What is the 'air compressor cut-in' pressure, and what is the minimum legal limit for it?

1. The pressure at which the compressor starts pumping air, which SGI says must be at least 100 psi (690 kPa)
2. The pressure at which the safety valve blows off, which is 150 psi
3. The pressure at which the brakes lock up, which is 20 psi
4. The pressure where the compressor stops pumping, which is 120 psi