How do I balance a hydronic heating system?

Hello the Wall,

My family owns a 1000 sq ft vacation house in Northern California built in the late 60s with a slab on grade hydronic system. No emitters, just six 3/8" loops in the slab. The boiler went a few months ago and in the course of replacing it, I discovered the system was not only poorly designed (no expansion tank, no air remover, no reducing valve, over-sized t&p valve), but also has been incorrectly serviced all these years--to correct the leaking t&p valve (no expansion tank), some bright boy plugged the discharge line with concrete; before that happened, in order to make up the water loss, the cold water inlet valve was left open to city water pressure (80 psi). The list goes on, but I think you get the picture.

Needless to say, there is also a slab leak. After fixing all these problems, I want to balance the system, as it's pretty obvious this was never done (the floor heats up quite unevenly). I've been searching online for a few hours and am unable to find a resource which would teach me how to do this. The supply manifold is buried in the slab and the six control valves on the return manifold are sweated on. The return manifold is just below the circulator (1/12 hp Grundfos). I know I have to measure flow in each loop to balance this system, but am sort of stumped as to how to go about it. Any help here would sure be appreciated.

Thanks and Happy New Year,

Scott Matthews