1992 Geo Metro front brakes sticking

1992 Geo/Metro 3 cyl mileage: 75,000. I was recently overseas for six months. While I was gone, I had my wife start my car once a week. Well when I got back it started right up and I was really surprised all it needed was an oil change. Then while I was driving it on my second or third day back I started feeling what seemed like the brakes were sticking. Turns out both front brakes are sticking. Thing is that they don't do it until you've been driving for ten minutes or so. Then the longer you drive the tighter they get. One time I could not get over 45 mph on the freeway. That’s when I figured I better really check it out. I took it to Midas because to be honest I had no idea. I am a decent mechanic but other than both calipers going out at exactly the same time, I was lost. Well the guys at Midas said there was nothing at all wrong. So I drove it home and on the way it started doing it again. By the way, I jacked each tire up individually to come up with both front brakes sticking. I talked to a couple people and tinkered a little bit and decided to get the proportioning valve from the dealer. After changing it the problem continued. Then I replaced the master cylinder after taking it apart. It seemed pretty worn. Problem continues. This has really got me stumped. Even if both front calipers went out they wouldn't get tighter and tighter the longer you drove it would they? Any suggestions would be great. Thank you! Oh yeah one more thing. Midas said the fluid wasn't contaminated but even if it was after changing the prop valve and master cylinder I've bled all the old fluid out of the system.