65 Valiant 4th or 5th master cylinder still leaking

I've got either the 4th or 5th master cylinder for my 65 Valiant with 9" manual drum brakes. I've tried remanufactured and new ones and each one of them leak from the same fitting on the bottom where there is a plug. I even tried wrapping it with teflon but nothing stops it. I'm currently running a reman Cardone master cylinder and I wrapped a coating of teflon on the threads of the plug and it still leaks. I'm using silicone fluid because the whole brake system was replaced and Right Stuff recommended it. My original one that didn't have an extra hole for a plug was sent to Cardone and deemed non-rebuildable and I was give one of their already reman units that is now leaking as soon as I put it on again from that same plug. The line to the top fitting is fine, no leaks at all. Just the bottom port where nothing goes it's just a plug. I remember someone telling me there was some kind of switch that went in that extra port for some kind of light. But I don't show it in my 65 Plymouth service guide. But here on EBAY I see one. What is that switch for? And here it's in the opposite hole. My line goes where that switch is in the picture. Maybe I need to get the switch and put it in. But what is it for? [ame="http://www.ebay.com/itm/1960-1966-Dodge-Plymouth-Manual-Break-Master-Cylinder-/201137848174?pt=Vintage_Car_Truck_Parts_Accessories&fits=Year%3A1965%7CModel%3AValiant&hash=item2ed4bfff6e&vxp=mtr#ht_226wt_1026"]1960 1966 Dodge Plymouth Manual Break Master Cylinder | eBay[/ame]