Switch the way a conveyor works?

So I am making a skylift and I am using velocity to move the parts. My only issue is that I want it so that it waits 5 seconds and then goes the other way. How would I do that with the script below

while true do
script.Parent.Velocity = script.Parent.CFrame.lookVector *15
wait(8)
script.Parent.Velocity = Vector3.new(0,0,0)
wait(5)
--script does the same as above but the opposite way.
end