I hear you about not wanting to spend megabux on replacement parts. This is why FCP Euro, Rock Auto, and volvopartswebstore.com are our friends. The locals want to charge $45 per head bolt (10 bolts needed), when a full set can be had genuine from the US plus express shipping for the price of 3......
This video may be instructive - it's on a 6 potter, but the fluid reservoir is the same on V8s.
https://www.youtube.com/watch?v=WPH8cK_eo14
There's a filter actually inside the reservoir, and they clog. Replacement reservoirs are about $60 USD plus shipping.
PS: Just looked at the bill from the S80's pump replacement to see what the locals charged us for a new reservoir - and, I've noticed just now how the idiot dealer didn't change it when I told them to! No wonder the thing is making noises again! The idiots should have changed it when they changed the pump.