I checked all the cylinders first, believing that the dump valve could not leak in one direction only.

I've not had a problem with my dump valve nor have I verified it is of the same type of the many I have personal experience with.  [30+ yrs in GE chemical business - Lexan plastics]

My point is that most ball valves have a 'floating' ball that uses only the downstream seat to seal.  So it would be easy to believe that the valve would leak only in one direction.  It is my belief that most leaking dump valves are caused by  foreign material scratching the seat [something that was left in the lines during construction].  This material probably also causes leaking cylinders.  I find it difficult to believe that in a clean installation the dump valve would ever leak in either direction.   The pressure rating relates to burst pressures - not sealing ability.  Of course, I could be mistaken...  Earl