braking problems

i have a 98 yukon. My problem is that sometimes when i hit my brakes and hit a bump it seems as if they grind or something. When it rains it does it all the time. Also i've been having a problem with them not stopping correctly(take a lil longer than usual). I bled the brakes today(all the way round) and found that when i got to the drivers front, nothing came out. I opened the screw almost til it came out and still nothing. What could be the problem? i have not noticed any leaks or anything elsewhere.

Tough to tell what grind or something means, could be premature ABS actuation, have someone knowledgeable drive the truck.

Any idea why when i tried to bleed the front driver caliper no fluid came out. all the other three points bled fine but when i got to the front driver, nothing.

Crack the line going into the flex line and see if there is any fluid.

those rubber lines go bad, may be the prob there. the noise maybe from the pads on the brake that was doin all the work is worn.