I installed new pressure switch and new tank, also new pressure gauge, also replaced the safety valve. Pressure switch is set to kick on at 40 psi and off at 65 psi, safety valve is suppose to open at 75 psi. How ever I get a small amount of leakage from the safety valve. What's my problem?

65 PSI is very high for the cut off pressure in a normal domestic water system. But if the safety switch is new I don't understand unless the tank is out in the sun and some way the heat is making the preasure rise in the tank after it cuts off at 65 PSI. Had I installed the new switch for you and it leaked I would have installed another one. So wherever you got the switch might replace it or refund your money.

This is a common complaint. If you have a backflow preventer or check valve installed on your water service with no expansion tank, (see image) then you will have this problem until you install one. Let me explain.
First you have to understand a expansion tanks function. When new codes forced us to install check valves and backflow preventers on water service lines that made your water system a closed system. That meant that when the heated water expended it could no longer expand back along the water service line and would build up pressure in the syatem. This would kick off the T & P ,(Temperature and Pressure Relief) valve aqnd the valve would discharge hot water out the relief line. To prevent this we installed a Expansion,(Air) tank on the heaters cold water supply line for some place the water to expand to. More questions? I'm as close as a click. Regards, Tom