I have a 79' F100 w/ a 351m and power brakes. The booster, master cylinder, steering gearbox, brake hoses, pads, drums/shoes, and rear wheel cylinders are all new. The proportional valve, and calibers are untouched and the originals. The brakes work great When I originally installed the components, I made sure to bleed each wheel to get all the air out of the fluid. This is not my first time installing new brake components on a dentside but it is the first time I have run across this problem. I usually fill the master cylinder with brake fluid up to about 3/8 to 1/2 of an inch from the top. Every now and then (not consistently) the brake fluid will ooze out of the master cylinder lid. It does not do this only once until the fluid level obtains a particular depth but it will continue to ooze out of the lid until the chamber is empty. It is only doing this in the front (small) chamber which is for the rear brakes. If I fill it back up, it does it again. It does not do it every time I drive, which is strange, but usually over a two month period. I am not braking hard and do not tow/haul any loads. Could it be a rear brake line that is stopped up? Could a wheel cylinder not be working properly? What do you think could be causing this?