For a long-running problem like this and having changed the calipers I would be looking at the master cylinder. Sometimes they don't fully release pressure when the brake pedal is released. The symptoms get worse and worse the more the car is run as the brakes and brake fluid gets heated up.
To test this you can go for a run and when you get back loosen the bleed nipple on a front wheel, briefly. If the brake fluid spurts out, rather than just falls out, fasten it back up and go for another run getting the brakes fully hot again. Then undo a bleed nipple on a rear wheel. If the brake fluid spurts out of that too it is likely to be the master cylinder.
Last edited by GolfingDuck; 11-04-2012 at 02:49 PM.