They are most likely leaking at the fittings. Very common if the installer used side-cutters to cut the air-lines instead of a sharp straight-blade.
I would highly recommend an air-compressor setup and in-cab controls. That way small leaks and normal air loss aren't a big deal, and it allows for load compensation. I did a full setup in my van and am very glad I did. There are now remote and wireless remote systems that make installation easy by eliminating the need to run air lines into the cab.