since it is only the RF it would have to be downstream from the T in the line out to the front brakes. we'll assume the hose ain't collapsing with it being new so I would open up the drum & check things out & check wheel bearing torque. I would also check booster rod clearance for the excessive pedal travel along with shoes to drum clearances. EDIT long(er) shot, a shoe hanging up on the 6 backing plate contact pads, you want em fairly smooth with no deep grooves. I'd adj the pushrod (want ~.020")/adj the shoes, all of em/check wheel bearing torque & good visual inside the drum on the RF that is locking up.