I had this with my V40, which unfortunately for me I couldnt use the gunson on as the master cylinder had a stupid take off for the clutch. This meant as soon as i pressurised it with the eazibleed it would piss brake fluid everywhere.
Anyway, i digress. I did one caliper on the Volvo and bled the old fashioned way - never let the master cylinder run dry etc etc and after i was done it was constantly shit. I just couldnt rectify it. I spent about two days rebleeding the caliper and got nowhere.
someone suggested that air might have got into the other calipers lines somehow, i scoffed as i didnt see it possible. Bled the front left caliper (there is a bleed sequence for each car somewhere) as this was the first one and it cleared it straight away.
I would suggest air has definitely snuck into the system from somewhere. I was paranoid it was in the ABS unit too, which can happen, but all that happens there if it does is that air from one line can apparently get into the other lines.
My advice? Bleed in sequence like a champion but use the old fashioned way. You could also find that the gunson is introducing air somehow, maybe through the seals between the master cylinder and the reservoir?