Disc Brake Hold Off & Prop Valves

"However I also tend to believe they help in keeping air out of the system which could enter at the wheel cylinders or calipers"

BigBlockMopar made that comment and I disputed that,,
well,,, I ran that by an old time mechanic that has much more experience than I do on these early brake systems, and he agrees with BigBlockMopar
said it is due to the design of the wheel cylinder seal, and they would suck air if not under pressure,,he said that in the mid 60's the wheel cylinder seals were changed to eliminate that issue.