Cessna 208 First Officer reported a brake malfunction during taxi after landing which resulted in a taxiway excursion. The aircraft stopped in the grass after striking an airport sign and sustained damage to the propeller.

Narrative

On our final flight back to ZZZ Airport; an incident occurred involving flight number XXXX. We were cleared to land on Runway XX. I was the Pilot Monitoring (PM); Person A; and Captain Person B was the Pilot Flying (PF).After landing; we exited Runway XX via taxiway 1 and were instructed to taxi via 1 to the FBO. While taxiing; Ground Control instructed us to hold short of taxiway 2. As we approached taxiway 2; I observed that the aircraft's speed was not decreasing as expected; and we were on a converging path with another aircraft on taxiway 2. I looked at the Captain; and at that precise moment; he informed me that he had lost braking control.He then asked me to apply the brakes. The left brake on my side was not responding properly; which made directional control more challenging. The aircraft subsequently drifted to the right; and I informed Ground Control; 'Unable to hold short of 2.' The aircraft left the taxiway and stopped on the grass after making contact with an airport sign; damage to the propeller was observed.Once the aircraft came to a complete stop; we notified Ground Control of the situation and focused on ensuring the well-being of all passengers.The Caravan does not have an alternate braking system. To mitigate this risk; it is recommended to verbalize and announce a standard callout at every taxiway intersection:'Crossing intersection; brakes OK.'