I am helping a friend with a Remodel of a hundred year old house, the main water lines were Iron, and rotting through, so the main lines to the second story were replaced with 3/4" PEX from a 3/4 copper in the basement. The line from the basement runs around 15 feet and then are split on T's reduced to 1/2 "and run to the different fixtures. There is no manifold, and I am thinking this may account for the low pressure. The lower level seems to have good pressure, the Pex is already run to the second story, and there is no budget to rerun with Copper. Does any one have any ideas on how to resolve the issue?

I doubt that copper will solve your problems unless you find a crimped tube when you are tearing the pex out. Water pressure is reduced approximately 1 psi for every 2 feet of vertical rise. So if your second story fixtures are 10 feet above your first floor fixtures they will have about 5 psi lower pressure. Now if you dropped to 1/2 in pex too quickly, if you have long runs of half inch and several tees in them, then yes, you will have pressure problems. A quick and dirty rule is run 3/4" to the last tee in the line and 1/2" for any branches to a single fixture. This is wasteful on the hot lines however. The plumbing code book will give you the correct way to calculate supply sizes.