With regular hoses the act of screwing on the end of the hose to the Schraeder valve causes it to open the valve... the same as on a tire stem... and you wait as you get close, crack open the valve at your guages with the refrigerant hooked to the middle...and purge your line of air as you finish screwing the fitting onto the R134a fitting ( in this case ).
So the same kind of situation may apply as you unscrew what was planned on being a permanent addition to the R12 fitting... that it will have the R12 valve opened to the inside by its proximity... and it should seal as you remove it further off the original R12 fitting.
The other kind of fitting involves a seperate control for the valve. YOu put the hose onto the other side of the fitting and then have the ability to turn the ( square I think ) rod end on the other side... it has inside a two sided valve like exists inside you guages .. forward and you seal it from one hose, backwards from another passage, inbetween and you have it connected from one hose to another hose.