Was this when the problem started? It sounds like (as Shane pointed out) the pushrod from the booster isn't fully retracting and allowing the fluid to return. The problem could be between the booster and the master, or between the pedal and the booster - both can cause the same problem. there is probably a factory spec for the pushrod clearances, but common sense is usually good enough. Checking and adjusting the booster-master pushrod is tougher but usually the culprit in my experience.
See if you can get it to intentionally lock up (jack up the front end, pump the brakes a few times, let off the pedal, see if the wheel will spin), then crack the bleeder. If it shoots fluid and releases the caliper, you're building too much pressure and not releasing it. With new lines, it's doubtful that the problem is in the fluid or lines.
