Update: the reservoir tank is fine, but as some have suspected, the hose is useless. I thought it was leaking, but the hose seems to be actually porous to brake fluid and allows it to slowly sweat out the sides! So I need to get a new brake fluid hose (unfortunately the original is just a tiny bit short to fit with any slack).
I’d still recommend this part, but don’t use the hose that comes with it!