This procedure is a combination from a posting on the ZRXOA message board by Walt #255 and the Kawasaki ZRX Service Manual:
Bleeding the air must be performed anytime the cooling system has experienced leaks or is drained. Here is a step by step procedure:
 

 

• Set fuel selector to “PRI” to fill the float chambers (This is so you don't need an external fuel tank later on in this procedure). Set fuel selector to “ON” or “RES”. With engine cold, remove the fuel tank.
   
• Put a suitable shim under the sidestand to place the bike near vertical (A 1 x 4 works for me!).
   
• Open the filler cap on the radiator and add anti-freeze/distilled water mixed 50/50 up to the top.
   
• Start the engine and let it run to circulate coolant. Run until no more bubbles appear. Tap the radiator hoses to dislodge air pockets.
   
• Again add coolant mix to the radiator up to the inner filler lip only.
   
• Start engine and allow to warm up until coolant begins expanding, going above inner filler lip. Install filler cap tight.
   
• With engine still running, place hose over air bleed nipple and crack nipple open. Continue running engine until coolant flows from hose. Close air bleed nipple.
   
• Shut off engine.
   
• Place absorbent rag or paper towels around hose and air bleed nipple to catch coolant in hose. Remove hose. Wipe up any spilled coolant from engine.
   
• Reinstall fuel tank.
   
• Start engine and allow it to run until cooling fan comes on. Stop engine. Allow engine to cool.
   
• With engine cold, check level in coolant recover tank and fill as necessary.
   
• Pat yourself on the back, you're done!!!!!