This 21 Oz. OEM Accumulator Tank, includes a mounting bracket and has 1/2" female ports (the RV version has male threads). You can get by without using this is a pressure system but if you have it, the system will be quieter, run more smoothly, and the pump won't turn on as often.

Materials: nylon housing, butyl diaphragm. Ports 1/2" NPT female. Dimensions 3.75" diameter x 8.75" long. Approvals NSF listed Max. Working Pressure 125 psi

Guide to Pressure Accumulator Tanks