Cold-- I would try that 45. If I'm not mistaken, you will need to add a bit more pipe to that 45. You will want to make sure that the lowest part of the exit pipe(be it the 45, or an extension of more pipe) be higher than the highest point in the elbow that bends upward. Hope that description makes sense to you!
Also, depending on your type of check valve, make sure that before you start filling it, that it is free of debris that may be keeping it from closing. Had this happen to mine more than once.
