You could put the loop together outside of the system and connect it to power and do some minor leak testing there but once it goes in the system you will need to leak test it again to make sure everything is still good to go
Well for leak testing, you get everything installed in the system and out a bunch of paper towels throughout the case and let the system run for about 12-24 hours, If there are no leaks then you should be good.
Okay, so full build implementation? For some reason I thought there was a way to leak test without all of the components installed. Custom loop is only for the CPU.
You could put the loop together outside of the system and connect it to power and do some minor leak testing there but once it goes in the system you will need to leak test it again to make sure everything is still good to go