Okay, i used my duct tape and got out a gauge, i cant measure the pressure due to the way the valve works, you cant open it unless you press it with your finger, in which case the adaptor piece wont fit....
I think the only way to measure this is to slowly increase the pressure on the pump/compressor until it meets the resistant pressure inside the tube, when the valve opens. Let's just forget all of this until i get a foot/hand pump with gauge.
Inside of valve, dont worry, it's only vaseline that you see on the inside:
Outside closed cover