DRG THE TAKEOFF ROLL WITH THE COPLT AT THE CTLS, THE ACFT BEGAN TURNING TO THE LEFT. THE COPLT CORRECTED WITH RGT RUDDER, BUT AS THE TAKEOFF CONTD, THE ACFT VEERED UNCONTROLLABLY TO THE LEFT. THE TAKEOFF WAS ABORTED, BUT BEFORE STOPPING, THE ACFT TURNED FURTHER LEFT & HIT A CESSNA 172 & A UPS VEHICLE, BOTH OF WHICH WERE UNOCCUPIED. THIS WAS THE 1ST FLT AFTER MAINT ON THE NOSEWHEEL STEERING SYS. A SYS CHECK REVEALED THAT WHEN HYDRAULIC & ELECTRICAL PWR WERE APPLIED, THE NOSEWHEEL STEERING SYS WOULD DEFLECT TO THE LEFT AT A RATE OF ABOUT ONE DEGREE PER SECOND. INVESTIGATION REVEALED THAT WHEN MAINT WAS PERFORMED, A ONE-PIECE ARMING/POPPET VLV WAS USED TO REPLACE A TWO-PIECE VLV, WHICH WAS PREVIOUSLY INSTALLED. DRG THE INSTLN, THE SMALL PART OF THE TWO-PIECE VLV (ABT THE SIZE OF A WASHER) WAS NOT REMOVED. WHEN THE ONE-PIECE VLV WAS INSTLD PROPERLY, THE SYS OPERD NORMALLY. MAINT INSTRNS & DRAWINGS DID NOT ADDRESS THE TW0- PIECE VLV & NO INSTRNS WERE FND TO INDC THE TWO PIECES OF THE OLD VLV SHOULD BE CHGD AS A PACKAGE.