ASKED BY THE PLT TO STEER THE C421 AFTER LANDING; LOW TIME PLT IN THE RIGHT SEAT IS UNABLE TO REMAIN ON THE RWY.

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I WAS ON A TRIP/VACATION WITH SOME FRIENDS; COMING HOME. UPON LNDG; THE ACFT LEFT THE RWY. ON THE WAY HOME I WAS RUNNING THE RADIOS SITTING IN THE R SEAT. WE WERE TALKING TO CHICAGO APCH AND CANCELED ABOUT 10 MI OUT AND WENT VFR FROM THERE. THE WINDS WERE OUT OF THE W SO THE PLT ENTERED FOR A L BASE TO FINAL FOR RWY 27. THE AIRPLANE TOUCHED DOWN ABOUT 1000 FT DOWN THE RWY WITH OVER 4000 FT LEFT TO STOP. UPON SLOWING DOWN; THE PLT ASKED IF I WANTED TO TAXI THE AIRPLANE BACK TO THE RAMP. I STARTED TO USE THE BRAKES AND NOTICED THE AIRPLANE WANTED TO GO TO THE R; AND I SAID SOMETHING WASN'T RIGHT WITH THE AIRPLANE. THIS WHOLE TIME THE PLT WAS WATCHING AND USING THE RUDDER PEDALS WITH ME. AS WE WERE VEERING TO THE R WE BOTH SAID THERE IS NO L RUDDER OR BRAKE. I REMOVED MY FEET FROM THE PEDALS AND A FEW SECONDS LATER WE WERE OFF THE RWY. THE PLT TRIED TO STOP THE AIRPLANE WITH THE R BRAKE AND CAUSED THE AIRPLANE TO SKID OFF THE RWY TO THE R. UPON REACHING THE RWY EDGE; THE NOSEWHEEL COLLAPSED AND WE STOPPED. THE TOTAL TIME OF THIS WAS MAYBE 10 SECONDS. AFTER THIS HE INSTRUCTED ALL OF US TO GET OFF THE AIRPLANE. AFTER SAFELY GETTING OFF THE AIRPLANE A MECH FOUND THAT THERE WAS NO L BRAKE PRESSURE. LOOKING BACK AT THE SITUATION A BETTER SOLUTION WOULD BE TO NOT LET OTHER TAXI AN UNFAMILIAR AIRPLANE. POSSIBLE PREFLT INSPECTION; OF BRAKE FLUID LEAKS? TESTING BRAKE PRESSURE WHEN GEAR IS IN DOWN POS. IF ALL HAPPENED OVER AGAIN GOING STRAIGHT OFF THE RWY WOULD BE A BETTER CHOICE INSTEAD OF TRYING TO SIDE-LOAD THE ACFT AND STOP WITH 1 BRAKE. I THINK IF THE PLT WENT STRAIGHT AHEAD THE AIRPLANE WOULD HAVE STOPPED IN THE GRASS WITH VERY MINIMAL DAMAGE. IN THE FUTURE ASKING SOMEONE TO TAXI AN UNFAMILIAR ACFT ON THE ACTIVE RWY WOULD ALSO NOT BE A GOOD IDEA. IF KNOWING THERE WAS NO BRAKE PRESSURE IN THE L; ANOTHER SOLUTION WOULD BE TO GO TO AN ARPT WITH A LONGER RWY; AND POSSIBLY USE A SLOWER APCH SPD.