(sorry for the long post, but I am trying to be thorough with my description)
This is my wife's car, so I normally don't drive it much. Car has 110000 miles on it. Does not have ABS.
Original symptom reported by my wife (I did not experience it myself): while waiting in the long line to drop my daughter at school, the brake pedal would slowly go to the floor when sitting and creeping at idle with the automatic transmission in drive. When I checked the brake fluid reservoir, it was not noticeably low. It was well above the min mark. I think it was at or close to the max mark.
There were no signs of leakage. I figured this had to be a worn master cylinder, so I replaced the master cylinder and bled the system at all four wheels (in order of decreasing brake line length) using a vacuum bleeder system. Used DOT 3 fluid. During a test drive it performed normally.
Yesterday we were 2 hours into a four hour drive and the brake light illuminated on the instrument panel. I opened the hood and discovered the fluid in the reservoir was to the min mark. I refilled the reservoir and we continued, with me paying close attention to brake feel. Up to this point I had not noticed anything unusual. From this point on the brakes performed okay but the pedal did seem slightly soft. At the end of the drive, the fluid level was again low -- about half way between min and max. Then my wife also reported that after I had replaced the master cylinder, the sinking pedal symptom had happened twice in about 5 weeks.
I am now trying to identify the leak in my garage. I see no evidence at all of a fluid leak. I have the car up on jack stands and the wheels off. I removed the rear drums -- no sign of leakage at the slave cylinders. No sign of leakage at the front slave cylinders, master cylinder or lines. With the engine running, I can push the brake pedal very hard with both feet and hold it, and I don't feel any sinking. I've pressed the pedal hard and easy many times, held it for a minute or so multiple times. The pedal might sink just a little but it is pretty hard to tell and it feels more like my imagination than anything real. In all my pedal pushing over the course of 30 minutes or so, the fluid reservoir level did drop some (about midway between min and max). That's not a huge amount of fluid but it is enough that I would have thought I'd definitely see it leaking somewhere.
Where could the fluid be going? Obviously I need to figure this out and repair it before I put the car on the road again. I'll take it to a shop if I can't figure it out but I'd prefer to do the work myself.