I noticed today that my brakes drop nearly to the floor when I press on them. The car just passed inspection so I doubt that they brakes are worn. Master Cylinder has the correct level of fluid and the pedal is hard when the vehicle is off, so I doubt that the master cylinder is failing.

How would I go about troubleshooting this problem?

 

Doubt is one thing.

Time to pull some wheels and open the hood and inspect everything yourself.

Including hoses at the wheels--look for bulges when under pressure.

 

When you step on the brake pedal do you hear any hissing?
If so, brake booster.

 

Nope, I think it's a rusted-through rear brakeline, giving me once again the invaluable opportunity to learn another automotive skill. Too bad it's raining

 

The rear brake line was corroded and had a hole in it. I have spent now 13 hours trying to install a new line that doesn't leak at the fittings and am SOOO frustrated! It seems that no matter what I do I get a leak under pressure. I would use the ones that have been pre-installed on the line at the auto parts store, but the fittings they have do not seem to work in the car - they are too large. I got an OEM double flaring kit from autozone, but that chinese junk can't seem to make a good end.

Open to suggestion.