Brake pedal slowly goes down to floor when right amount of pressure given
 

Hi,

On my 1980 w116, when I put light pressure on the brakes (about what would be needed to keep the car from rolling forward when in drive at a stoplight), the pedal sometimes starts to slowly move down to the floor, at which point the brakes still work but feel unassisted. Releasing the pedal fixes it, until I hold the brakes lightly again. I can also "play" with the pedal, pushing lightly and releasing just right, and get the pedal to go to the floor.

I looked at the reservoir and it appears to be filled properly. There is a vacuum leak but it's not severe (i.e. the door locks work fine for a few minutes after shutting off the engine, the engine shuts off properly and quickly, the AC works).

I have not checked the brake pads yet.

Thanks!

Checking out this article now.

Most likely : worn mater-cylinder seals
flexible hose pipe is swelling up under pressure.
Air ;re-bleed with the front raised up.
lots of advise on this topic if you search
regards
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Thanks...I am sure that's what it is too.