Ken, its a simple device that will not let the fuel rack to travel above the low idle spot until the oil pressure is above a pre-set psi, if the oil pressure drops below this pre-set spot even if the engine has been running at full speed on the next movement of the fuel control rack (throttle) it will peg up the rack in the low idle position,D8'sand 9's run at around the 30 psi spot, the 33oC and 3306 will run O.K. without damage in the low 20's psi, the oil pump on the 3304-6 has a pressure control valve built of hard domed washers, these washers are known to crack resulting in the oil pressure dropping quickly to usualy just above the 20psi spot, I have known Turbo D6's to suffer this sudden drop on the gauge and continue to work hard for some months without spitting the dummy out, on pulling the engine down I have found no damage to the engine innards and just simply fitted a spare prv to the oil pump, and checked out the oil filter head valves (alloy filter head)
hope this helps some. tctractors