If it's leaking into the barrel the exhaust valve has not sealed.
If you can't build up pressure in the valve the check (inlet) valve is at fault.
Try putting a couple of drops of oil directly into the pump and pumping up to pressure.
If it holds it's pressure put the gun to one side and leave it pumped up overnight.
This normally beds the valves in.
There are other causes such as weak valve spring and the central O ring leaking but I'd try the oil trick first.
Have you tried cocking the gun before pumping?
Was the inlet valve the same as the old one (they have changed)?
All the best Mick