Disco1 300tdi abs brake woes

Hello chaps, many thanks for so many useful posts over the last few years but now i have got stuck.

My P reg disco with the 300tdi and abs has crap brakes. The pedal has a nice firm feel when the engine is off, turn her on and the pedal sinks down to the stop. A nice hissing noise accompanies this. It is possible to pump the brakes up somewhat.

When driving it stops but not as quickly as I would like given the brake pedal hits the stop. It is probably not what i would regard as safe with a heavy trailer. In the wet it can be rather 'exciting'.

I have bled the brakes and got minimal air out.
The wheel bearings feel nice and tight.
The pads appear to have plenty of material all round.
There is exellent vacuum from the pump and no obvious vacuum leak from the forward aspect of the servo.
No brake fluid loss.
The abs warning light goes off as one would expect.
The servo nonreturn valve appears to function.
The rear discs have light rusting suggesting perhaps they are not currently working well.

Does anyone have any suggestions for likely sources of the problem, or for further diagnostic tests that could help? So far as i can tell it is either the servo or the master cylinder. As either seems to be a £100 and an afternoons job i would ideally like a clear diagnosis before spending time and money on a best guess.

If you pull the vacuum line off the servo whilst the foot is on the brake the pedal lifts up.
Some vacuum seems to be maintained after the engine is shut down.

I am starting to think the master cylinder is the likely culprit.

It sounds more like a problem with the brake vac servo, though, the rusty rear discs could point to an abs malfunction, the servo should not be applying pressure to the master cylinder without the pedal first being depressed.
Lift the rear wheels off the ground and try rotating them whilst someone applies the brake to see if any braking force is reaching the rear calipers, they could even have seized pistons, it is not unheard of.