It's leaking anyway, so you have nothing to lose by removing the capping.
Or get some rubber flashing and glue it to the wall/lead flashing and overlap the capping.
Yes, a rubber coupling should do the trick. You need a soft material to make the seal.
Or better still, get a longer section of pipe so that there are no joints.
But it would be able to do that even if the pressure in the system was say 0.3 bar
My question was, why operate at 1 bar system pressure (as most boilers seem to) instead of say 0.3 or 0.5 bar?