Maddog parking brake

Hey guys, I need some help with the Maddog. I can't set the parking brake for some reason. I hold down the toe brakes, but I can't engage the parking brake. I use FSUIPC for button and axis assignments. I have the same problem with the PMDG 737 with releasing the parking brake, but I use a button to press the brakes in the 737 to release them. When I use a button the press the brakes in the Maddog, I can't click the parking brake. Does anyone know a solution for this problem?