Air is coming out of the oil breather on a t35 air compressor-any ideas why?   

A Powermate 5hp 80 quart compressor has significant amounts of air coming out of the crankcase vent. I believe it is a B5900 compressor head. Is it piston rings? They seem in good condition. The rings don’t appear broken. The springs are strong. Despite the fact that the cylinder bore was still a mirror finish, they still had a good seal to the cylinder body (and it took as much force as it should take to pull it off). Other than that, I was guessing the build-up around the reed valves might be causing a problem if it prevents the valves from sealing correctly as the previous owner used it for years without an air filter. However, my guess is this would much more likely lead to the air flowing back out the air intake, not the crankcase vent. Once cleaned the reeds look fine as well. I’m currently cutting new gaskets for it and hoping to reassemble soon. I’m also hoping to keep the price of this compressor in the amazing deal zone and not put $100 dollars of unnecessary piston rings into it unless truly needed. Are there any measurements/ diagnostics that I could do to confirm the piston rings are good/ bad? Are there any other problems that could cause the air in the crankcase?Thanks,