Several owners have reported issue with the above, the clue is in the title. How do you know you have used the wrong fluid. When you soak the master cylinder seals, you will notice that they swell to a size roughly a third bigger than original, rendering them useless. If you soak the diaphragm, it likes like a monster compared to the original size.
Initially thought to be a problem with the products from AN, seals from other Norton spares venders that AN do not supply were purchased by one owner and the same happened. From the first occasion I was made aware I suspected the oil, the amount of seals sold by the various vendors would have raised the issue quicker.
A product from Fuchs silkolene was one brand that someone used, at the time I was able to find the product was still for sale at motorcycle spares shops but not from Fuchs Silkolene themselves. There have been instances of other brands having the same effect.
Beware, DOT 4 brake fluid is DOT 4 brake fluid, DOT 4 Compatible fluid is not DOT 4 fluid and not compatible with all seals, test it before topping up with it, when overhauling, the result is easy to spot.
I suspect you're referrin…
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I suspect you're referring to DOT 5 fluid which was introduced as it was less hygroscopic. However, as you mention it was not kind to some seals.
DOT 5.1 was then introduced and it is effectively updated DOT 4 so should be safe with all seals. I/ve never seen any fluid marked as DOT 4 compatible.....