If the air is escaping as you unscrew the pump, but the valve holds whatever's left when your done, it's neither the pump or valve, but an unfortunate combination of the two.
Pump heads (hoses) have a valve depressor that opens the valve so air can flow. If the valve stem or depressor are high the valve opens before the gasket seals the top of the tube, so air can leak out. The fix is fairly easy. Add an O-ring or rubber washer to the gasket, so the seal happens sooner, and releases later with respect the valve being open.
Go to the hardware store and buy a 5/16" (8mm) OD O-ring, and drop it into the pump head, and things will get better. Toy might have to tighten the head on hard to compress the O-ring enough to engage the valve depressor.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.