92 Roadmaster Brake System Problems

I am 2nd owner; bought it 4 years ago. Car has been running good. Replaced both front brake pads/shoes then 3 days later the brake light came on and the pedal went all the way down with a WHOOSH sound. Since then, we have replaced the MC (3x), power booster (3x), front brake calipers, rear cylindars, rear brake sensor and a thingamagig that connects the sensor to the steering column but hooks to the sensor. The whooshing sound is gone, the brake pedal works, we have bled the lines several times but can't seem to get the brake fluid to go to the back wheels when bleeding. We went back and re-did everything to make sure all was on correctly. Thought we had it today when the brake light went off and we road tested it. Everything seemed to work but then we jacked it up to make sure the fluid was going to the back and the light came back on and we lost fluid pressure in the back again. Don't know what else to do. Can anyone help? Thanks.

What about the proportioning valve, have you replaced it?

The proportioning valve is the next thing we are going to get. It was the most expensive and we couldn't get it locally so we left it for the last, but we needed confirmation that you guys agreed this might be it. Also, we haven't taken it to get an ABS scan; do you think we should do this first before ordering the PV? Thanks.