Transcrete now has an optional air clean out system on their pumps that works very very well.

what is the sorce of the comppresed air ?

Over 95% of Transcrete pumps sold in Australia are fitted with a 16 CFM air compressor and 40 gallon air tank.

The remote air blowout also has it's own back up air tank for the control circuit. Air in and out has "dead man" control.

Nothing but good words for this system but PLEASE people what's wrong with you all?? Put these hoses DOWN on the ground and take some distance when blowing out with air.  No reason to hold it in your hands. Put it down, preferable in a straight line (no bend). If you are afraid concrete will blow away you can put a wheelbarrow up side down on top of the end of the hose. Works great.