Personally, I'd change the expansion cap almost as a diagnostic measure first as last, they're cheap as chips anyway.
How hard are the hoses getting? Very hard hoses could be an indication of engine gasses finding their way into the cooling system.
Air locks can cause coolant loss, but if the system has been OK for some time, then suddenly starts losing coolant I would be tempted to suspect the failure of some part or other rather than just an air lock.