[86loco]

Does it look or sound like it is coming from one of the dump valves. I have had a valve stick before after the coach had been sitting for a while and with a little lubricant sprayed on it, it quit leaking. Sometimes dirt can get lodge somewhere on a opening relief or dump valve. I'm not exactly sure of the type on your year. Spray bottle with water works wonders at finding air leaks. It is also possible it is coincidental problem. 20 minutes is a descent leakdown. The shop could have disturbed another line or something as well.