Most of the little so called water regulators are preset at 45 psi, I've got an adjustable regulator set at 55 psi & have never had a problem. Most rv water systems are tested to 65 & rated higher to around 80. For blowing them out to winterize 40 should be sufficient, but normal water pressure 40 is too low in my opinion, most city systems are 60-65 psi.
Danny & Linda
Sold our '13 Thor Redwood 36 FB
Currently RV & truckless