Having a brain fart today.
What is the device of the main. It is rusted and needs to be replaced.
If itis a regulator they usually have a control screw.
Thanks
Back check or pressure regulator.
The doo-hickey
The thig-a-ma-jig
The wha-cha macallit
Then again, back check-pressure regulator sounds good too. 
Mic
I think you may be referring to an “RP” valve or Pressure Reducing Valve. There is a screw adjustment on top where if you release the lock nut you can change the water pressure to the home to what you want by turning the screw, then reset the locking nut. They typically come preset from the factory at about 55 psi but can be raised to about 75 psi and lowered to 35 psi. They are used alot on sprinkler systems too to step down the pressure. Cost about 40 bucks at Lowes or HD. Probably more than you ever wanted to know about an RP but there you go.