Bit if a weird one, that I've not seen as a mechanic. I'll start from the beginning... Fitted S3 front brakes, noticed they were binding. Checked on brake tester, and left caliper effort was slower than right, but overall effort was actually higher. They also binded a bit. Stripped out pistons, which were fine. Cleaned and lubed sliders, obviously bleed brakes. Binding was fixed, but still left side was slow to apply. I was thinking a collapsed flexi maybe, un likely, but would explain it. So put original brakes back on for now as I've been off work last week. Today, out of curiosity, I put car in brake tester with standard brakes on. Exactly the same results, left side applies slower, but with higher overall effort. Both standard and S3 are imbalanced enough to fail mot. Bit both feel fine under braking. Doesnt pull at all. So this has ruled out calipers, carriers, flexis. Anyone ever had anything like this? I'm now down to master cylinder, steel line, valve (assuming it has one) on bulkhead, ABS unit.