P0456 can be from a loose cap, but most often it's from a bad Evap Vent Valve or a bad o-ring holding the valve into the canister. P0448 is not from a loose cap, it's from one of the following:
• EVAP canister vent control valve
• EVAP control system pressure sensor and circuit
• Blocked EVAP plumbing
• EVAP canister is flooded
Given them both together, it's very likely your canister has an issue.