Thought I would share with everyone the parts I used to build my own Hydraulic Pressure Gauge. It took a little while tracking down the right QD Fitting and thread adapter to hook everything up. Here are the links to what I used. Plugged it into the QD for my FEL to test. My dealer indicated the master relief pressure for my L3540 should be set between 2630 and 2770 psi per his documentation and that the relief pressure for the FEL or 3ph should not be set above 2770 psi. I wanted to see at what pressure kubota had set my tractor. So, here is what I found. It was about 2500 psi at about 1100 rpm's, and at 2700 rpm's, I measured a full 2770 psi. I usually operate at about 1900 rpm's, and the psi measured a little over 2600 psi at this speed. So, everything looks to be set from the factory just as I would have it.

QD connector (Gotta love the non-std threads)
Pioneer 3/8 in. ISO B Interchange Coupler, Kubota/John Deere Tractors - 1820160 | Tractor Supply Company

8-1/4 F5OG adapter (3/4"-16 UNF male to 1/4" NPT female)
Goodyear Rubber Products

Pressure Gauge (Grade B with 3/2/3 % accuracy)
Valley Instrument 2 1/2in. Stainless Steel Glycerin Gauge 0-5000 PSI | Filled Gauges| Northern Tool + Equipment

If you build one though, just be sure to use it safely. Here are the 3 major things to watch out for.
1 - Watch out for leaks. At high pressure they will open you up just like a sharp knife. Use cardboard to search and don't stick you head right in front of the gauge to read it.
2 - If you use teflon or thread dope, don't get it in your system. It can mess things up like your valves. You don't always need these. If you do, it goes on the male side only, and avoid the first 2 threads.
3 - Don't deadhead your pump. They need to keep flowing.