You mean drain valve which the part you manually relief pressure in tank? It's not a must on AB compressors in my opinion, air can be discharged by water seperapors' valve or the hose can be seperated. As i said my compressor has an outer protective case, you must disassemble it to see it's inner parts, if there's a drain valve you can't use it.

By solenoid i mean pressure switch, tanks for correction, it's some kind of bad technical talking habit.