did i buy a broken machine [breville infuser, kingrinder k2]

so i just bought a used breville infuser off of ebay, and ive been having issues with the pressure. this has been driving me insane qs i have now spent hours trying to troubleshoot this, so im really hoping someone can tell me if this is truly a mechanical issue, and not user error or a dialing in issue.

my problem is that when brewing a shot the pressure gauge will not go past the pre infusion mark, and the shots taste terrible. i have tried so many different grind sizes and doses and this does not change. i have tested on both the dual wall and single wall baskets with the same results. i have also noticed that when a shot choked, the pressure still stayed within that range which i think is an important detail. you’d think that choking it would raise it to max pressure. but when i tried back flushing with the rubber disk the pressure actually raised to max which is confusing.

i am using a kingrinder k2 so i know the grinder is not a problem. i have already replaced the worn gasket and have cleaned the group head and descaled. i am currently waiting on a scale to be delivered, and this seems to be the last factor that could make this user error since i was dosing by eye before (approximately a few mm below the basket rim after tamping). but i thought that the dual wall should be a lot more forgiving with grind size and dosing issues, but again i was still getting the same results with that. i have just done so many test runs (one and a half bags of coffee to be exact) that you’d think id get it right at least once.

i’d really really appreciate any help or advice.