After further consideration, I decided to eliminate the "trap" in my sprinkler run by digging down to the horizontal branch of the trap and running directly there from the valve (see picture). Now my only concern is draining the branch from point B to the auto drain. Will there be a vacuum created at B that will keep that branch from draining? To the right of B it will be slightly downhill to the various spray heads, but the total drop will be a foot or so - none will be anywhere near as low as the horizontal run under the wall.
Is there a such a thing as a valve that closes when the line is pressurized, but is spring-loaded to open when the water is shut off? I'd like to put something like that at B, and I don't think the usual atmospheric vacuum breaker works that way.
