With the timed breather, I measured the pressure in the rocker box on my Atlas, and found it to be negative 18 inches of water from 2000 rpm up. I then fitted Mikes XS reed valve to the timed breather port, with the rotary disc still in place. I then measured the pressure again and found no difference.
Thus it would seem to be no benefit to fitting a reed valve on the timed breather port unless you split the cases and remove the disc.
I have since blocked the timed breather port, fitted Comnoz's sump plug breather, but as yet made no further pressure measurements as I have not yet cranked up after rebuilding the head.
You might read again reply #4, concerning fitting the reed valve to the timing case, while leaving the rotary disc in place.
Slick