I've spent ages trying different lengths, i know what happens if it's too long and did fall into this trap before. I put a smear of blue hylomar on the end and basically adjusted it until I was only just getting a little on the piston face. That's the best I could do. As I said brakes are there and are actually good but it's the pedal travel the mot tester has an issue with. I've now replaced servo, master cylinder, all brake pipes, flexis, fluid and bled multiple times. There's enough space with the pedal as far down as it'll go for me to fit the toe of my boot under easily so I don't know what his issue is.