Today I took the ole Benz for a ride for the first time in a few weeks and noticed the brakes were very mushy. I pulled over and the rear part of the reservoir, closest to the brake booster, was empty. The grommet at the bottom of the reservoir going into the master cylinder (right?) was a bit wet.

I refilled the brake reservoir and went for a drive, its a bit better but not all the way, I will bleed it this weekend.

Other than replacing that gasket, what else should I do?

Anyone who has replaced that gasket have a few tips for me?

Thanks!

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1985 300D (CA, 173K)

Dynamite Biodiesel Co-op, Phoenix,AZ

It could be the grommet that the reservoir mates to the master cylinder has gotten old and needs replacing, but it's also possible that the cylinder itself has finally gone bad. I had to help a buddy replace one this year, they are good parts and don't fail often, but they still do wear out over time. Luckily they aren't too expensive and are easy to change. It may be a good idea to just replace the master cylinder and then bleed the brakes, last thing you want going out on a hill is the brakes.

if the grommet is wet, new ones are pretty simple to install, just smear some brake fluid on the inside of the grommet, and pop them into the MC, then reinstall the reservoir... see if that takes care of it. you may have to bench bleed the MC to get fluid back working in it.

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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 have heard a knocking sound from that area from time to time... very loud... clanging for a minute or two, I stop, turn off car and drive away it goes away.

Does that sound like the MC to you?

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1985 300D (CA, 173K)

Dynamite Biodiesel Co-op, Phoenix,AZ

Or there could be a leak somewhere else in the system.

Left to their own devices, master cylinders are pretty quiet.

Could be, and it could be something as simple as the bleeder screw not closed 100%, but mushy brakes, wet grommet, and this new info of a loud clunk leads me to believe the master cylinder may have lived its useful life and kicked the bucket.

IIRC a bad booster usually results in a hard pedal, so luckily that isn't the case here, they are a lot more expensive.

I cannot imagine what the clunk could be in relation to the MC...

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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!

Evidently, it's the sound of the master cylinder "kicking the bucket."

Apparently reading comprehension isn't so strong with you, don't froget the qualifying "may" in there. Ntuarally, it could be a host other things, but the leaking at/around the grommet and mushy brakes are good indications that the prudent course of action on a 26yo car is to replace a cheap part to help ensure brakes are functioning at peak performance. Good luck with your reading comprehension goals in 2012.

BTW, when the master cylinder died on my old Chev Subruban back in 09, it sure made a nice bit of noise quite similar to a clunk...what are the odds?

I dont think you got the little joke there. No need to throw insults.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

A spongy brake pedal is a symptom of air in the brake lines. Air in the brake lines is a natural consequence of an empty reservoir. An empty reservoir in the natural consequence of a leak. None of which is necessarily an indictment of the master cylinder itself. I would hate to see the OP replace his master cylinder (age unknown) only to find that he has a leaking caliper seal or brake hose.

The prudent course of action is to positively identify the source of the leak.

Ditto!

Leaky grommets on the master cylinder are on the non-pressurized, reservior side. I would be very suspicious of a flexible brake hose at the caliper. If you were getting all of that fluid loss at the master cylinder, wouldn't you also see more tell-tale signs of fluid loss than the leaky grommets? For instance, fluid in the surrounding area would be very evident. Even with leaky grommets, the reservior would remain nearly full, in the relaxed state. Gasket change? Are you talking about the gasket between the booster and the master cylinder? I don't follow how the fluid loss and this relate. It would be better to resolve the fluid loss first.

I've recently had wet grommets at the master cylinder and there was absolutely no effect on braking power. Yes, determining the source of the leak will be money ahead, not only for the frustration factor but for safety reasons.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Is it a manual or auto transmission? If a manual, the leak could even be in the clutch system.

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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 don`t read the possibility of the fluid leaking into the brake booster from the MC.

I had the peddle go to the floor one time. the rear section was empty. when I removed the MC, the booster was full of fluid to the top of the opening in the center.

I had added fluid once in a while, but maybe this had been going on before we purchased the car.

The booster is pricy, but being full of fluid, and not knowing if it would cause problems with the rubber inside it, I replaced it and the MC.

Charlie

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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works