Define mating flanges? Do you mean the flared fitting? Or the NPT part? If it is on the flared fitting don't use great stuff, use a single layer of teflon tape on the tapper and tighten down. Greatstuff can not seal the pressures of a power steering system. That is why they use a flared fitting.
If the fluid is hot get a bigger cooler.
It isn't fittings that are leaking. I'm talking about these two flanges.
