Is there a way I can inject dye into the A/C system without the compressor running?

I'm working on A/C in the car listed above with a 305. As soon as I turn off my vacuum pump, the gauge drops from -20 almost instantly to zero. So I'm assuming there's a large leak.

Is there a way I can inject dye without the compressor running?

Or do I need to jump the pressure switch, or the compressor, to make it run and accept and distribute the dye?

Should I maybe change all the o-rings first? (All the connections are all under the hood and easy to reach) and then try to vacuum it down again to see if it holds.

Thanks in advance.