07 International 4300/Durastar Hydraulic brake problems.

If this has the WABCO HPB (2 Pumps and Accumulators) the best route would be to connect with the TOOLBOX software and check for faults. If the rear pump is failing it'll show a fault for that circuit. Additionally, make sure the brake fluid isn't overfilled as it can cause the MC cap to cause a vacuum/pressurize and that causes issues for the motor/s. If you really want to test the cap/motors, deplete all the pressure (Pull fuse, pump brakes a bunch, remove cap, and then put fuse back in, when it builds back up if the light goes out just replace cap and ensure the reservoir isn't overfilled)