Brake pedal suddenly went soft

[MrWampus]

Yesterday driving home from work I came up to a traffic light and my brake pedal was suddenly very soft. The car stopped OK, but my heart jumped when the pedal went as far as it did. It pumped up and stayed firm at the light. As I drove the rest of the way home I noticed the continued softness. When I got home I checked the usual supects: resevoir is full, no apparent leaks on or around the master cylinder. I have not yet checked individual brake lines, but I'll do that today in the daylight. I have not done any work on the brakes, so I don't think it is mechanic induced.

What else should I check? Do I simply have a master cylinder that's failing?

[MrWampus]

I may have discovered and fixed the problem. I recall yesterday running over a piece of nylon string stretched across the ground at the entrance of a gas station. As I pulled out, my car snagged the string and I noticed in my mirror that it stretched until it broke. About 10 minutes later is when the brake pedal went soft. When I looked under the rear of the car this morning I saw a piece of that string wrapped tightly around the rear axel at the right side. It was wrapped around the hand brake cable connection too. When I cut the string loose, the car nudged forward a fraction which tells me that this must have had some effect. Could it be that the heat of that caliper heated up the brake fluid enough to boil it a bit and cause it to behave the way it did? I'll take it for a drive today long enough for everything to heat up and see if that was the cause. But there is a very strong correlation between running over the string and the brake fade.

[narfire]

If or when you have a moment,perhaps a quick brake bleed starting at the rear pas. side to rule out any air in the system.