20% on a leak down is unacceptable for a "healthy" engine. It does not necessarily however indicate engine damage. What I would do and have advised before is, remove the rocker shafts. Remove the plugs. Pressurize each individual cylinder one at a time and listen to where the air escapes. Through the exhaust, an exhaust valve is not seating. Through the carburetor, an intake valve is not seating. Through the valve cover area, hole or crack in a piston or bad or broken rings. Through the open radiator cap, head gasket, cracked head or block. Leak down testers are a good tool, but often times they are "too specific" and cannot diagnose the "where" of the problem. Been there too many times with it. I don't even own a leak down tester. I personally think they SUCK for a diagnostic tool. They are more of a performance indicator. If I had a full time race car, I'd own one.
Having said all that, If I had to "guess" I would guess that your valve springs already sucked and you over taxed a few and now they are weak.....that is, since you didn't find any bent pushrods. I would also guess that with no bent pushrods, you'll likely have no bent valves. Pushrods normally bend first or right by themselves. I cannot remember any engine I ever saw where the valves were bent and the pushrods not. However, I have seen several with bent pushrods and no bent valves.
