The brake pedal acts up on the 220S. If I'm holding it on a hill, stopped in traffic, I can feel the pedal stiffening under my foot and then braking efficacy fades. One time while cruising around in some hills, I completely lost the use of the them! Fortunately it was a desserted area, at the bottom of the hill.

Also, after much use of the brake pedal, as in a hard stop, the next application of the brakes involves a pedal that has a lot more travel to get things slowing down.

The master cylinder is new and the booster has been rebuilt recently. I bled the brakes after I did the work and later asked my mechanic to bleed them, but he said they didn't need it.

I'm using DOT 3.

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6

The 2nd symptom may come from using Dot 3 - it boils at a lower temp than Dot 4, which is what should be in there.

#1 sounds like your booster is failing, like it's leaking and/or not recieving enough engine vac to keep it going.

But it does also sound like your master cylinder is bad... you say it's new, but it doesn't sound like it's working right. I'm going to assume the rebuild to be bad before the new master cyl, though.

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Two or four wheel disk brakes?

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Front wheel disc.

The master cylinder was a new Mercedes replacement. Maybe the vacuum to the booster is at fault.

There is no loss of brake fluid either.

__________________
63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6

If you have the remote booster, make sure you have all the air out of the secondary master cylinder, else you get strange things going on.

Otherwise, if your boost fades away under constant pedal pressure, you have a booster problem or a bad check valve or leaking hose -- intermittant boost can be either, as well. A loose intake manifold fitting can drive you nuts. The vac fitting on my old Dodge Aries came adrift periodically, causing similar symptoms.

If you have not done so, replace ALL the brake hoses, too -- they can come apart internally and act as one-way valves. Usually causes the calipers to lock on, but occasionally will cause one to get no fluid when you step on the brakes.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Whilst I don't think this could be the same problem, I had a similar thing happen in my fintail.

After having to brake hard following some idiot pulling out in front of me, my pedal went to the floor, then back to normal, then to the floor again.

It turned out that one of the pads on brake shoe of the rear drum had separated from the metal plate. It was turning with the drum but when in the right position the brakes worked fine...

Hello,
The brake booster is the *hydrovac*(remote) type, right?
The booster is losing vacuum AND there is air in the hydraulic circuit. Pressure bleeding (NOT vacuum bleeding) in the the correct sequence will get the air out, but it may be an internal leak in the booster. Experience with right hand steering Alfas and BMW that have TWO hydrovac servos has thought me that it takes a lot of patience and work to get the air out of these brake systems.
Hope that helps.

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Nachi11744

Thanks for the help.

It has a remote booster.
I'll replace all the lines, replace fluid with dot 4, check the drums and check vacuum connections. I'm not using any fluid, so I'm hoping the booster is okay.

__________________
63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6

If I let the brake fluid get too low in the old Toyota Crown (man, that was a car!), the front brakes would lock up from the air in the remote slave cylinder. Had to stomp on the brake pedal to get it to let loose -- usually got a "boing" out of the booster. I could have just replaced the leaking proportioning valve, but hey, I was 22 and poor.

Something floating around in a drum will certainly cause the pedal to jump up and down, but unless it's brake shoe matterial, it will almost always grind or screech, too. Worth a look. Rusty brake shoes on the rear brakes sticking on the backing plate can also do this.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!