Looking for some other methods to try. I can not get any pressure at the lever on my 06 SY250. The pistons are moving freely. I've checked all the rubber seals including the small between both halfs of the caliper.There are no fluid leaks anywhere. I have back bleed the system using a siringe. I have remved the lever and used a screwdriver to push the mastercylinder piston as recommended by Nigel Birlett.

Help please as I'm struggling on what to try next

try removing the master cylinder off the bars and put it lower than the caliper then bleed it works for me every time

Try pushing the pistons a bit further back into the caliper and sliding a thin spacer each side of the disc so that there is less fluid/air trapped behind them.

Also just to be sure, check that your hose hasn't been damaged at some point allowing it to expand when pressure is applied.