I talked to the MOT cahp at te grafe.
What they do, is wash off all te grease and even brush the pipes with a wire brush to see if the corrosion is severe enough to warrant failure.
The ministry approved chap who retested my car after a garage tried to use the MOT to scam me, said that they are within their rights to fail anything they cant tell ought to pass as well.
(By the way, you are within your rights to remove a car from a garage even if it has no MOT, and they have failed it, if the purpose of taking it there was to get an MOT, and if it still *has* an MOT, then retaining it on the grounds that it is 'unsafe' until said garage spends several thousand on it (unnecessarily) is actually a criminal offence of extortion).
Or fail it outright.
I think getting my rear brake pipes replaced last time was around £80. I've done it myself when younger on a Saturday afternoon for about £20 including a brake flare tool.
A small price to pay - less than a tank of diesel then - for knowing that they were sound, and that the brake fluid in the system had also largely been flushed out.