The beer was carbonated in the kegmenter to 15psi, I transferred it to the dispensing vessel with only a slight pressure differential, but once the keg was full the duotight flow stopper leaked.
See the video on youtube for my example with the PET bottle:
I connected it to the PET bottle for testing as I didn't want to wait until another keg was filled. Essentially this is replicating the liquid level reaching the bottom of the gas in dip tube.
I pressurised the PET bottle to 15psi and then screwed the spunding valve all the way in. I then slowly wound the spunding valve out to allow flow.
It looks like the flow stopper won't work under a pressure of 15psi, even if the spunding valve is only just cracked enough to allow flow.
The float ball is clean, but has a slight seam on it from manufacturing.
Thanks for that video that really helps us. You might need more pressure difference over the float valve so you have enough pressure to push the float valve hard up against the seat to ensure it seals. At the moment the blowtie spunding valve is only just slightly open meaning you dont have much pressure over the float to develop enough pressure to activate the float to seal properly.
Can you try either removing the blowtie unit all together and see what your results are or allow more gas to escape using the blowtie.
Having such a slow flow through the blowtie will also mean that you have extremly slow flow rates during filling which is not ideal either. I think all you need is more of a pressure difference over the flow stopper.