I have a 2011 duramax with 56000 miles that i've had to the shop about 12 times for coolant leaks. so far resevoir has been changed twice, block heater replaced, radiator cap replaced, some kind of hose update that put a t off the hose that goes from radiator to the resevoir and now hooks to heater core, factory spring clamps replaced with regular hose clamps, and then hose update kit leaked and had to be replaced. thought leaks were all fixed so i went ahead and lost the dpf, installed cold air intake and went with H&S mini maxx. now i got an oil sample back containing high levels of potasium, but glycol was negative and high levels of aluminum and copper. i put a coolant pressure tester on and got up to temp and it never made more than 8 psi in cooling system. if i pressurize the system to 20 psi, which is what the new cap is, it leaks down to 5 psi in less than an hour. tried another pressure tester and it did the same. dealership just scratches their heads, anybody got any ideas?