1979 300D - Brake Pedal Just "Sunk to the floor", but still works a little bit?
 

OK, So just yesterday I noticed that my brake pedal charateristic movement just changed in a very noticable way. It used to seem like I could press on it and within only one centimeter of travel the brakes would start engaging.

Now the pedal travels almost to the floor before it starts engaging the brakes. I can still stop, but much less quickly and firmly. It's an uncomfortable feeling to say the least.

I just happened to replace my front disc pads and give it new rotors in the process, but I don't know if this new problem could be related to something I did wrong. I hope not.

I'm thinking that this brake system is probably dual-circuit.. so one element in one of the circuits blew a leak. This would be why I still have just a little braking power left. I popped the hood and looked at the power-brake cylinder/booster thing and could see any leaks on or around that, so my suspicion is that it's one of the calipers or perhaps a brake line to the calipers?

Anyway, can anybody suggest where and how to diagnose this new situation?

Oh... and the pedal, as I press it, make a kind of jittery, "rubber slipping" sound, before reaching the point where brakes start to engage.

Thanks for the help.

It sounds like your master cylinder has failed. I had the same symptoms when mine failed.

when you installed the new pads, did you by chance do a full flush of fluid and use someone pushing on the pedal and someone else bleeding the wheels?
odds are good that the pedal pusher, pushed it right into a rough spot on the MC and damaged the piston seal.
this is if you are not low on fluid in the MC. if you are, obviously, start filling it and looking for the leak!

How much to repair/replace a master cylinder?
 

So it sounds as if it's my master cylinder that has failed. I did not full-flush and re-bleed my brakes when i did the pad/rotor change, so I don't think it was someone pushing on the pedal and moving the master-cylinder piston seal across a rough area...

Anyway... if it is a master-cylinder issue, is it difficult to get into it and replace the seal/piston and re-hone the cylinder?

Thanks for the input.