This vehicle does not utilize a proportioning valve. This vehicle is equipped with a dynamic rear proportioning (DRP), which is a control system that enhances the hydraulic proportioning function of the mechanical proportioning valve in the base brake system. The DRP control system is part of the operation software in the electronic brake control module (EBCM). The DRP uses active control with existing ABS in order to regulate the vehicle's rear brake pressure.
Knowing this vehicle I would first recommend that you completely bleed the front blakes to verify that there are no signs of air pockets. I would also examine closey the rear calipers to verify that they are fully releasing and that the slides are not corroded. If the calipers may be hanging, you would note more wear on one of the outer or inner brake pads. Feel free to contact me with any questions or problems! Good luck!
The EBCM is simply the electronic part of the abs. The control valve below the ebcm could certainly be a factor in this problem. Here is an illustration. Note #3
There is some testing that can be done to determine if this problem is being cause by the valve assembly itself, but you will need a scan tool to command certain operations of the valve assembly. Without that testing being done it will be next to impossible to verify if this problem is regarding the master cylinder or the valve. Honestly the master cylinder would be a cost effective item to try before getting yourself behind the labor and price of use of a class 2 scan tool. I am not into throwing parts at a vehicle, but I would figure you could find a master cylinder for less than what is going to cost for a repair facility to do this testing. If you have access to a class 2 scan tool that can command ABS operations, let me know and I can get you info on this testing.