Forget the constant pressure pumps as they are a nightmare. Troublesome, complicated and require replacing often.
Consider replacing with a conventional pump and a Pside-Kick. The Pside-Kick comes with a small WX-102 tank and everything else you need. This small tank is all that you need. It will extend the life of your pump, tank and pressure switch plus it will give you constant pressure like city water. Your family will love it. http://www.cyclestopvalves.com/products.html.