I've got to a point where I'm properly a-scratching my bonce. I've bled the brakes once the conventional two man method, and twice with a pressure bleeder (awesome bit of kit, best thing I've bought in ages!), and I've even replaced the master cylinder, but still spongey.
I've gone through two bottles of Dot 5.1, so the lines are clean and there are no leaks. The springs on the calipers are in place, no kinks.

The problem is, the brake pedal is rock hard when the engine is off, but soft as sh*te when it's running. I'm looking at the servo as we speak, do these leak air at the pedals, also there is a bit of rust behind the m.cylinder, not much though.
It must be the vacuum, but I've been told it's not. It's pretty obvious it must be, can someone advise??

It dosnt sound like the servo mate as if it was leaking Vaccum then the pedal would go hard, not spongy....

Bleeding Brakes :

Make sure the Master Cylinder resivoir is topped up at all times during the bleeding proces.

Start with the bleed nipple furthest away from the master cylinder, once this one has been bled then bleed the diagonally opposite bleed nipple. Once this one is bled then go to the remaining front bleed nipple and bleed that one, this should only leave you one rear one left to bleed, the one on the same side as the brake master cylinder but at the rear.

=====================================

A simple way to test for air in the brakes, pump the brake pedal up so its hard with the engine off, leave it for about 2 mins and then press it again, it should feel the same as you left it (Rock Hard) if its dropped or become spongy in the 2 mins then there is a brake fluid leak or air in the system....

I can have a quick look for you if your stuck mate, let me know......

There's no air in the system at all though. I did the 2 minute test, rock hard it was.
Someone suggested clamping each brake hose to pinpoint the problem. Are you any good with calipers?!

Yeah mate, there is no point in clamping the hoses off coz you have not got air in the brakes, if you did have air in the brakes then the pedal would have gone down when you pressed it after the two mins as the air would have expanded over the two mins period.

Are the rear brakes disc or drum ?

Discs on the back. I meant clamp the hoses to isolate one of the calipers.
I know I'll kick myself when I realise what it is.

I guess there is no harm in trying mate