Came a step closer to fixing this fault today. Pulled the two solenoids and pistons. Inlet side was clean and sliding freely. Outlet side (problem side) the piston was seized and full of crud and sludge. Cleaned it up and reinstalled. I don't expect to see results for a while if any given the length of time I have this issue and god only knows what else is seized but it's definitely a factor as it was stuck solid. It only took 40 mins using my homemade solenoid removal tool, looks ignorant but did the job perfect.

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