Is your pit properly sealed?
Is the check valve working properly? Was it mounted horiz or vertically?
I'd look at these first.
If the pipe break were large enough to send enough effluent through your earth to the footing drain to your sump to stink, then you'd likely notice something above ground on your lawn too (either bright green patches or spongy ground, or sewage smell). Also, these pipes are deep in the ground, so a crack would probably have occured during installation and would have been noticed very quickly. If your pit is only recently starting to stink, I suspect one of the above to be a more likely prob.
I'm not a pro