check valves

CHUCK THE PLUMBER

I recently got called to the college gym on A boiler leak. The steam boiler was pouring water from the combustion chamber.First i shut off the power, then i shut off the water that fed the condensate return tank through a float mechanisim, then i opened the boiler drains. Before the water level reached the sight glass, the leak stopped. Upon inspection i found two sections had holes the size of an egg at the higest point.
I've since replaced the sections, but had a problem with flooding. This is a two pipe system with thermostatic float traps on every branch There is a return manifold that every float trap drains into, each drain has a swing check on it. Between the return manifold (rm) and the condensate return tank (crt)there is an additional swing check.
My question is "Is the swing check between the RM and theCRT necessary and why?
Also the boiler manufacturer told me the holes were caused by oxygen errosion. Is this possible? I thought oxygen errosion occured closer to the water level inside the boiler