adjusting my vac

Im trying to adjust my vac...i cant get higher than 5 but the needle is moving like crazy....when i put a higher pv in like a 6.5 it steadys out a bit and then likes to be put leaner...but with the 4.5 in it wont steady out...its a 750dp with 74 jets up front and 82 in the rear...do you think its a leak?