Harold we do not use a “mechanical check valve” the cap is just that.
A cap to prevent vandalism of bums throwing debris down the vent terminal BUT because of its design it doesn’t allow wind to go straight down the pipe.
The return bend again is designed to keep debris from being thrown down the vent piping BUT it would also prevents down drafts.
The vent height varies with each code BUT even the standard plumbing code states in part “All roof vent pipes that extend through a roof shall be terminated at least inches—“
EXCEPT other then weather protection the vent shall be run 7 feet above the roof.
The local Code I use says 24″ for weather protection ONLY