom of the brake booster through the main vacuum line. I then used my MityVac and withdrew about 8 ounces of brake fluid. Ugggh! Unfortunately I think I now need to replace the booster since that is about double the amount that the CD says can accumulate without covering some of the non-brake fluid resistant rubber. A word for the wise, if you start to loose brake fluid check to see if is accumulating in the booster. It's an easy 5-10 minute job and if you catch it in time you'll save yourself the cost of a new booster (I wish I had).

Mordecai · #5 11-10-2003, 07:06 PM

What kind of transmission? Some MBZ hydraulic clutches share the master cylinder reservoir. I had a mysterious leak that turned out to be my clutch slave cylinder.

M.

lrg · #6 11-10-2003, 07:18 PM

It's a standard US spec. auto box. Speedy was right, it's definitely being sucked into the brake booster.