That Lucas stuff is more of a stop leak. If it's been sitting dry for a while, I'd think you'd need more of a solvent flush to get any residual gunk and gum out of the valves in the rack.
If the pump turns out to not be the problem (and it doesn't sound like it, since if the pump were the problem you'd have similar issues turning both ways) then I'd recommend trying a more agressive flush of the rack. See what a pro recommends, or try something like Seafoam Trans Tune - it works for PS systems too. If you it turns out you need to replace the rack anyway there's not too much harm in throwing the kitchen sink at it first.
Trans Tune - Sea Foam