A904-30Rh Fresh Rebuild. No 3rd gear, Please Help

Often enough that I check every single drum and I've been rebuilding them since 1980. I wouldn't call it stupid; just a waste of time. I thought you said you heard a clunk but then the hiss of air? If you're wrong, then you have to take it out again. If you're right then you'll stay on a solidified path of incorrect thinking.