Brake pedal drops when parked overnight
 

Hi all, I'm a new member to this forum from L.I., NY. I drive a '94 E320 w/90K on the clock. I do most of the work on the car myself as a hobby. My present problem with the brakes is this; when parked overnight or extended period of time, the brake pedal drops to to floor, one pump and the pedal pressure is fine. On extended highway drives, the pedal sometimes also drop very low when I step on it by a toll booth or exit ramp, one pump the pedal pressure returns to normal. This been happening several months. Master cyl seems fine, it holds pressure while stepping on it hard with engine off and with engine running. Brake booster seems fine, normal pressure needed to stop car. I bled the brakes a few months ago, pedal pressure fine a few weeks, then reoccurring problem. I then changed the front rotors, pads, rear pads, flushed brake fluid; pedal fine for a few weeks, now it's dropping again. Anybody got comments on this? It's driving me nuts. Can't figure out where's the problem.

Master Cylinder. If there are no external leaks I can't see how anything else could be at fault. I believe the condition you are describing is typical when the internal seals are begining to fail. When cold the seals shrink a little allowing the fluid to pass around them, after you pump it a couple of times and some heat is generated the seals swell back to close. It's dangerous to drive it in my opinion and should be replaced, TODAY.

absolutely the master cylinder is starting to fail.

i wouldn't drive it... this happened to me before in another car.... that said.. i did drive it to a dealer..and they switched it out in under 30 minutes.

My VW was really bad about that sort of thing, and it was always the master cylinder. For some reason, either it was a bad design, or the extreme weather changes through the seasons would cause the seals to prematurely fail.

low brake pedal pressure
 

There could be several reasons for this. a)s mall leaks in master cylinder, b) small leaks disc brake cylinder seals, c) inadequate bleeding brake lines. Does fluid level change in master cylinder change? Hope this is possible answer. Abe G

Thanks guys, I'll r/r master cyl asap. There's no loss of fluid in system, no leaks from wheel cyl's, and good pedal pressure after one pump. I'll change the master cyl and report back any change in pedal drop. Anyone got a link to r/r'ing a master cyl?