While taxiing in after landing an instructor pilot and his multi-engine student lost control of their PA34 when the brakes failed; running off the taxiway and striking a sign.

Narrative

Upon landing on Runway 9L; I exited the runway and brought the aircraft to a full stop to go through the after landing checklist. After receiving taxi instructions from Ground Control; my commercial multi engine student proceeded to taxi the aircraft. I noticed that he was making a turn in the opposite direction of the runway. I asked if everything was fine; and was advised yes. Upon correcting the aircraft in the correct direction as instructed to taxi; I noticed the aircraft starting to veer to the left of the taxiway. I asked my student again if he had control of the aircraft? He said yes; and I noticed the aircraft still veering to the left of the taxiway; and I assumed control of the aircraft. When I took control of the aircraft; I pressed the copilot brakes; and immediately noticed that the left landing gear brakes were functioning; but the right side were not. The aircraft was veering to the left; because when my student pressed the brakes; the left brakes worked but the right did not. When I pressed the brakes; the aircraft veered more to the left because of right brake failure. I immediately pulled the emergency brakes; and there was no pressure. The throttle was in idle as I took control of the aircraft; so I placed the mixtures to cut off. The aircraft was still in motion; and went off of the runway; and struck a taxiway sign.