1986 Nissan 720 Pickup, brake light on dash won't go off

109,000 miles on my Nissan 720 pickup. My truck won't pass state inspection until the brake light indicator on the dash goes off correctly when the parking brake is released. He says that the level sensor switch needs to be replaced, but that you can't buy the switch by itself and must replace the entire master cylinder -- about a $300 repair. Is there another way to fix this problem? It appears this is just a sensor connected to the cap of the master cylinder that tells the brake when to go off. Help! Thanks.