ronobrien
Guru
If cylinder compression is getting to the cooling jacket, then coolant will be getting into the cylinders when the engine is stopped. Evidence of coolant in the cylinder(s) will likely be there and tell you of a head gasket problem, which would explain the pressure buildup in the coolant jacket. This would verify a head gasket issue in that area without pulling the head. I have been doing diesel repair/rebuild for 40 years and can tell you it this works (assuming the injectors aren't in a pre-combustion chamber). Another way is to use an exhaust gas analyzing test bench and sniff for CO at the coolant filler cap. Though not many lay persons are going to have test bench.