Firstly, thanks to all that have offered an opinion here. Its all gratefully accepted and I'll look at each idea in turn. Unfortunately I'm working away so will only get one day over the weekend to try these.
@DiscoMike. There are none of the three amigos lit up. I recently had them and was traced to a faulty ABS sensor which was replaced and everything ok after that. The brake problems have been ongoing long before that though.
@RoverMasterTEch. As far as I can make out, there is no hissing sound from inside the car, engine bay etc. Mind you with a solid flywheel conversion you can hardly hear yourself speak!
@FrankieBoy. I had DOT4 brake fluid flushed through the whole system in an earlier attempt to solve the problem.
@TRIALII. I had the calipers checked when I replaced the discs and pads. They apply and release ok, a mechanic took the pads out and replaced them, reported nothing wrong after with their previous fitting.

Now I have another theory that I will investigate when I get back at the weekend. The EGR valve was taken out and blanked out. I want to check if the vacuum pipe from that has been blanked off properly?
If I'm lucky (probably not) that might be an easy fix. Other than that I'll be checking each of the possible solutions offered here.

 

I know this may sound silly, but Midas or similar might give you a Free inspection. That might lead you in the right direction, and if not, it was free.

 

Midas?

 

Midas!!!!!!!!!!!!!!!!!!!!!!!! I wouldn't take my grand son's little red wagon to them.

Our Disco's don't have an EGR valve, are you referring to the oil separator, is so that should never be removed.

 

The EGR valve is on top of the exhaust manifold. It 'recycles' a percentage of exhaust gases into the intake air (the quantity of exhaust is controlled by vaccuum/engine revs). Later versions are done electronically.
I believe its all to do with emmissions.

 

No dude there is no none notta zero zilch EGR valve on a D II NONE PERIOD

 

Maybe Midas can find the EGR valve or add one.

 

Maybe you are referring to the SAI? On a petrol engine, it is a small vacuum can on top of a flange connected to stainless tubes that feed into the heads. There's one on each head. It allows extra air to be injected into the exhaust when it's cold so that the catalytic converters will heat up quicker.

I seen them once when I was at Midas.

By the way, what do you mean by a solid flywheel coversion? Is this a TD5 that you replaced the dual mass flywheel in? If so, then your vacuum is supplied by a vacuum pump mounted on the front of the alternator. If there is a vacuum leak in that system, it will not impact engine operation at all. And in that case, yes you do have an EGR, or at least you did until you had enough with the black smoke and blocked it off. It is also possible that the pump is weak and not supplying enough vacuum for repeated braking. You can tap into the system with a vacuum gauge and repeatedly hit the brakes and see if the vacuum drops to zero.

 

@lRsilver2001. I dont know what you drive about in but I can assure you mine had an EGR valve. One of the most common mods is to remove it and blank it off. If its not an EGR valve I dread to think what it is sitting in a box in the back of the car, whatever it is the car runs better without it.

@04duxlr. Yes I replaced the dual mass with a solid flywheel. Looking back I wish I hadn't. The engine isn't the most refined unit in the world and with the solid flywheel it sounds dreadful on tickover. I'm aware the pump is on the alternator. Someone else suggested the vacuum guage method for testing the vacuum pressure. If I can get hold of a guage I'll try that one. I'll start with the cheapest method of sorting it and work my way through. The vacuum pipes will be the first check when I get back.

 

Doctor smudge, I am in the states and drive a petrol now! I am sorry I ASSumed you drove a petrol aswell. I do know of the EGR delete your referring to and how it keeps the dirty air from being run back threw. Please post in signature that you drive a td5 so others know how to answer your questions when you have them in the future!