As it was explained/sold to me by the oil company who supplied the lubricants for the factory bike race team back in the day; DOT 5 (silicone) is better in terms of paintwork damage (important on bikes given the amount of painted plastic on them & any accident normally ripped a reservoir off so it went everywhere ! It has a lower boiling point V’s DOT 3-4 but also as correctly stated above wasn’t as good in compressibility terms, quite important at the extremes of performance ! However it didn't absorb water as quickly over time in service (not relevant to anything race wise) but more importantly in a race situation when it did 'boil' it didn't degrade as quickly. DOT 4 boils at x degrees, but as it re-absorbs the moisture it then means it degrades again therefore will boil again at say half x degrees, and so on, hence it gets worse rapidly in a race/day. DOT 5 has a much slower degrade after overheating or through natural moisture absorption. The seal swelling is certainly true with different DOT’s. Most new volume selling cars run DOT 4 because .. it’s cheaper and more easily available to us owners for home maintenance. Working for a vehicle manufacturer I can assure you that’s the case. Given it seems the original problem has been solved, which was the reason for posting, maybe not too relevant but all adds to the knowledge base for a paddock bar debate !
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