Plumbing Questions? Ask a Plumber for Answers ASAP
I wouldn't attempt to do this underwater as it's a pneumatic tube, so water shouldn't be allowed to enter it.You should isolate the water supply to the cistern, although that's not vital, and remove the flush valve. You do that by twisting the entire valve anti-clockwise and lifting it out. You'll have to do this anyway, to replace the flush seal.Many concealed cisterns are installed without much room above them, so you might struggle to get the valve out. You could measure the gap above it first and let me know, and I'll tell you if I think you can get it out without removing the cistern.