Just a quick one for future reference because I have come across this myself after draining the system down and then repressuring. I too have come accross the problem of the system losing pressure and the water was releasing from the tell tale pipe outside.
I corrected the problem by simply turing the pressure release valve fully and letting the spring loaded action reseat the valve properly and this stopped the water leaking past the seat of the valve.
I know this sounds simple but its worth a try before you take the long way round and take it apart! The reason why it didn't reseat properly was that I was relaesing the system pressure by just easing the valve open and not opening fully.
Hope this helps.
Many thanks for the tip - that is exactly what happened to me, and your fix fixed it.