I have a couple pics in my gallery. The unit I have ran about $800 for the manual version. Its a spring loaded butterfly valve and it works!!! I have a spring loaded toggle in the cab with a protection loop on it. Just pop the switch and the valve slams.. Has to be manually reset.

My biz partner has the electronic control version (monitors RPM for shutdown) and that added about $300...

 

Ive got 2 fully automatic positive airs, from roda, the one sometimes goes off by itself, at startup, when moisture gets in the connections, but are trouble free other wise.