Mine was doing exactly the same recently, the bottom one was leaking but had the valve built in. I replaced them both with new metal type fittings. Now Ive got the old one off the bike Ive played with valve and stopped it leaking. Could be just a bit of crud has got in stopping it seal,or the valve has jammed off centre, try gently pushing a round blunt object in against the valve a couple of times, it might reseat properly.