Brakes

I have A 2003 Crown Vic with ABS and traction Control. I had the master cylinder replaced last year, as it was beginning to leak. No problems. Brakes worked fine. Then, on Thanksgiving Day, the pedal went to the floor. I've driven the car all winter with just rear brakes only. Last weekend, I replaced the master cylinder (not cheap), and still no brakes. No fluid loss. I was told that the proportioning valve almost never goes. I was also told that the power brake booster could be bad. How could it ? If the booster is going bad, there would be a loss of boost, yes ? No problems there. I can't pump up the brakes, unless the motor is shut off, but after a few seconds, it goes to the floor. Any suggestions ?

you have mentioned 2 things that actually do go bad and they act up in different ways.
also, the brake cyl's at the wheels need to be checked.
you are in a moisture prone location, brake fluid absorbs moisture and deposits it on the internals. rusting it all.
do you have any fluid leaking into the vacuum(engine hoses) or any tranny conditions as in faulty vacuum modulator???
if it is as simple as the master cyl. failing again, remember this, cheap rebuilt parts usually dont get fully diagnosed, just kitted and resold.
so you have several things to follow up on here. let us know how it goes for more help.