hey guys, for the last 3 weeks i have been losing brake fluid pretty quickly. My car is an 81 280zx, so i have the dual brake master cylinder. Each time, the rear cylinder (closest to the cabin) is nearly empty, with just a little bit of fluid in the bottom. I can put brake fluid in it,(about a pint) and the brakes feel great, no mushy feeling at all. I just finished checking all of the brake lines, calipers, and bleeder valves, but everything was bone dry. now, about 4 months ago, i rebuilt my front calipers (after which the brakes were greatly improved) , but like i said i could find no leaks. I am thinking that the master cylinder may be going, and leaking internally... Thanks for help in advance