Closed fermentation questions

[fozzybeer]

Hi there, I've recently started fermenting under pressure using a Fermentasaurus http://www.oxebar.com.au/fermentasaurus/
Basically I pour the wort in, pitch the yeast, attach the lid and attach a Spunding Valve. The lid also has a Pressure Release Valve.
What I'm noticing is that the Spunding valve gets up to about 10 - 12 PSI during active fermentation and then starts to drop - eventually to zero.
My question is why is this happening?
Could the spunding valve be leaking or is this expected behaviour?
I think I understand that the CO2 produced during fermentation fills the head space and then starts to get absorbed by the beer. Is this why the Spunding valve starts to drop? Is it reading the pressure of the head space only (where the pressure will be dropping as fermentation nears the end and therefore CO2 production slows right down)?

 

[Mase]

It sounds like a bad valve. It seems to be sticking on the return after the blow off.

 

[Ozarks Mountain Brewery]

its possible that the pressure is less than the valves depression point meaning the pressure is high at first then falls below the point where the valve releases so its reading 0 when that happens

 

[fozzybeer]

Thanks guys, so in theory the pressure in the head space should remain the same even though it is getting absorbed by the beer and the CO2 production is slowing down?