I'm finishing up a complete 4-wheel drum brake overhaul on a '70 Super Bee (manual brakes). I had the original MC resleeved and rebuilt it myself using a Raybestos kit. Seemed to go together very nicely. All pistons and seals were replaced, as well as the copper fitting which go inside the outlet ports. I'm fairly certain I seated them correctly. Since I don't have a vice and didn't want to scratch the MC I installed it on the car and connected the brand new brake lines I purchased for it (from Fine Lines). Instead of connecting them to the distribution block, I attached some rubber hose rerouted back to the MC pots. Effectively, to bench-bleed it.
I noticed right away I was getting an air leak at the outlet ports on the MC, even though I had tightened them what I would consider "sufficiently". So, I cranked them down some more. Still sucking air back in.
I've now tried Permatex Thread Sealant on both ports and cranked them down pretty hard. There does not appear to be any leaks around the outlet ports now, and I cannot hear the air hiss that I had before; however, I'm still getting some occasional, small bubbles back through the hose when pumping the brake pedal.

If a MC is properly bench-bled, should there be any bubbles whatsoever coming back from the hose which is in the fluid??

It bothers me that i had to crank the fittings down so tight, and use thread sealant in order to get them to seal. I'm thinking perhaps the threads on the ports were galled/pitted some because the car sat so long and the MC was quite crusty (before being refurbished). I'm worried the leak will return when everything is hooked up and there is real pressure on the system.

If it makes a difference, I'm using DOT-5 fluid.

Thanks.

the reason to bench bleed, is to eliminate air from the master piston set up. if you still see air, you have some trapped, or the master is sucking some air in somewhere. with the thing on the bench, or in the car with the short bleed lines attached, you should get to a point, where there's no more bubbles. bubbles from the bleed hoses means bubbles being sent to the brakes.

X 2

Sounds to me the problem is the lines or the flare fittings in the master.

No sealant will help the problem as the threads don't seal anything the flares do.

If you have to crank them down extremily hard then something is not right.

Are thess the same lines or knew ones. Mild steel or SS. Stainless can be a real pite to seal on a flare sometimes.

Often, I've had to tighten and loosen new fittings firmly several times to get a good seal on new lines. You should notice the threaded nut turning in more-and-more as the flare finally seats.

I also tap the master cylinder with a small hammer to help expel the smaller air bubbles while on the bench.