Mechanic says the cp3 cannot maintain the rt pressure. He wants to replace the cups, the cp3 and stud it, one at a time, at a cost of 5500. This seems to be a little steep to me but im new to diesels
we dont have enough info from you or the mechanic to make an accurate diag on this.
But from what i gather from your posts, either the mechanic needs to diag further into the truck or you need to ask more questions.
get every injector balance rate, then get the main injection rate of the truck at idle, in park, no a/c or acc running.
next, the tech should tell you how he diaged its the cups leaking. cooling system pressure test should have been ran and checked for combustion in the system.
3rd, a return rate test should have been ran on the CP3 and injectors to determind if the CP3 is indeed bad or if its just injectors that are returning too much fuel. Just saying its not holding rail could also mean air getting in the fuel system like a bad fuel filter, fuel hose sucking shut, or fuel filter head leaking (extremely common). any of those three can cause this when you have no lift pump.
if the truck is 100% stock, studs are a waste of money. they will not prevent an old prone to leak HG from leaking. If hes is GOING to stud it like you make it out to be done, hes either pulling one bolt at a time and installing a stud without removing the head or hes pulling the heads in which case, they should be sent out to verify they are flat and dont need any more attention. from what i remember, you cant get 1 or two studs in at the back of the heads with the heads on the truck. they have to go in with the head so keep that in mind
if the tech cant get you the correct info or answer you back knowledgably, take the truck elsewhere.