curious as to why you use a pump. is it because you do not have head tank to create flow? i am replacing my flex copper, the flare fitting is leaking and i am going to shield it and insulate the ends. like you have found, shielding makes a big difference. if i could get a good seal i would try to run in the stack but with a blower i am afraid i would just make a mess if i did not get it sealed 100%