Thought they were all sprung to relative pressure. Why not get a Heavy sprung (higher pressure) from an auto factors, even Halfrauds do them. Peugeot partner/ 806 / Boxer, LDV Pilot all have 20Lb caps so ought to fit the bill (your standard cap Should vent before these)
The one on the swirl pot is not sprung, the expansion tank one is. Car Builder Solutions do them all, Demon Tweaks pos. Shouldn't be too difficult to find.
Unfortunately the 3 motor factors that i visited today were all pretty useless, couldn't help unless i knew which car it was from. Didn't have a chance to visit a Halfords where i could have just matched it myself.
