ISX 15 cm 2250 injector rebuild

[JaAdam]

Apologies, my bad I didn't describe/start properly...
Mileage is 550k, the head is new style - metallic plungers (I think they have more than 400k).
I am chasing a Fuel pump high pressure side leak... its drop from 29k psi to 300psi in 20seconds.. I brought it to a cummins local dealer dealer for troubleshooting.. they told me that the lift pump is out of specs.. I changed it (new gaskets, torquing to specs).. nothing changed. Toke it to couple Local Mechanics too… one told me that can be injectors or fuel release valve... (figured out then that just a gussying- he don't have even the fittings and the adapters to troubleshoot properly the problem... but he was willing to charge me $400... so I bought the proper fittings, gauges, adapters to troubleshoot it by my self… “Did my home work”… partially ..)
So that what I got…
Measured the air in fuel - no air in fuel.
Measured the fuel inlet restriction (at the gear pump )-found 0psi (Specs are less than 10psi).
Measured the fuel lift pump pressure - found around 25psi (specs 10 psi minimal)
Did High-Pressure System Leak Down Test 29k psi to 300psi in 20 sec.
DIDN’t did the High-Pressure Fuel Pump Performance Test- don’t have the tool…
Did the High-Pressure Fuel Pump Return Flow Test -found 12oz (specs 18oz minimum)
The manual is saying “If 525 ml/cc [ 18 fl-oz ] pump head drain flow is collected in less than 30 seconds, the fuel pump head has malfunctioned and must be replaced.”
I THINK THATS MY PROBLEM - What do you think? Do you recommend to change only the head or the shoe fuel pump?
Did the High-Pressure Injector Return Flow Test - found 4oz (specs are less than 7) so no leaking injectors.
The pressure relive valve, High pressure pump actuator are installed before the measures (Peterbilt dealer), (the actuator is reman).
(I got your message about rebuilding the injectors- Thanks)

Thank you in advance.