420SEL almost stalls when brake pedal is released
 

I remember reading something about this awhile back but my search turned up zilch.


If while sitting in park with the idle at about 600rpm, I press the brake pedal, the engine will almost stall, then jump to 1000rpm before returning to normal (600rpm) upon release of the pedal.


I'll stipulate to being a bonehead novice but where do I start with this one?

IF you just sit and hold it, does it have the problem or when you pump?

The brake booster works off of a vacuum supply. So, if the seal fails, it could leak vacuum.

Do you hear it hissing in the car ?

Fredsegal,

I'm the one who posted this topic some time back, and I can tell you for sure the problem is with the Brake Booster Pump! Affirmative.

Read my post in this thread:
http://www.peachparts.com/shopforum/mercedes-benz-performance-paddock/18698-upgrade-master-cylinder.html?highlight=brake+booster+pump

I've replaced so many other parts (brake rotors and pads, idle control valve, ignition control module, plugs, cables, cap and rotor etc.) that I'd almost wanted to give up. But I was lucky at the end, albeit much poorer. ;)

Go replace your booster pump, it must be leaking like mine was.

I don't hear a hiss and I can not duplicate the problem so far today. Yesterday it only happened when I held the brake down and released , not when I pumped it.