corrosion on brake line fail

Totally agree with the above. By "cleaning" the corrosion you have actually made it worse as material has been removed. However,as they said on my refresher in April, if it comes back covered in grease,you have to pass and advise. You cannot remove the grease. Unfortunately common sense isn't high on the list of the DVSA test standards. Like the steel plates that I saw screwed, yes screwed onto the back of outer sills on a Mazda. Now I know that some clever git is trying it on. But according to DVSA it a P&A. Non standard cover. Can't fail on inadequate repair,you can't see its corroded,so you can't say it's corroded. Complete joke.

Exactly what I said. But you cant argue the "repair" is inadequate, you'd have to be guessing that corrosion exists behind the plate. Bloody daft.