on my spa re-build project I tested for leaks and found the blower check valve was not functioning correctly and was feeding water back to the blower. I will try flushing it with water to see if that solves the problem, but if it does not, can I just install another check valve behind the current one without removing it? The current one is too close to the elbow to remove and replace.

as long as the valve opens freely, you can add a second one in series. What most likely happened is the gasket in the valve has failed. One thing you might want to try, no guarentees though, is to take a long needle nosed plier and break out the weak supports inside the existing valve and remove the parts so it's basically a straight through passage. One thing that concerns me though is the valve is there to prevent accidental water backfeed, not to hold back a significant amount of water. If you have water backfeeding, look into stopping that flow first. Typically, even with a check valve, most spas have a loop of plumbing that goes above the water line after the check valve, before the tub. This is to prevent the water from being forced out. If your's isn't this way, see if you can add that loop of pipe above the waterline. That alone is often enough to prevent water from backfeeding, and the check valve is a "just in case" device.