My 16 year-old's car. Most of the time the brakes work fine. Occasionally (and becoming more frequent) the pedal will become mushy and travel much further down and the brakes will perform poorly. This is usually accompanied by a "BRAKE" warning light in the cluster. It doesn't always do it so I've sort of ruled out air in the lines. The fluid level is correct and I could find no leaks.
Master cylinder? I would think that a vacuum booster failure (which I have already experienced on this car when it was purchased) would involve high pedal pressures and reduced braking effectiveness and wouldn't be intermittent.
Thoughts?

Thanks in advance. I need to get this fixed asap.

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1983 M-B 240D-Gone too.
1976 M-B 300D-Departed.

"Good" is the worst enemy of "Great".

Sounds like a master cylinder problem to me.

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

the little rubber seal is gone but it just doesn't know it yet. Sometimes it flaps and seals and sometimes it doesn't. Borrowed time.

You do not have enough info or background to rule out anything...
This is not a casual situation...
You need to do a snap up brake bleeding and change out your brake fluid immediately.... if that does not fix it.. THEN come back and talk about it as you will then have a solid basis to work from...

( I used to be a mechanic at a Spring and Brake shop.... )

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/showthread.php?t=156207&highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

And do it now before your son has a brake failure and hits something!

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

I ordered a master cylinder and it should be here in the AM. The changeout will also involve bleeding (hopefully not me) and fluid flush. Hopefully that fixes it. The rubber seal idea makes sense since it is a random occurrence.
Thanks guys!

__________________
1983 M-B 240D-Gone too.
1976 M-B 300D-Departed.

"Good" is the worst enemy of "Great".

Contaminated brake fluid can give that same symptom..... if you replace both at same time you will have no idea what was wrong...having assumed it was the master cylinder....
Hint, if some seal is leaking sometimes... why is the fluid level still correct ? Or why has it not hydrauliced the piston due to liquid on other side ?

__________________
1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/showthread.php?t=156207&highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

The master can leak internally and bleed pressure back to the reservoir. I've had this problem on my cars.

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Yes, but since bad fluid can feel like this also.... if both are replaced at the same time you don't know which was causing the problem...

__________________
1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/showthread.php?t=156207&highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html