85 300sd intermittent low brake pedal
 

I'm going crazy trying to figure out the brake problem on 85 300sd. I started when pretty much all my calipers started leaking, so I replaced all 4 calipers and hoses. After that I realized the pedal would sink so I assumed I over stroked the master cylinder and got a new ATE one. I bench bled the master, then bled from the fittings on the vehicle, then power bleed a gallon of super dot 4 through all the calipers. Now the pedal will intermittently go very low, in my garage it feels pretty good and firm but if i drive it around sometimes it will go almost down to the floor. I am stumped what to do next since I don't see any hard lines that are leaking and I also replaced the booster a few months ago.

Wrong section, but did you make sure there are NO air bubbles as the system was bleeding? You need to see a solid stream of fluid thru the tube before closing the bleeder screw.

I suspect you may be losing fluid in your reservoir, thus air may be getting in somehow. Re bleed the brakes and see if you can burp the system of any air bubbles. Also, try to clean the calipers and brake hoses if they are dirty with brake cleaner and then stomp on the brake pedal, check to see if you have any fluid leaking from the hoses or from the seals of the caliper. I was losing fluid and did the "stomp and see" procedure mentioned and found out my rear right caliper was leaking. I didn't bother with a remanufactured caliper and just bought a seal kit for dirt cheap to fix the issue.

If you're not losing fluid from the brakes or lines, then check to see if the fluid reservoir is sealed properly. If you stilk cant find a leak, then you'll have to see if the fluid has gotten into the brake booster. According to the FSM, if a small amount leaks into the booster you're fine. But over a certain amount and the manual demands that the brake booster gets R&R'd.

You say you bench bled the MC. But as I remember it has two compartments for fluid and the one in the rear is hard to get filled.

Might check that.

I also seem to remember that there is a rubber button/switch on top of the MC and you are supposed to push it to reset. Not sure what it resets. Maybe it is for your low brake fluid light. Of course that would have nothing to do with the pedal going to the floor.

Good luck.

Check your wheel bearings. I had a similar problem and found that one of my rear wheel bearings had loosened after it was replaced.