Lets start with the complete machine serial number please.
A flashing PB light usually indicates a command for park brake release with no feedback indicating it is released.
Any message in the monitor when it is flashing?
With the engine running, press and hold the menu button until "SERVICE MENU" is displayed in the monitor, then release the button. Press next until "DIAGNOSTICS" is displayed, press select. Press next until "TRANSMISSION" is displayed, press select. Press next until "PARK BRAKE" is displayed, press select. press select at the submenu for PARK BRAKE PRESS SWITCH., When engaging and disengaging the park brake, does the status of that switch change? If not, then there is no pressure getting to the brake or the pressure switch is defective.
Also, while you are in diagnostics, stop and look at codes. Post any active and stored codes the machine may have.