Later HLP models are fitted with a silver hexagon allen drive cap at the back of the pump. To bleed these models, whilst the pump is running, you simply unscrew the cap until you get a dribble of water coming out of the back of the pump. Now using the shut off valve fitted on the PRESSURE side of the pump, close it and open it sharply 2-3 times. You should see bubbles of air being forced out the back of the cap. Continue this until no more air is released and then tighten up the cap. Then do the same with the tap on the SUCTION side and you should see any air that is in the main housing of the pump being removed.
With earlier models or with pumps installed where it is difficult to get to the back of the pump there is a different procedure.
Close both shut off valves and remove the pump from the system. Take the pipe connector off the suction side of the pump and look inside. You should see in the centre of the impeller a screw which currently blocks a passage through to the bearing section of the pump. Remove the screw and install the pump back into the system. Now once again with the pump running, sharply close and open the tap on the SUCTION side of the pump. This is the opposite of the above method as you are trying to suck the air out and not force it through the pump. Each time you sharply close the tap you should see air exiting the pipe. Continue to do this until no more air comes out. Now do the same with the PRESSURE side a couple of times and then back to the SUCTION side. By now there should be no more air coming out and the pump should be running quieter with no air sounds and cool/not hot to the touch at the back.
Now switch off the pump and remove it once again, put the screw back in the centre of the impeller then reinstall it back into place.
The pump should now run normally.