Brakes Sticking and Bad Brake Booster?

I have a 1999 Buick Regal GS and it feels like I'm continuously riding the brakes at random times and engine sounds like its working harder than usual to get the car to move. I thought I had this problem solved by replacing the master cylinder but I seems to be happening again. The reason I replaced the master cylinder was because I came out one morning about 2 months ago and started down my driveway to get the mail (I live on a steep hill) and I thought I had a broken line or something because I had almost no pedal and it was sinking to the floor. What confused me the brakes seemed good the day before when I drove about 40 miles round trip to town and then parked it overnight. (except for the brakes sticking feeling) I had the car put on a rack and found nothing more than a slow leak at the proportioning valve but I noticed on the way to the shop the car was no longer hard to move and was smoother rolling than it had been in a long long time. The brake fluid was a little low but when I filled it to the max level the pedal still felt soft and would sink unit I had the master cylinder replaced, With the sinking pedal and all I was thinking its probably a bad/sticking master cylinder that maybe could have been applying slight pressure to all the brakes at once and now it was released. I purchased a reman master cylinder from advance and had it put on and it seemed to work great and my car was still rolling freely. The proportional valve is still leaking (small drip when pressed) but I didn't think it could cause all these problems. I usually have to pump the brake before I leave of a morning or my pedal will be soft and sink but not all the time. Recently I noticed my car is starting to feel like it is sticking again and my last mpg estimate was 17mpg, I might be a little hard on the throttle at times but not all the time. The rims feel equally warm after driving but not real hot. I have replaced a lot of parts over the last 2 years so most of the braking parts are near new. All the brake lines have been replaced along with the hydraulic hoses. Front calipers are new Rear caliper I rebuilt myself Master cylinder was replaced over a month ago. Could it be the abs unit or the brake booster? A week ago the car rolled freely but the pedal was starting to feel hard like the symptoms of a bad brake booster.

Make sure you have free play travel in the brake pedal, at least 1/4".

You don't think a leak shouldn't be fixed? fluid goes out, air comes in......if you have ABS, don't you feel it activating when you apply the brakes?