Sinking brake pedal, 300SD - PeachParts Mercedes-Benz Forum

the heap · #1 04-18-2009, 04:40 PM

So my buddy complains of sinking brake pedal on his '83 300SD, I agree to change brake master cylinder, which was just accomplished. Bench bled the cylinder before installation, master cylinder is new. With the engine OFF, the brake pedal is firm. When the engine is running, the brake pedal is sinking, actually a bit worse than before. The brakes themselves have not been bled yet.

Is this a brake bleeding symptom, or does this suggest a leaking brake booster? I'm thinking with the engine off, than air in the brake lines themselves would manifest themself as spongy brake. When engine is running, the vacuum now adds the booster into the system. So sinking pedal with engine on points to booster, and firm with engine off suggests the brakes are fine without bleeding?

Suggestions?

'77 300D 355k mi
'95 C200 180K mi

tangofox007 · #2 04-18-2009, 05:33 PM

Quote:

Originally Posted by the heap

The brakes themselves have not been bled yet.

I would recommend that you remedy that situation.

leathermang · #3 04-18-2009, 05:59 PM

You have to bleed the brakes first... like TF007 said...
but after that I swear I read somewhere once that a sinking pedal was often caused by a brake booster going bad...
But the last time this symptom was posted everyone yelled Master Cylinder...

compress ignite · #4 04-18-2009, 07:01 PM

Leathermang,

You're correct!

However,in the O P's situation...He has not completed his R+R tasks ...so
There be no way to tell what's in store for him until he does!

leathermang · #5 04-18-2009, 07:49 PM

Compress Ignite, Where were you on that last thread ? LOL, I really felt strange that NO ONE thought I could have been correct... but you know how stubborn I am... I did not retract it... I just kept hoping someone else had read the same thing... I even Googled it and could not find anyone who said the same thing....

the heap · #6 04-18-2009, 08:32 PM

So, I bled the brakes using Motive Power Bleeder, starting with the RH rear corner, ending with the LH front. Some bubbles, but not many extruded through the clear plastic bleed hose. It's no different than before, still sinking. It's perhaps somewhat worse than when we started. The new master cylinder comes with a new O-ring between MC and booster, which was installed. Going for a test drive through the quiet neighborhood now, will advise results.

I'm thinking booster diaphram.

77 300D

leathermang · #7 04-18-2009, 08:46 PM

Quote:

Originally Posted by the heap

I'm thinking booster diaphram. 77 300D

Good Thinking !!!!

phyxer · #8 04-18-2009, 09:02 PM

A bad booster should give you a hard petal. Just bleed them again. When you think you have bled them enough, do it some more.

leathermang · #9 04-18-2009, 09:09 PM

A totally bad booster usually does..
But one that has a small leak will pump up... then at a stop light slowly go to the floor..

phyxer · #10 04-18-2009, 09:17 PM

Check the booster with a vaccum pump to see if it holds vaccum.

tangofox007 · #11 04-18-2009, 10:08 PM

Quote:

Originally Posted by leathermang

A totally bad booster usually does..
But one that has a small leak will pump up... then at a stop light slowly go to the floor..

Which is exactly what a leaking master cylinder will do.

leathermang · #12 04-18-2009, 10:17 PM

Did you read the part in the first post where a new MC had been installed ?
Playing the odds here.... between bad new MC and Booster giving way....

tangofox007 · #13 04-18-2009, 10:33 PM

Quote:

Originally Posted by leathermang

Did you read the part in the first post where a new MC had been installed ?

Affirmative.

the heap · #14 04-19-2009, 12:14 AM

Thanks for the advices here. I read the shop manual for my 300D, looking for a brake booster test, some way I can isolate the symptom. If I disconnect and successfully plug the vacuum hose coming from the vacuum pump to the booster, and tap my Mityvac into one of the tees that come off of that hose, can I use hand applied vacuum to depressurize the booster and see if it holds? Or is there a slow bleed port on the backside of the diaphram, so something else I don't know about?

Diesel911 · #15 04-19-2009, 01:54 AM

Quote:

Originally Posted by leathermang

Did you read the part in the first post where a new MC had been installed ?
Playing the odds here.... between bad new MC and Booster giving way....

Not taking sides on the MC verse Booster issue. But, was it New as in never rebuilt; or is the term New used to mean another MC.

Installed an Rebuilt MC and after 2 bleedings the pedal would still creep down a little but I left it on and about 1 year and 4 months later the Pedal went to the floor.
I replaced it with a Brand New one that cured the problem about 2 months ago.
My point being that the rebuilt MC was not good to begin with. So if the New MC is really a Rebuilt MC it could be a bad rebuild job.