C12 problems.

I have a 2006 Cat C12 covered 500000 miles. Recently noticed moisture in the breather pipe. Pulled a valve cover and its the same story. Its using nearly a gallon of coolant in 1000miles. Been keeping an eye on the temp and oil pressure on the CAT ID as I have no oil gauge and thats down to 26 ibs running and 12 ibs idle. Yesterday I felt the Jakes were losing their effectiveness .Verified this with a mechanical gauge too. I have also pressurized the cooling system to 14 ibs and pulled the pan and cant find any internal leaks. Can this be 1 thing like a cracked head giving me this problem or has my luck gone bad?

 

Had a couple of hours on it today. Had the oil pressure relief valve out. Its got some scuff marks on the plunger but unless I strip the filter housing off I cant tell what the bore is like as the chassis wont let me see in the hole. Had the valve covers off to see if there was a cracked Jake housing or loose solenoid again it all looks nice.
Thanks for your replies so far.

 

Bit of an update. Friday the oil pressure was down to 20lbs, I had 2 very local loads to do so did those and back to the yard. Pulled the pan,got the pick up pipe off and had a good check for cracks, next off with the oil pump, the end cap showed very slight wear so that was cleaned and put back together. Pulled no 5 con rod cap and the bearings were decent,put a dial gauge on the crank and end float was .008” so no problem there. I got the oil filter housing off next. Still thought the oil pressure relief valve didn’t slide too well in its bore so I got it in the lathe and polished it up with wet and dry. Put it all back together with fresh oil and filter. Pressure was at 38 lbs tick over on initial start up. I am going to get the old oil sampled. Still have moisture in the oil breather but at least oil pressures back.