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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