If you go to the manfacturer of the couplings and tell him that you are running 3000psi on series 60 brass coupling then they will tell you that they are wrong.
The steel coupling are for this pressure, they are zinc plated with chromate on top to give a yellow appearance. To the layman in the street, might say they look like brass which they are not.
I have run brass coupling off my ninja on a trckmount, overtime with the outside temp, and quick expansion with hot water they cracked.
From a cost point, the steel couplings are cheaper and run at a far greater temp. Since changing all coupling s to steel I have had no problems.
Regards
CATMAN