How do you add brake fluid to a 1990 Toyota Camry CSI Twin cam?

You add it to the master cylinder mounted on the firewall, drivers side. Do not overfill, and only add fluid from a sealed container. Use the proper fluid as listed on the master cylinder cap. If it is low of fluid it means that you either have a leak or the brake pads have worn down, and they should be checked. You may need to have your pads replaced. Fluid does not evaporate and those are the only reasons it will be low. This car is 16 years old, and if you have not had the fluid changed it is way over due. Have the entire brake system flushed and fresh fluid installed. Brake fluid should be changed at least every 3/36. This is an often overlooked service.