i dont think the suction of the pump is collapsing teh dubing, doesnt make sense to me.

it's pushing water through the tubing, at teh same rate it's pulling it in, there fore there really isnt any suction of that sort to cause collapsing of the tube(so long as the water is being placed back into the tubing it wont do that). the only way i could think of it doing that, is if there is a clog in either
A) pump outlet
or
B) in one of the blocks