So, I had a caliper in the back start to drag yesterday, so I replaced it today along with both rear hoses and the rear pads. Bled out the brakes and everything seemed fine. Piled the family in the car and drove about 3 miles and the car started feeling very sluggish on the interstate. As I started looking for an exit I could smell the brakes and as I was exiting I could see smoke. I went about two more blocks so I could pull onto a side street and out of traffic and was having to mash the accelerator just to go 10 mph or so. The rear brakes smoked for quite a while as we waited for a friend to come pick us up. I went back an hour or so later and the brakes had freed up, but I checked a few things first. I had overfilled the master cylinder, which I have read can cause the brakes to drag. I drained some fluid out and drove home without any problems (though I didn't use the brakes until I got within a few blocks of my house.)

Can anyone explain to me why an overfilled master cylinder can cause the brakes to drag?

thanks,
ben

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1985 300D 170k

First hydraulic fluid no doubt expands when it heats up. Meanwhile the MB brake fluid reservoir has flow and breathing channels designed to both trap grime (driver's side thin triangular channel between fore'n aft double compartments) and equalize pressure between the two, yet allowing each 1/2 rez to operate independent of the other. Finally the rez mounts slightly tilted so if ye overfill it to the cap then the back 1/2 reservoir caint breath at all.

General rule for all MB fluid levels: *less = better than more*... Engines burn more oil when topped entirely up; trannies kick like mules when even slightly overfilled; hydraulics niether flow nor breath properly when maxed out over top line, basically choking the channels between each 1/2 rez.

Pulled the rez and took it to kitchen sink to clean it out when doing master cyl work on the 240D and was astonished at its complexity of inner channels and the grime trap passage especially.

I just got back from a second test drive and the rears were locking down when I got home (about a 5 minute drive). I cracked the hard line coming out of the master and when some fluid bled out the wheels spun freely again. I'm thinking I need to rebuild the master as the fluid is now at the max mark (not over). I guess I've got nothing to loose if I suck a little more out and try one more time to see if it tightens up the rear wheels. I'm going to rebuild the calipers now because of the exesive heat (looks like the piston dust boots might be a bit cooked) so I may as well rebuild the master at the same time.
AHHH, the joys of owning an old car...
ben

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1985 300D 170k