OK ..... I gave up trying to fix this, had dozens of people provide suggestions. Replaced loads of spare parts, taken it into Indespension twice - still unresolved.
Instead I modified the Coupling so that the 'Brake actuating lever cannot move' i.e. the hydraulic coupling cannot move backwards.
I did this without touching the sliding tube, damper or seals.
Instead I drilled the frame such that a 12mm high tensile pin can be inserted immediately behind the 'S Pawl' (Brake actuating lever).
The white on pin is just primer I was pushing into the hole .... the red band is to make sure it goes in for correct insertion depth.
Not a through hole ... though both walls of the square section and 3/4 through the handbrake assembly plate .... didn't make it through hole as it would go though the swivel part of handbrake assembly, and was too close to grease nipple.
Sorry I can't post pics ... forum does not seem to want to accept links from FLIKR today
Insert pin for reversing up hill, remove and brakes function as normal.
https://flic.kr/p/JPrAtD
https://flic.kr/p/276Fj9v