Brake Pedal Responsiveness w201
 

Morning everyone,

I have the following setup on my 190E:
400E Front Brakes (Textar pads / ATE Rotors)
500E Rear Brakes (Textar pads, ATE Rotors)
420SEL Booster (same as 500E)
500E Master Cylinder
Stainless Steel Braided Soft Lines

I started noticing this a few days ago, the responsiveness of my brake pedal has been diminishing. I had the fluid flushed a few weeks ago and the pedal felt great. Brakes engaged and stopped high in the pedal travel.

Now the pedal is softer, and the brakes engage lightly high in the pedal travel and when the pedal is 1/2 way down it feels firmer and by the time I stop the car, the pedal is nearly to the floor (I can put my foot between the pedal and the floor)

My brakes should be much more responsive and the car should be stopped well higher up in the travel, shouldn't it?

Does it seem like this might be indicative of a bad master cylinder? or something else?

Thanks,

Kris

fading brakes
 

check for leaks first. check for brake fluid in the brake booster.after that i would suspect the master cylinder.

Have you checked the level of the fluid in the master cylinder?

The reservoir is full to the max mark on the side of it.

I think I'm going to replace the soft lines and bleed the system and see where that gets me.

Whoever did the bleed may have let it get too low in one of the cavities. When you do the re-bleed it is important to follow the sequence on the location for the starting location to the finish, and not to let the cylinder get too low.

Kris, I guess you are aware that the master cylinder has a front & rear chamber. It's necessary to keep both full when bleeding the braking system calipers in sequence: RR, LR, RF, LF. Also, it's best to use a pressure bleeder rather than pumping the brake pedal.

Yes I'm aware of all that. I have one of those Motive Products Pressure Bleeders. When I replace the soft lines (currently stainless steel braided) I'll rebleed the system (obviously) and see if that fixes anything. If not...it might be time for a new master. I mean this one did sit on my shelf for a year or so..

For some reason I replied to the email that notified me of the new posts...WEIRD

FWIW, I've replaced both my W123 master cylinders due to similar problems within the last couple of years. The pedal started feeling soft and would travel near the floor before getting hard. Apparently one of the brake circuits had failed.

Craig, thanks for your input. Did you have a lot of leaks? I mean if the circuit failed wouldn't there be leaks or no fluid coming out of a bleeder during a bleed?

Very likely a bad master cylinder. Though, we recently experienced similar pedal response issues and found it was a stuck piston on one of the front calipers. Swapping it out with a good one resolved the issue. (Master cylinder also replaced with a good German one)

I've contemplated replacing the front calipers...

They're around $90/each rebuilt..
A new ATE master is like $150

I too suspect the master...

It didn't have external leaks, I think it was just bypassing internally. I didn't even try to bleed it because I hadn't done anything to introduce any air. Honestly, I just took it my indy and said, "fix this thing." A new master cylinder cured the issue.