Expansion tank cap is it working ?

Booked a sniffer test on my coolant next week to see if i have any exhaust gases in i but i want to ask about the cap on the expansion tank. i'm loosing some coolant and dont know where. In the morning before I start her up the top hose that goes from the heads to the rad is flexible with no pressure in it. When i undo the cap i get a loss of pressure (hissing from the cap) and a sound of bubbles coming from he engine and the coollant level rising slightly which means its under pressure cold. Is this ok or do I have a faulty cap ? Its a p38 4.0 thor engine

Sounds about right. Not much pressure in the hose and a lil hiss when you
open the cap.
For peace of mind get a new cap (£7) if the seal isn't holding pressure you'll lose water and it will evaporate without you seeing it as you drive.

Don't over fill your expansion tank. Just below half full should be fine.

where are you getting your caps from as the cheapest i've seen is about £14

Bought mine from eblag.