Diesel Bombers - just a little help

I have a 94 7.3 idi tubo from factory, new injector pump i turned up two flats,had injectors rebuilt, turned injector pump timing up just a hair all is goin good but still seems like it's bein held back, i put the air intake up in the firewall gettin good cool air from outside also. now my switch for the fuel tanks is messin up, i can burn all the fuel out of the back tank, switch to the front but its still suckin a little from the back, this sounds bad to you but listen to this, while runnin on front tank and suckin a little air from the back tank i get power, big power, it runs like nothin holdin it back, can really hear turbo workin and all. my question is when i put a new tank switch and all is workin like it should, how can i get this power that its havin when its suckin air from the back tank.