Brake Proportioning valve Question

[cleszkie]

My buddy was having some issues with his brakes, so I helped him dig into it to diagnose the problem. Seems he had a rear brake caliper piston that was leaking brake fluid, and it set off the brake warning light triggered by the proportioning valve. When I opened up the master cylinder, the reservoir for the rear brakes was full, and the reservoir for the front brakes was dry, much to my surprise. I was expecting to see the reservoir for the rear brakes dry since the leak was in the rear. And yes, I verified which reservoir went to front and rear brakes when I retracted the rear caliper pistons and saw the fluid level rise in the reservoir that was already full. I had always thought that the prop valve only warned when there was unequal pressure in the front and rear systems, but that it didn't transfer fluid between the front and rear systems. It seems to me that somehow fluid got transfered between the front and rear systems, as there are no leaks in the front system, but the reservoir went dry (I know the front brake reservoir had fluid in it because I helped him change front brake pads a while back). Can any shed some light on this for me?