First off, thanks for all the input on my question(s).
So the compressor I rented is a Sulair 185. The rental house informed me to watch the pressure gauge so that the pressure doesn't drop below 100psi on the gauge attached to the unit.
They were told by Sulair that if it does, then the oil separator for the screw pump won't separate out all the oil and potentially I could blow oil into the irrigation systems.
So, I rigged up my own pressure gauge. I couldn't find one that I liked that had a bigger opening to not restrict CFM's during a blow out. I've got a bigger 9 zone system that I need to do tomorrow and the access cap in the line is 1.5".
Here's the pic, and I'm also disappointed in the installer for not putting in a better blow-out valve. I ended up having to remove a test port and use an 1/8th inch adapter to get it connected.
The gauge worked great and never cracked the valve enough to go past 75psi. The machine kept a constant 120 psi on it's gauge.
Even with the 1/8" fitting, I still felt I was getting sufficient air flow through the system.
I also used a zip tie to hold the hose in place to make sure not to break the fitting.