Brake fluid disappearing

[Sieg]

Off the PT topic but relates to ABS brake systems........

'05 Toyota Tundra pickup. Brakes have been trouble free since new. Still running stock rear drum shoes, replace front pads 6 months ago. Twice in the last two months the brake warning light has come on. Checked fluid in the reservoir the first time and it was close to minimum, visually checked the lines for leaks and refilled with 5.1 fluid.

A month later it comes on again, pull the rear drums and inspect wheel cylinders...dry, adjusted the shoes as I'd noticed the parking brake tension was questionable, check lines and junctions, no leaks.

Yesterday the light comes on again and the reservoir level is close to minimum. I go under the truck with a flashlight and check every brake/abs line, reservoir, rotor/drums for any sign of fluid both visual and with my hand. Couldn't find a trace of brake fluid on the vehicle, nor has there been any on my clean concrete driveway.

Vehicle has not had the sytem flushed so it is OEM fluid from Dec '94 build date. Filling with 5.1 which is supposedly compatible.

I'm close to stumped, it's like it is evaporating.

Anyone have a similar experience?