Need someones help with my brakes!!

Hi all, Would really like someones advice. I have a 300tdi with no pressure in the brakes. I have pressure when not moving, i can rev it, leave on idle for ages go back and the pedal is fine. Soon as i move forward i loose pressure and the pedal just goes to the floor! I can pump it (10 times ish) and build back up again, but as soon as i move... Yep, nothing! I have done the callipers, Pistons, seals and pads. also just got new master cylinder but still not fixed the problem. Its got all new fluid in the system and been bled 3 times, just to make sure. I have never lost any fluid either suggesting no leaks. Thought it might be the vacume pump so checked the pipes which look fine and the pump is sucking pretty strong. Could anybody help me with my nightmare of a problem!!!

have you renewed pistons and seals your self ,seems like your still getting air in there .the pedal should feel harder before starting as vac pump makes it easier to press pedal ,is it abs disco 1 or 2

Thanks for the reply. Its a 1996 model. No abs. The pedal will be hard as long as i turn it off with pressure there. if i push pedal too floor and switch off it stays with no pressure but builds up when pumped. Done seals and piston myself thinking it might be the problem. Pretty sure i done it ok. Done it before on another car. With engine running and vechile stationary i can push pedal hard and dont get any movement in pedal. Was told pedal might slowly sink down if air in system. Have bled it few times now though.

just to rule out obvious calipers bolted on tightly discs in good order no wheel brg play, pistons in right way round,when you say i can push pedal hard and dont get any movement is that after free play.whem pedal pumped to you get splashing in reservoir

il have a look after work. Check them. The pedal will go hard after i build pressure up and stay hard, normal, like it should be. Until i move the car. Even a few inches. There was no build up of a problem. Just happened as i came off a slip road. No brakes! But il check it later. Thanks.

Check for distortion in the disks and wheel bearing play

also unlikely to be at fault but won't harm to check, the front brake flexy hose to make sure there not balooning when they are under pressure.

Checked it all. all calipers are fine, Discs are nice and straight and not in bad shape. Cant feel any play in any wheels, Bled the system and checked all visible pipes and hoses, No loss of any fluid. Cant really listen out for anything over the engine. Ive rang up ang got someone to come and have a look. Works for land rover so should figure it out. Thanks for the help! Il be sure to post what this pain of a problem is. Cheers.

Sounds like a faulty servo or servo check valve.