If it's only a decrease in volume, it just that maybe the current flowing through the switch are diminished by a reduction in contact area resulting from mechanical stress (misalignment, chipping, etc) or rust. Use the following alternatives before replacing it with a new switch:
1. cleaning - use WD40 or contact cleaner to loosen and remove rust and make sure the contacts are aligned
2. check the pickup, especially the black vulcanized fiber - look for signs of water, the black fiberboard will suck and hold water longer especially if the coils are potted with wax. This may create shorts that reduce the magnet's power; inspect the copper eyelets (magnet wire terminals) also.
3. silver plate the contacts to widen/build-up the contact area - jewellers in the local palengke will accept the job for less than a hundred bucks; a silver plated contact will sometimes sound even better than a new switch!
4. check the circuit path of the bridge pickup (wires, solders joints, capacitor, if any) Rust under solder is a usual culprit to the symptoms you mentioned
5. Make sure that the pickup in question is really a bridge pickup - you might have had this guitar checked by an amateur tech and unknowingly switched the pickups. The Bridge and Neck pickups are not interchangeable, the bridge pups are normally stronger with bigger coils. If these are original OEM pups, you can verify their resistances using an ohmmeter and compare with standard values. Again, the bridge coil's resistance is bigger than that of the neck.
6. Everything else fails, replace the switch.
Goodluck.
