I'm not sure what the u-channel bracket it exactly. All the kit came with was the set of rocker arms, the new studs(allen studs by the way), and the so called washers that are half moon on one side and flat on the other. I didn't use the new bolts or the washers and from what you're saying, I'm thinking that the rocker is just too far down on the valvespring and it's holding it open. The instructions said to but the washer, half moon side up, on the factory bolt, and then put it on the rocker and then mount it. The whole washer idea seemed really weird though so I didn't bother with it. I kind of think that is my problem.
'02 GMC Sierra 4.8L
CAI, Granatelli MAF, 90mm TB, 1.8 roller rockers, JET tune, true dual Flowmaster race mufflers.