cause fluid to decrease in one reservoir section and increase in the other?
NO The master cylinder may be faulty. I have seen many right out of the box that were faulty. The proportioning valve reduces the pressure to the rear brakes. Regardless of what type of brakes a car has, the rear brakes require less force than the front brakes.
Your brake master cylinder as two sections for safety reasons. The sections are two sizes. the largest section is for the front brakes. And I guess you have disc brakes. As your front pads wear down, the brake fluid will also drop. This is normal. If the rear brakes are disc, they will do the same. If you have drum brakes, the fluid height should stay the same or if you have a leak,the height drops. If you do have an increase in the small section, someone either added fluid or you have an internal problem in the master cylinder and may require changing. If your unsure and know a good mechanic, take it to him to be safe. The proportioning value works to adjust braking to weight being carried in the bed of a truck. It should not affect fluid height.
The vehicle master cylinders I am familiar with are constructed with the common wall between the reserviors a slight bit lower than the external walls. This enables the brake fluid to remain at equal levels in both reservoirs until a serious leakage in one part (front or rear) of the system causes the fluid to drop below the level of that middle wall. With such a leak the brake fluid will be low in one reservoir, while remaining at a higher level in the other, but not full to the required mark. So, IOW, probably a leak in either the front or rear brake system. If the front section of the reservoir is low, you have a leak in the rear brake part of your brake system. If the rear section is low, check for fluid leak in your front brakes.
