My 1965 production Hurst line lock locks the flow both directions when energized. Pedal cannot be depressed if energized.

I have seen some that are simply a directional check valve when energized. Probably B & M?

GD, weak voltage at the solenoid, or there is some junk on the valve / seat.
Personally, I wouldn't use any tape or sealant on the fittings.

I might be a little different, but I pump up my brakes until the brakes gets hard and the hit the button. I have never had a issue with doing it this way.

But if your pedal is going away after you engage the line lock. Then you are losing power or pressure is bleeding off while line lock is still engaged. I would think wiring or bad line lock. I also agree with Ed on the air in the lines. If there is any air it would cause this, but I think it would also be doing the same thing if you do not have the line lock engaged.

I set my line lock exactly the same and never had an issue.

That said there is air in his lines based on what he said and didn’t say.

I have also some issues with linelock,in my there was an O-ring inside valve that was broken,replaced o-ring and works again