Master cylinder plug leaking no matter what brand I buy

[wh23g3g]

Just wondering if anyone here running the original type single port master cylinders that have an extra outlet that comes with a plug in has had problems with it leaking. I've got a 65 Valiant with the single port master cylinder that I've redone all the brakes on and I cannot get it to quit leaking. I eventually am going to switch to the later dual port master cylinder but right now I'm just trying to get it roadworthy so I can take it a few miles away to get the front end bushings changed. It's got 9 inch manual drums. I've replaced everything in the brake system except the junction block and the tee in the rear. Everything else has been replaced. I've tried a Cardone Reman master cylinder, a new Dorman master cylinder, tried to send my original that only had 1 fitting to Cardone and they just sent me another junky reman master cylinder that's leaking in the same spot. It leaks in the bottom outlet no matter how tight you tighten the plug or use teflon. I even tried buying another fitting to make sure it was a 1/4 pipe thread. You can bleed it and it has good pressure and you come back the next day and that little plug is wet with fluid and it's lost pressure. All 3 I've gone through have all leaked in the same spot. There is no flare there it's just an open hole. I'm using Silicone fluid does that make it leak easier? Even wrapping it in teflon doesn't work. I've got a new Bendix brand coming that I don't even know if I should try. Who could I even take this to and try and figure out why this plug keeps leaking? A machine shop? Dorman and Cardone tech lines are of no help. They just say probably a bad casting. But the past 3 in a row.