Brakes not working very well

[CarSick]

I have a 1988 Pontiac LeMans with 245,000 miles on the odometer. I noticed that when sitting at a stop light, the brake pedal would drop a bit a few seconds after I came to a stop. So I thought that it had a bad master cylinder and replaced it with a rebuilt one. I bled the MC on the car as the instructions said to do and then replaced the fluid and bled the brakes at all four corners. I then drove it for about a week with the brakes feeling normal (not great, but normal). Then, one day at lunch they stopped working for the most part, but the pedal is firm. I used the e-brake to drive the car home. I expected to find the MC low on fluid, but it wasn't. There were no obvious leaks around the MC. I tried bleeding the calipers again, but there were no bubbles. I thought that maybe the booster died, but I should still be able to press really hard on the pedal and have it brake -- but it doesn't. The only thing I can think of is that a bubble wasn't removed from the MC bleed and ended up "moving" someplace important. I dunno. Any thoughts?

[CarSick]

It turned out that the MC wasn't bled completely of air. I used a baby wipes container (a rectangle, that perfectly surrounds the MC) to bath the MC in brake fluid and when I was done there was not a bit of air and the pedal worked great.