I have a 1998 F-150 with a 4.6, auto, Front disc & rear drum.When I apply the brakes when I first start the truck for the day the rear brakes make a noise & then lock up. As I drive the truck the condition seems to go away (brakes don't lock up anymore). But the brake pedal seems to go down further & it seems the front brakes are hardly working. I did a front & rear brake job not to long ago & everything was working fine till now. Could it be the master cylinder maybe? Chief Mike.