Just my two cents, I do run 2 seperate air systems. One for my retracts and doors, and a 2nd one for my brakes. The reason is that with retracts requiring air to lock in the up position as well as the down, I want to make sure my air pressure is going nowhere but to those retracts. While it's true that having pressure for brakes is pointless if the wheels don't come down, having brakes linked into the same air system as retracts gives just that many more connection points for failure. I like having as few pieces of airline and T fittings as possible in an air system. True, using 2 systems does require an extra filler valve and tank, you don't HAVE to have a pressure guage in the line, we got along for years without them, but it does allow for 2 individual simpler systems as opposed to one bigger complex system. You have to use 2 servo's and valves anyway, so the extra weight and expense is not much at all.
I've never used glennis brakes or wheels so I can't comment on the amount of pressure needed, all I know is I've always run 120psi for my retracts and 100psi on my brakes and never had any problems.