Disclaimer: I'm not a plumber...
On my MegaFlow system there is a procedure to follow when draining and re-filling the cylinder to re-charge the air gap that creates the flow pressure.
An unvented cylinder for use with a conventional boiler and electric Available in 8 sizes ranging from 90 litres to 400 litres, with the largest model capable of satisfying the […]
www.gledhill.net
Looking at the manual for your cylinder, Page 22 mentions a drain down procedure which must be followed, Might be worth doing this to the letter then closing all valves/taps and re-filling and see if it solves the problem?
file:///C:/Users/sithl/Downloads/Gledhill-Stainlesslite-Plus-Instruction-Manual_compressed.pdf
Quote:
IMPORTANT - DRAIN DOWN PROCEDURE
1 Switch off both the boiler and the immersion heater
2 Open the nearest hot tap and run all hot water until cold, then close it
3 Close the incoming cold main at the stop tap
4 Hold open the pressure and temperature relief valve until water stops discharging into the tundish and leave it open
5 Open the cold taps starting from the highest point and working down to the lowest tap, leaving them open
6 When the cold taps have stopped draining, open the hot taps starting from the highest and working down to the lowest tap
7 Open the drain cock and ensure the pressure and temperature relief valve is held open until the cylinder is empty
