1972 W114 250 - losing my brake pedal at idle - PeachParts Mercedes-Benz Forum

rgnprof · #1 05-11-2023, 10:40 PM

Just developed this problem - while idling in gear the brake pedal slowly goes a bit soft on me and I find myself having to press harder and harder to stop from moving forward.

If I pump the brakes quickly it seems like my pedal gets firmer but then slowly I start to lose it. I never find it going all the way to the floor, but it is definitely disconcerting!

I've had this 250 for 20+ years and other than replacing pads, I've not done much with the brakes.

Any thoughts?

Thanks,
ryan

meltedpanda · #2 05-12-2023, 09:30 AM

Do you have the ATE T50/26 Braking System Servo on that car? I'm thinking not but thought Id ask. if so ignore the rest of this message

It sounds like you have a master cylinder going bad or a line that is allowing air into the system. Seeing any leaks ?
The R/R is pretty simple, the hardest part is getting the lines off the master . Get a good set of flare nut wrenches !

Frank Reiner · #3 05-12-2023, 10:05 AM

There are two common scenarios that would apply:

1) If there is a loss of fluid from the reservoirs, look for visible external leaks (calipers, corroded brake lines), or the vacuum seal between the master cylinder and the booster is leaking and allowing fluid into the booster.

2) If there is no loss of fluid, the master cylinder is bypassing internally.

Sugar Bear · #4 05-12-2023, 10:27 AM

Due to the age of the car a thorough complete brake inspection is in order if one hasn't been done recently. The symptoms described are a failing master cylinder. Pull the master away from the booster to confirm brake fluid isn't leaking into the master cylinder, it can damage the booster.

Good luck!!!

rgnprof · #5 05-12-2023, 07:42 PM

Thanks for the replies. I was also thinking master cylinder - I see no external leaks and when I went to fill the MC (thinking it would be low due to the pedal issues), brake fluid level in the master was fine.

Would you guys go with a Beck/Arnley? I believe ATE is listed as OEM...

Sugar Bear · #6 05-12-2023, 07:45 PM

I'd try for Ate; I think Beck Arnley is a re-boxer not a manufacturer. Not certain though.

aluminum · #7 05-13-2023, 04:46 AM

Before spending Money >
Check Your " Power Brake Booster " is holding Vacuum >
After sitting motor off a Bit like an Hour or longer for test purposes remove the Vacuum Line / or plastic fitting from the Booster
( is a check valve you have deal with )
and you well Hear the Vacuum leak out > if not you need a a new Brake Booster its leaking Vacuum . ( they are fun to change out BTW )
* When the booster go's it can seem like a Master Cylinder is some respects * Just saying

Todd Miller · #8 05-13-2023, 03:31 PM

Quote:

Originally Posted by rgnprof

Thanks for the replies. I was also thinking master cylinder - I see no external leaks and when I went to fill the MC (thinking it would be low due to the pedal issues), brake fluid level in the master was fine.

Would you guys go with a Beck/Arnley? I believe ATE is listed as OEM...

Pedal fade at idle, under lighter and constant foot pressure, is always going to be failed cups inside the master. Their nice edges get warn off, and in situations where the conventional brake fluid hasn't been flushed and replaced every 2 years, the sludge build up (due to brake fluid being hydroscopic), will gum up the cups and make them function poorly. You're "re-stepping" on the brake pedal proves it. That action causes the edges of the cups to expand, grab more fluid, and hold for a while.

The Beck unit IS probably an Ate, so IMO, worth getting. If you don't own a Mityvac with a brake bleed kit, this would be a nice tool to have. It'll allow you to suck fresh brake fluid (and air) through your entire system, flushing the lines and calipers nicely.

subforry · #9 05-19-2023, 05:02 PM

Start applying penetrating oil to the brake line connectors at the MC. If you have to disconnect the brake lines: As suggested, get a flare wrench. I have been told that vice grips can be used to slightly distort the nut and free it from the pipe.

rgnprof · #10 05-19-2023, 05:08 PM

Haven't had time to start this job yet - been working on my motorhome and my son's C240.

Thanks for the help.

meltedpanda · #11 05-19-2023, 06:02 PM

Spray all connections with pb blaster or similar. Let them soak

Idle · #12 05-24-2023, 04:37 PM

Sixty years ago this was a common, and very expensive, thing.

The cups in the MC had worn out and you needed to rebuild the MC. This involved replacing the cups and honing out the interior of the MC. This was more of an art than a science.

MC new were laughably expensive. Rebuilt units were sketchy. Uses ones were just as good.

Then new units prices started to come down and eventually everyone just bought new ones. And when was the last time you heard of someone honing a MC?

On removing the brake lines.... Yeah, you can use vice grips to remove the lines. But you will have a tough time installing them again after you have chewed up the flats. If you don't have the proper sized flare nut wench then get one. I have used vice grips to break the nuts loose but then put the vice grips away and use the proper tool.

Don't put excessive pressure on a flare nut wench. You will open the flare and ruin the wrench.

rgnprof · #13 05-27-2023, 12:50 PM

Getting ready to tackle this job - sprayed the lines and have the proper flare tools.

Any hints? Bench bleed the MC? Do I have to then bleed all 4 tires?

meltedpanda · #14 05-27-2023, 01:02 PM

yes both , a proper line flush with refill ( old school procedure ) will need to be done , unless you have a mitty vac system

lightly tap all around the caliper brake line nuts using a small hammer and nail set( or similar) May help ....

rgnprof · #15 05-27-2023, 01:18 PM

Thank you.

Just noticed that the ATE MC did not come with bleed fittings...I'm going to have to track some down.

Can I bleed the MC after I install it - as long as I keep the MC level?

Or better to do on the bench?

And I do have a Motive - need to see if it will fit the MC???