RPMs increase when brake pedal pushed

[gugel]

I realize this topic has been discussed before, but I have to say the discussions left me puzzled.  

My idle speed increases by about 200 rpm when the brake pedal is depressed.  It seems to me this has to be a problem within the booster -- if there was a leak in the vacuum hose or the connection to it, then that leak would presumbly be there all the time, not just when the pedal was depressed.  Has anyone actually fixed this problem by replacing the vacuum hose?

If the leak is indeed within the booster, then where is it likely to be?  Only the diaphragm, or are there other possibilities?  The brakes work fine otherwise.  

Clearly I'm trying to avoid rebuilding the booster if possible.  The original booster was replaced with a used one from Gernold a couple of years ago, so I find it somewhat hard to believe that it needs replacing or rebuilding again.

Chris Earnest
'65 Euro 230SL

[al_lieffring]

Chris

If the idle picks up when the pedal is moving then drops back to normal when pedal is held down, the problem is there is too much pedal travel.

If the idle comes up and stays up the entire time the pedal is held down the booster is leaking internally.

If the first is true you may need to check the adjustment of the rear brake shoes, they do not adjust automatically on these cars and require periodic adjustment.

I had to replace my booster when I first got my 230sl because It was leaking when the pedal was held down. That was in 1976, I'm sure they cost more now than the $200 I spent back then.

Al Lieffring
66 230Sl
Jones'n for a new gas tank

[WTF]

This topic reminds me of an incident that occurred early this summer while driving our 280SL. I was just leaving the autoroute and turning a tight circle.I depressed the brake pedal in a normal fashion, but as the vehicle hardly slowed, I immediately pressed as hard as possible not to hit the guard rail.Fortunately I retained control during this manoeuvre.When I released my pressure on the brake pedal, the car surged ahead, without even my foot on the gas pedal! What a shock!I hit the brakes again, and pulled to the side of the road, out of traffics'way.I promptly shifted the automatic transmission into neutral to eliminate any further drive motion and again noticed the engine accelerated to about 1000-1500 rpm without my foot on the gas pedal. I quickly shut everything down.Once I had stopped shaking,I got out of the car,and lifted the hood to see if I could identify a likely reason for this nightmarist performance.(I'm very happy I was driving and not my wife- who may have had serious trouble avoiding an accident.)What did I find? The Mercedes-Benz original small plastic check valve installed in the vacuum line that goes accross the engine to the Vacuum Brake Booster had simply split into two pieces. Consequently, no more vacuum boost to assist the braking, and an excess of air flow  into the intake system that caused the engine to accelerate as it did.Obviously 38 years of service was probably more than MB had in mind when the valve was installed.For the sake of safety,and at little cost, I recommend that all of us should replace the original valve with a new one. The peace of mind is certainly worth it.

W.T.Forsythe