Scion XB brakes

I have recently acquired a 2006 Scion XB which has sat for the last 8 months. When I come to a stop the left front brake lets go and then grabs again just as I am about to stop. I drove this car about 2 months ago and after driving it for about 100 miles the problem disappeared. After sitting the past 2 months the problem has reappeared. What should I be looking for? Thanks