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I drive my '77 280SE every day.  About six months my brakes were really spongy so I replaced the master cylinder.  Everything solid ever since with nice tight brakes.

Last night I parked my green goddess (she's silver green) per usual, but this morning as I backed out into the street, I put my foot on the brake pedal and it went all the way to the floor.  I still have brakes, but they are extremely weak.

Assuming it's NOT the master cylinder, what would cause such a dramatic overnight loss of brake power?

Well, the brake power is the same, just that the pedal went to the floor.  That can mean only two things assuming there is no air in the system (I assume this because the brakes were fine the night before)

1. The master cylinder is bypassing.
2. The calipers are bypassing.

You won't see a leak from the MC but you will from the calipers.

Regards,

Carl.

I was thinking of the piston sticking in the master (which would still allow the pedal to return). Carl, the bypassing, is that like the piston pushing fluid back and forth in and out of the reservoir, basically bypassing the seal and back into the reservoir? And I guess the bypass in the caliper seal then leaks out as you say.

My money would be on some sort of problem in the master for you to have such dead pedal though.

Of all the 116s I've owned, only one I neglected the brakes long enough to wear right down to the min pad thickness.  On that car the pedal got very spongy, and the brake place explained that the caliper design resticts the pad from wearing so far down as to have metal-on-metal.  The pedal gets spongy by if you push hard the car will stop just as well because there is still brake material on the pads, but you are pushing against the resistance of the metal backing plate.  I didn't give it much credence at the time but it does make sense... if you ignore the brake warning light (or don't have them fitted as was the case with me) then the car attempts to warn you but making the pedal feel different.

I suppose I would start by making sure all your brake pads are of decent thickness, the warning wires are connected, and that the brake fluid hasn't dropped too low and introduced an air bubble into the line.

I.

Here's my guess......the normal min and max fluid levels on a reservoir should approximate the difference between new brake pads and worn ones, the idea being that with a 'reasonable' amount of wear, ie, not down to the metal, will draw down more fluid to compensate for the caliper pistons being nearer to the rotors.
The level then rises to the max again when new pads are fitted.
This is why you should never 'top-up' brake fluid, if the level is down there is a reason; either a leak, or evaporation of the moisture already absorbed, therefore requiring a flush out and new brake fluid throughout.
If this case shows no leaks, and let's assume as the master cylinder was replaced recently, mabryt, the most likely cause is that you have worn the pads down enough for the level to get low enough to allow air into the system.
Check the fluid level, is it low?
If so, and you are going to have to jack it up and get the wheels off to check the calipers are not leaking anyway, so you may as well get some new pads ready to put in if they are needed, (if not, put them back on the shelf in the shed for next time).
Leaking caliper? There's your answer.
Now it is back to bleeding the system, but if there is a chance air got in at the m/c, you will have to bleed the pipe connections from there first, and then at each wheel, preferably starting with the wheel furthest from the m/c.
The normal symptom of a master cylinder not holding pressure is to feel the pedal creeping down when holding the car stationary, if you have had this recently, say when sitting in traffic, or when in drive at a red light for example, then there is your culprit, but it still unusual for a master cylinder to fail completely overnight, and the other possibility for it failing after only a few months is if you were pushing the pedal too hard when you were bleeding the last one fitted as this can distort the seals if they bottom out. If bleeding by pumping the pedal, do not go all the way to the floor.
Good luck and let us know what you find.

RIGHT- THANKS TO ALL very much for your careful advice.

HOWEVER- I realize now that I FORGOT to mention something.  My break pads and rotors were replaced a few months ago and the brakes were bled as well.  However, the brake warning light would not go out.  My mechanic - and excellent old Czech gentleman who knows his stuff - said he couldn't figure it out, when all of a sudden it turned off.  The light would still work when I set the parking brake, and turn off when it was released.  But a few days ago, the light came on again.  I thought to myself- I know the brakes are in good shape- I wonder if I can just disconnect the wire coupling to the MC so the dumb light doesn't go off?  If you look at the photo, the small wire on the left side is what I disconnected- and it worked!  Brake light went on and off when it was supposed to, i.e. when the parking brake was engaged or not, and no other trouble.

And then, as I said, the very next day, I have spongy brakes.  Is that wire more than just a sensor?  Oops?

Also, if there were leaking calipers, wouldn't I see leaking brake fluid under the car?

THANKS FOR PUTTING UP WITH A BONEHEAD POST THAT FORGOT A KEY PIECE OF INFO!

I am pretty sure that the wire to that sensor is to indicate an imbalance of pressure between the two circuits ie. front and rear brakes. Best not to remove such wires since they are telling you something is wrong. Is the photo current? If so you haven't lost any fluid and leaks are not a problem. For air to get in, the level would have to be way down the bottom of the reservoir for it to get in that way. If it was me I would be looking for a fault in the new m/cyl. Perhaps a poorly fitted seal that is letting air past it? a seal between the two circuits fitted the wrong way around so no leaks from the outer seal being fitted right? Just suggestions.

I agree with Craig.

I would change the master cylinder with a new one. (Not recond)
(It may have an internal leak.)

If there are no leaks at the front or rear calipers and the pads are good, that is all that is left.
Make sure it is bled well when you change the M/C and it should all be good.

The other week I had a small problem with the 6.9. Brakes worked well, but sometimes when I braked, the pedal went down about 10-20mm more than normal.
It would only do this sometimes, so I fitted a new master cylinder and all's well.

(The brakes were telling me there was someting wrong and it wouldn't be long till I had complete brake failure.)

On some master cylinders the pressure imbalance switch switch screwed in to the side of the m/cylinder has to be removed during  bleeding to allow the sensing piston to move to the OK - pressures are the same position. It is possible to damage the switch on these master cylinders if this in not done.

However, there is a problem somewhere in the system, not just a wire.

koan

Again, MANY THANKS for all the careful replies.  I think a new MC is in order, which is a shame since this one is only 7 months old, but first a couple of final questions:

1) I was wondering if the fault could be with the pressure switch?  Could I just replace that instead of the much more expensive MC?
2) What is the normal position for the pressure switch?  I assume that it would pop out only if there is a problem, yes?  And then after addressing whatever problem may be afoot one could depress the switch to reset it?

Again, much appreciated everybody!

To summarise I think you have a physical problem and not a sensing problem. Changing the sensor that works the light is not going to solve your pedal problem... sorry. If it helps to describe the sensor, in the old days you pressed the pedal that pushed a piston into the mcyl that pushed to all the wheels cyl/ calipers and pushed those pistons out.......... if you got a leak and lost fluid..... no brakes. Then a smart cookie thought what if we have 2 pistons in line and 2 outlets that worked the front ones and back ones seperately....... if one leaked you would still have the other ones to stop you. Even better when you put your foot on the brake the pressure to everything is the same but if one starts to leak and loses some pressure if there is a switch between them that pushes towards the side of lesser pressure and switches a light on your dash then you get and early warning of losing your brakes. Did that make any sense?

Anyway, I still think your sensor is working correctly in warning you there is a problem and your pedal is confirmation that there really is a physical problem and my money is still on the master.

My experience is based around drum brakes, but when there is an issue with "firmness" in the pedal, in my case it's always been down to a rubber seal in one of the brake cylinders.

No fluid loss = internal seal in the master cylinder.

Fluid loss = leaking seal in one of the slave cylinders.

Once again, gents, thank very much.  Craigb- the lesson on the engineering of the twin piston brake master cylinder is much appreciated.  And yes, since no fluid leak that I can see anywhere, it seems a mechanical problem in the MC itself.  Taking her in on Friday and will let you know what's what.  MANY THANKS!

So get this- I take it in for a new master cylinder, which was really disappointing since it had been replaced only seven months earlier. 

But based on everyone's advice, I thought that must be the problem, since the brake fluid reservoir was full, so there could not have been any leaking calipers, etc., right?

Turns out that the reservoir only LOOKED full!  There was a kind of high water mark that tricked the eye, while the reservoir itself was empty because of leaks- the right rear caliper needed to be replaced and the right front caliper was rebuilt, and the system bled (of course).  The master cylinder is just fine.

Not too expensive for the work (about US$200) and my brakes are now tight as a drum.

What a palaver.

Glad you got it fixed mabryt.

Mechanics can indeed be palaverish, intermittent problems, faulty new parts, two causes for one problem, overlooking, in retrospect, silly things.

Nice feeling of accomplishment when one is back on the road though.