master cylinder or ABS controller
 

My son, 17 and new license holder, was driving my 85 500sel (W126) Friday night and he called me all in a panic. He felt the pedal slip, "going to the floor". He was going slow and in a parking lot no less, so he pulled into a spot while pumping the brakes and parked the car.

So I showed up with a fresh bottle of fluid an flashlight and found nothing out of the ordinary; no odd pedal behavior or fluid puddles, nothing. So I drove it home (a bit more than a mile) nice and slow and the problem didn't repeat itself.

I was going to give the car new shocks and rubber hoses because it's relatively new to me with about 160k miles on it, the minute I have some free time for it.

My limited experience with ABS controllers suggests that there's nothing that would go wrong with it to trigger the feeling of the foot going to the floor. That means if there is something wrong, it's likely the master cylinder and I should add that to the work list I already have planned out of an abundance of caution.

Sound reasonable?

-CTH

Sounds like internal bypassing in the master cylinder; fairly common. It's also an off again & on again condition.

It is the classic symptom of a failing M/C due to an internal leak. The symptom can sometime be reproduced by keeping very faint pressure on the pedal e.g., waiting at a stop light and preventing the car from rolling.

Check everything e.g., for being due such as hoses also but replace the master if you cannot find any external leaks.

Good luck!!!

Thank you both. I am a big believer in the basics and for this, a new master is one of those.

-CTH

be advised mb has a bulletin about new master cyls sinking after install and being returned as defective. the new ate cyls will sink almost to the floor with slight pedal pressure but work fine for normal braking. i have it someplace and if i could EVER find it, i'll post it. applies to 107s, 123s and 126s.