The dive shop where you bought a used set of gear. Will have the resources to get you a manual. Remember what you were taught in class on the care and maintenence of your gear. Have the gear inspected/annual maint. before you use it. That would make your dive shop more responsible if the gear doesn’t work correctly. You should not have paid over $80 for that old used set. Rubber/Plastic & Orings deteriortes over time. Esp. sitting in the sun at a dive site. It’ll cost you $125 for an annual rebuild. But not from that same diveshop. Your better off buying a new cheapest 1st/2nd stage model from a name brand manufacturer.