If it holds vacuum for 2 minutes then it should hold refrigerant for a few minutes as well. Lets make sure your equipment is even good first.
Check the rented gauges are in good working order. I have seen them with bent pins and screwed seals. Check the pins for obvious damage and proper range of motion. Check the seals are even there, and they are not torn or worn, do NOT over tighten the hoses to the gauge, finger tight only and not even that tight really. I have seen rented sets missing seals, or damaged. I over tightened a set and that pushed a seal into the gauge causing a restriction into the system, and a leak out of the hose. I did not tighten it that much at all, it was really easy to do.
Out of the 20 or so times I rented gauges, 2 of them were damaged and did not work. I had a few more with bad seals that gave me issues. people around here must be really hard on stuff.
Connect high and low side hoses.
Screw the knobs down to open both high and low valves (blue and red). Sometimes screwing them down too much causes issues, be careful and back off a turn maybe?
Open the blue and red knobs on the valve manifold
(If applicable) open the center valve that is connected to the vacuum (usually Yellow).
pull a vacuum for around 15 minutes.
With vacuum pump running close the center valve (usually yellow) then turn off pump.
note vacuum level on paper or take a pic of the gauge.
Let sit for 15 minutes and look at vacuum level.
If the gauge visibly moves right after shut off then you have a major leak. trying to fill with dye refrigerant should still be tracible because its still coming out. The Schrader valves are a super common leak point if there is no leak detected at any other component.