I've got little to no brake pressure in my 93 Celica GTS and need help, please!

Background: I've only had this 93 Celica GTS Liftback for about 6 weeks and, so far, I've had to replace the distributor, the clutch slave cylinder, the rear struts, and the brake master cylinder. Parking brake linkage needed some work, too, and I'm thinking it needs new calipers/rotors/pads all around, as well as front struts. (Wife is wondering if I bought a lemon, but I told her that lemons don't make it to the ripe, old age of 19. Right?)

After I completed the rear strut work, I had to reconnect the rear brake lines and bleed the system. Halfway through the bleed process, something happened (?) And whenever the pedal was released (after being held down) I could hear a small whistling, sucking noise. I could never get good, solid pressure at the pedal after that. Driving around the neighborhood, the pedal will just continue down to the floor and the car will slowly come to a stop after a few seconds. Not safe to drive, obviously.

The brake master cylinder didn't look very old, but I went ahead and replaced it last night. Which didn't change a **** thing. Pedal still won't maintain pressure. Does this sound like I've got a blown line somewhere? How can I find it? I can't pinpoint where the whistling sound is coming from, and I don't see fluid spraying out anywhere, either. Please help!

(Car is all stock, as far as I can tell, and does have ABS. Calipers all look like original '93 Toyota parts.)