I have a questions about these lauchers. Do they have a tendancy to leak when fully extended? If they do, does anyone know if anyone rebuilds them?

Air cylinders in general shouldn't leak and are generally good for well over 1 million cycles. What kind of pressure are you sending to them? If they are truly bad you need to know the OD, retracted and extended length (aka the stroke) and you should be able to replace them for a reasonable price.

Mine never did that I know of. It was a Stroud kit with a K&R compressor

Nope

Bimba valves , go to the Bibma website, easy to get