Still sounds like air in the system, alternatively the master cylinder might be failing.

I'm taking a gamble and I've ordered a new master cylinder.

Linked diagonally if I recall so bleed both fronts.

+1

Linked diagonally if I recall so bleed both fronts.

NO, split is definitely front to back. Rear brake limiting valve wouldn't work otherwise and there's only one pipe going to the back axle where it splits to each wheel.

Now most of my experience is with the L200's,

And I was going to suggest brake pipe clamps on the flexi pipes to eliminate or more accurately pinpoint just where the fault lies.

BUT if I'm right and the braking system is the same as the L200's then check all the vacuum pipes under the bonnet,

and the solenoids they run to.

If I recall right there's a number of small bore vac pipes the run fron the vac pump in branches,

round the back of the head to the booster.

A simple test is start it,pump the brakes,kill it then pump the pedal several times to exhaust the boost res,

then while holding the pedal hard down start it up.

If the boost vac and servo are operting right the pedal "should" dip to the floor under your foot.

Try that then clamp of the three flexi pipes one by on while re testing,

if you find a pedal improvement you've narrowed down where the fault is.

No it's never been like that. I may have given the wrong impression. I know that if you press on the pedal and it feels hard and then if you start the engine the pedal goes down. That is not what is happening here. If I start the engine and then press on the pedal it feels hard initially and then sinks very slowly further down. Best theory I've heaard so far is that when the rear circuit failed then the piston would travel much further down the master cylinder than it normally does. The wall of the piston in this area may be corroded or have a lip on it. This could damage the seal and allow fluid to pass. No fluid would leak out of the system but pedal would sink. based on this theory I've ordered a new master cylinder and crossed my fingers.