Overboosted brakes?

[waltklatt]

Hello all,
I know this has been discussed many times about the hard pedal travel.  
But what about the opposite?  
The pedal almost sinks to floor while driving long and hard.  In stop and go traffic the pedal feel is normal with a good firmness.  
It's still working fine, but am puzzled to the sinking pedal after a long drive.  
For now I have loosened the 19mm captive nut fitting on the vacuum line at the booster.  So the vacuum can bleed out and I can keep the 'normal pedal feel.
Any thoughts to what is wrong?
Walter
1967 220SL-diesel

[rwmastel]

quote:Originally posted by waltklatt

The pedal almost sinks to floor while driving long and hard.

I'm no race car driver, but isn't this what the pros refer to as "brake fade"?

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[waltklatt]

Rodd,
Yes that is absolutely correct, but in this case the brakes are not boiling over or hot.  Just a long straight drive without braking and then the pedal lacks feel till all the way to the bottom.  I have put the back of my hand to each wheel to feel the heat and all are reasonably cool/warm.
Might be the vacuum pump is overboosting the vacuum.  I will try another larger *booster to see what happens.
*note: I think this is a conversion problem with some incompatible parts.  Besides all the parts coming from MB's, but the booster is from a W124 series, the MC and engine with pump are W201 series.  Think I'll shoot for a W201 booster to see what happens.
Thanks,
Walter
1967 220SL-diesel

[al_lieffring]

Walter, It doesnt sound like a booster problem to me. Too much vacuum assist would make the brakes grab with very little pedal pressure. It wouldn't make the pedal travel farther before the brakes engage. Check the lateral run out of the rotors. I can only guss that you have disk brakes in the rear. If the rotors have any side movement as they rotate it will push back the caliper pistons and when you push the pedal fluid has to move them back out again untill they make contact with the rotors. Loose front wheel bearings could be a posibility too.
Try pumping the pedal a couple of times, and see if it hold at the normal height on the second or third pump. If so it could be a rotor run out problem.

al


113-042-10-014715
built 11 Jan 66
904/396 blue, Ivory Tex
condition- rust bucket

[jeffc280sl]

Walter,

Do you have access to a vacuum gauge?  I know we have talked about intake vacuum pressure before on this site.  I can't imagine a circumstance where too much pressure would result.

Jeff C.
1970 280SL 4-speed