WHILE PERFORMING THE TKOF CHKLIST; THE SPD BRAKE WARNING CHK WAS NOT SATISFACTORY. BEING #1 FOR TKOF AND ALREADY DELAYED FLT; CAPT DECIDED TO TKOF ANYWAY.

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WHILE TAXING OUT OF AN FBO AT CHICAGO O'HARE ARPT HEADING TOWARD THE ACTIVE RWY IN USE THE CREW BEGAN PERFORMING THE TAXI CHKLIST. WHEN THE CREW WAS CHKING THE SPD BRAKE WARNING AS PER ACFT OWNERS MANUAL; THE CREW FOUND THAT THE WARNING HORN WAS NOT ACTIVATING PROPERLY. AT THE TIME OF THIS DISCOVERY OUR ACFT WAS APCHING THE HOLD SHORT LINE OF THE ACTIVE RWY IN USE. WE WERE HOLDING SHORT OF THE ACTIVE RWY TRYING TO ACTIVATE THE WARNING HORN BY PERFORMING SEVERAL TESTS WHEN THE TWR CLRED US FOR TKOF. BEFORE ANSWERING THE TWR I (FO/PNF) ASKED THE CAPT IF HE WOULD LIKE MORE TIME TO ASSESS THE PROB! THIS WAS MY WAY OF SAYING WE NEED TO ADDRESS THE PROB AND PULL INTO THE BLOCK AREA. THE CAPT'S RESPONSE WAS; 'NO; LET'S GO.' AT THIS TIME I ACKNOWLEDGED THE TWR AND AT THE SAME TIME PERFORMED A SILENT SELF-CHK OF ALL ITEMS AFFILIATED WITH THE TKOF WARNING HORN. WITH ALL ITEMS CHKED WE STARTED OUR TKOF ROLL AND CONTINUED OUR FLT TO DEST WITHOUT INCIDENT. SOME OF THE FACTORS LEADING UP TO THIS EVENT: TRYING TO GET BACK ON SCHEDULE WHICH LED TO A FEELING OF BEING RUSHED; LOW TIME/INEXPERIENCED CAPT; CAPT'S DISREGARD OF CREW INPUT IN DEALING WITH SIT. THE MOST IMPORTANT CORRECTIVE ACTION ITEMS IN THIS SCENARIO ARE TO NEVER LOSE SIGHT OF ITEMS CONCERNING 'SAFETY OF FLT;' AND NEVER RUSH IN ANY SIT. NO MATTER HOW LONG IT TAKES OR WHATEVER LEVEL OF EXPERIENCE AN INDIVIDUAL POSSESSES; ALWAYS INCLUDE IN YOUR DECISION MAKING PROCESS YOUR CREW INPUTS AND UTILIZE YOUR BASIC COCKPIT RESOURCE MGMNT PROCS! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RECEIVED CALLBACKS FROM BOTH THE FO AND THE SO OF THE FLT. WHEN ACCOMPLISHING THE BEFORE TKOF CHKLIST; THE SPD BRAKE WARNING CHK WAS NOT SATISFACTORY. NORMALLY; WHEN ALL TKOF PARAMETERS ARE MET; FLAPS; SPD BRAKE AND TRIM; AND THE SPD BRAKE WARNING HORN IS CHKED; IT WOULD NOT SOUND IF ALL WERE SET PROPERLY. THEN; IF THE SPD BRAKE WAS BROUGHT OUT; WITH A THROTTLE ADVANCED; THE WARNING WOULD SOUND. BOTH FLC RPTRS WERE CONCERNED ABOUT THEIR CAPT'S LACK OF HEEDING THEIR INPUT TO TAKE MORE TIME TO ASSESS THE PROB. THEY HAD ALREADY BEEN DELAYED; THEY WERE #1 FOR TKOF AND THE CAPT CONSIDERED IT SAFE TO TKOF; IN SPITE OF THE FACT THAT AN IMPORTANT WARNING SYS WAS APPARENTLY NOT WORKING PROPERLY.