A couple of weeks ago, I notice that my brake pedal was somewhat soft, almost going to the floor board. This happened about twice over a three week period. But now its fine. Does any one know, what might cause this.

Drew:
It may help to know the year and model MB you have, also mileage. Any brake work done recently or no? Whem was the brake fluid kast changed?
Did the pedal change from normal to soft and back again suddenly, or just one of those things that changes without notice?
Gilly

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Sorry, it a 1985 300SD. Have not done any brake work since I got the car. I checked the fluid level and it's o.k. When I depressed the pedal, I had to pump the pedal a few times in order to stop the car.

Is your brake light on. If so, I am sure that is what it is. My suggestion is to swap is out soon. When mine started to go south, it degraded quickly and made the car a danger to drive.

The master cylinder is a fairly easy, suprisingly fix, at least it was on my 1977.

Here are some checks for your brake system.
With the motor running and the car stopped press and hold down the brake pedal, keeping pressure hard and firm , if the pedal slowly sinks to the floorboard (and you are not loosing brake fluid when you check the res.) You have a bad master cylinder. If you are loosing brake fluid that would be a wheel cylinder or brake line ( easy to find because there will be brake fluid at the leak.)
If your brake pedal does not sink all the way to the floor board, but holds firm , your master cylinder is good.
If your brake pedal sinks low at first but you can pump it up firm,
then you need to bleed your brakes. I would also suggest to flush out the old fluid and put in fresh brake fluid (dot 4 only).Hope this helps.

Air in the lines does not just go away. A leak will use fluid. I think your master cylinder is going bad and fluid is leaking past the main piston. I have had the same thing happen in several different USA cars and trucks. Next time it does this may be the time you need to stop in a hurry and you will only have 1/2 of your system to stop you. It is an easy fix.