Rebuilding W123 Master Cylinder (seal kit?)

[johnathan1]

Yesterday, while in LA traffic, I went to stop and the brake pedal suddenly went all the way to the floor, and I had to pump it several times to get any feel back in the pedal. After that happened, the car still seemed to stop ok, but the brakes would engage with the pedal MUCH lower than it ever has before.

I'm assuming the master cylinder is failing? Are these difficult to rebuild? I've heard of other members replacing the internal rubber seals, and they make it sound like it's a pretty easy job to do. Can anyone comment on how to do this, and where I might find the seal kit?