Only a guess.
You may have a pressure valve in the system just past where the water enters check it out it white with knurled knob on top. Turn the system off and turn this knob clockwise to close the valve. Turn the sytem back on and open one cold tap. Then open the valve until it starts to audibly 'click' stop turning and allow water to flow free without splutter then turn off the tap, if this is it problem solved. Look for leak on inlet connections inside c/van. Same if motor home.
Nobby
April 2009