I have got to swap a differential pressure valve on a main combi 24he. The part at the front with the flow switch bolted on. Its leaking from the spindle which makes the microswitch as usual. I normally just replace the diaphragm but my customer is insisting the whole lot is replaced for some reason. I have ordered one and my suppliers said it says to order a small oring with it as well. Does anyone know what this oring is for? I thought it was just as case of disconnecting the microswitch, the 2 pipes on the top and then undoing the grub screws and then the new one should slide on? I don't ever remember an oring in the past?