E150 brakes

I have a 2002 E150. At 22K the brakes were rebuilt due to pull to right. At 28K the problem reoccured. Dealer says dirg on caliper slides. Reoccured at 33K. My inspection reveals no clear problem. Any suggestions ? Anyone else see this problem.
Thanks

Check the wheel bearings. If the bearings get too loose, the rotor will be at an angle to the pads, causing unusual wear and reduced effectiveness. So, check the LH side particularly well, as the "pull" will show on the "good" side.