Brake pedal "fade" after start up

[smallblock450sl]

My 1978 450sl has a very "strong" brake pedal feel when the engine is not running, when I start the engine the pedal goes "soft". Sould there be a "check valve" from manifold vacumn to brake booster ? Any help would be great. Thanks Dave

[ScottinSoCal]

Originally Posted by smallblock450sl

My 1978 450sl has a very "strong" brake pedal feel when the engine is not running, when I start the engine the pedal goes "soft". Sould there be a "check valve" from manifold vacumn to brake booster ? Any help would be great. Thanks Dave

There is supposed to be a check valve, but the normal feel of the pedal with the engine running is kinda soft. Certainly nothing like my other two cars (both of them 21st century technology).

I am getting ready to change the brake lines. The ones on it are original, so they're 30 years old. I've been told that changing them to braided cables (standard lines, but with stainless steel braid wrapped around them) firms up the pedal feel quite a bit. I haven't decided if I'm going to go that route or just replace them with more OEM lines. OEMs made it through the last 30 years, and the stopping power is great, even with a soft pedal.

[Mike Miller]

I went the whole Monty on my 1974 3.0 CSi BMW. I put Kevlar lined, stainless steel braided brake lines on it. Actually, first I replaced all the lines with OEM. It made a difference over the old lines, but then someone told me about the braided lines. For the price, it was worth the second change. Much stiffer pedal and nice solid braking "feel". Your MB should react the same way.

A simple job at a relatively cheap (by MB standards) price. GO-4-It.

[ctaylor738]

When you start the engine, you get vaccuum assist, which makes the pedal easier to push down and will amplify any problems with air in the hydraulics or a failing master cylinder. So you might try bleeding to see if there is air in the lines. A sign of a master cylinder going south is having to pump the brakes to hold a stop at idle, like at a long red light.

When doing brakes, I've learned to check the pedal with the engine running before taking the car off the jack stands. I have had a rock hard pedal go soft with the assist.