I recently just used the blowout method with an air compressor to clear my sprinkler lines, but I still need to get the water going from the house into the back-flow preventer(BFP) out along with the water in the back-flow itself. I turned all my valves outside on the BFP to 45 degree angles, but I am unsure of what to do inside on the basement. Right now the sprinkler valve is shut off along a valve that controls water to a valve outside that allows me to connect a hose. The pipes in my basement branch off into 2; one for the sprinkler and one for the outside valve for a hose. I've tried to drain the water from the hose valve because the drain(the part that looks like a tire cap) is located on that pipe and higher than where the 2 pipes fork and when doing so the water pressure was too great and I was forced to stop. Anyone have a tutorial on what to do? Would greatly appreciate the help.

there should be a shutoff inside the house with a hose valve next to it.
you hook your compressor to the hose valve and blow air from there.