The brakes on my 77 300d require MUCH higher pedal effort than the brakes on my 85 TD. I have all vacuum components disconnected except the ignition switch and the brakes. The vacuum is at about 18" which I think is appropriate for my altitude (5000'). I can also pull a 20" vacuum on the brake booster and it holds it. Vacuum seems to respond approrpriately to brake use. Drops a few inches for each use, then rebounds. Rebuilt the vacuum pump a few months ago.
Why would the pedal effort be so high? Jacked up the car and both front and back brakes (on one side at least) seem to be performing correctly.
I'm pretty sure this big difference in pedal effort between the two cars is a pretty recent phenomena, although I only noticed it after driving the 77 after not driving it for a few months. (My wife usually drives it)

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Can you perform an experiment?

Drive the vehicle with the vacuum properly connected to the booster. Observe the braking behavior carefully.

Then, disconnect and plug the vacuum to the booster. Drive the vehicle again.


You're looking for a significant increase in pedal effort without vacuum. If you don't get it.......the booster has failed.

Be very careful when performing the second test.........the vehicle might not stop without significant pedal effort.

I haven't performed precisely that test, but the booster seems to be working (at least somewhat). With the engine off, I can pump the brakes, getting rid of all the vacuum in the booster, then start the engine. I can feel the booster kick in as the pedal sinks a fraction of an inch as the vacuum comes up.\
I know what a total lack of boost will probably feel like, and it's not like that. I don't have to put both feet on the pedal and push like hell to get it to stop. But on the 85 I can use toe effort to stop the car. The 77 requires foot effort. I wish I had a little scale I could put under my foot to get comparative numbers. Something is not right but it is not completely wrong. I really don't want it to go completely wrong when I'm driving it
It doesn't seem like a master cylinder problem since there is no sponginess and the pedal does not pump up.. It just seems as though I'm getting about half of the boost I'm getting on the 85.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

The SD is somewhat similar. It requires you to use your foot to stop it. The SDL only requires a very light toe pressure. The SDL has relatively new pads and the SD has pads that are ancient. I have a suspicion that the condition and age of the pads is relevant.......but.......no data to support it.

I'm not sure what year they started putting wear sensors on these cars, but my '74 and '75 Diesels did not have them. The symptom of worn out pads was not a metal-to-metal scraping sound, but a HARD PEDAL. It seems the pads limit on someting in the caliper and will only extend so far beore they limit. Is it possble you just need new pads?

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)

It's got wear sensors on it. (front only I think--Is that correct?). i replaced the front pads 18k miles ago. Perhaps it's time for rear pads?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Nope. Rear brake shoe have plenty of material. No problem there.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

What's the chances that the pads are glazed? This would increase the required force to stop the vehicle.

I don't know. What would cause glazing?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I'm not positive of the cause myself. But, if the pads take on a sheen and have somewhat of a polished surface........they are glazed. If they are the original rough surface, they're fine.

It may be uncommon on M/B pads.........but, might not be impossible.

I keep going back to things that actually happened to my '74 and '75 since this is a '77 being discussed.

On both my '74 and '75 the rear brakes stopped working with plenty of pad available. The symptom again was hard pedal and ineffective braking. The problem was that the calipers were stuck. This was easy to spot because the rear rotors eventually rusted over. I did not notice because the problem came on over a long period of time and both sides were identically stuck. Go figure!

If the rear rotors appear 'normal' this would not be the case. Good luck!

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)

I thought that perhaps either the front or back brakes were not working correctly. I jacked it up, spun a wheel and hit the brakes. Brakes seemed to grip normally.
Is there any adjustment in the system at all to regulate the amount of boost? What would make the boost reduce in strength?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I put a post up just last night about brake failure on my W124. Might want to read that. Someone replied back that my problem might be due to the brake lines needing replaced.

jeff
1991 300d, 105k

you could try swapping the pads from the 84 to the 77 and see if it improves. also, you could have thin rotors. so the possibles quoted here are glazed pads, frozen or partially siezed calipers either front or back, bad lines and thin beyond spec rotors. check all this and report back.
John

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I don't believe that thin rotors would cause a hard pedal. The capability of the vehicle to stop under heavy braking loads is compromised because the thin rotor won't be able to handle the heat........but........it should not create the symptom of a hard pedal under normal driving conditions.