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IIRC it's not good to mix different DOT brake fluids in disc brake system, Will I have to flush the system completely and how much new fluid will this take? can you advise?
Cheers
Dot 4 and 5.1 are not incompatible. Not good practice to mix them but it wont cause any disasters. However you should bleed right thru with new fluid every year or so anyway. takes about 50 ml with my hope minis.
If you're changing fluid, even if you stay with the same type you want to replace it all anyway, so it's pretty much the same process. Remember that standard 5 isn't compatible, but 5.1 and 4 are.
Dot 4 & 5.1 CAN be mixed safely, but you're not getting any benefits of the 5.1 if you do.
Dot 4 doesn't need changing quite as often as 5.1, but has a lower boiling point, so if you have a overheating issue, get some fresh new 5.1
Don't use an old bottle either, make sure it's fresh.
Dot 5 is different stuff completely & needs avoiding-it won't do your brakes much good at all.
Generally (if you're doing the job properly) a brake bleed is a complete flush out of old fluid anyway. It won't do it any harm if there's a few ml of residual DOT 4 left in there though, 5.1 and 4 are fully miscible with no damage to the braking system.
There is so little fluid in hydraulic bike brakes that any attempt to bleed them will flush out the existing fluid 5.1 or 4 anyway.
😀 thanks - looks as if the bleed is on for tonight...
+1 4 and 5.1 CAN BE MIXED 😉
And dont let people tell you that 5.1 is some kind of panacea for your brakes, a good quality 4 can be better than 5.1
Which is why (for wanting of a better example) halfords sell both a basic and a race 4 and 5.1 , with the race 4 selling for almsot double the basic 5.1 .
FWIW dot 4 bought by the litre in the car and good quality dot 5.1 by the half pint in the bike as the bike gets more of a hammering (when was the last time you had to do repeated stops from 100mph in the car?). And it doesnt do it good to be left out for years so a 250ml bottle can flush through most brakes several times a year. If you've several bikes get a vacuum brake bleeder, I can bleed my hope minis in 5 minutes with it, its awesome, just pump it up, open the nipple, and pour 30ml or so brake fluid into the resevoir.
And just to confuse things even more, race/track cars often (or at least used to) used Dot3 as it boils at a higher temperature, but this drops quicker as it absorbs water, so not to be used unless your planning on bleeding the brakes more often.
(when was the last time you had to do repeated stops from 100mph in the car?)
This weekend.
dot 5.1 and 4 mix fine, though just easy to fluch through with new fluid. also only use clean fresh fluid - get small bottles - brake fluid (not mineral) is hydroscopic so over time will aborb water - water in fluid is bad for cars, expecially if you do track days/motorsport where the brakes het hot. the issue is that any water in the fluid turns to steam and thus as that is a gas the hydraulic nature of the brake system is compromised!
not sure that this is issue for bikes, perhaps long alpine downhills?
Just resurrecting this thread as I'm thinking about switching from 5.1 to 4 on my Mono Minis in another attempt to get some lasting power... Mine only feel good for the first 2 or 3 rides after bleeding, despite a recent re-build with fresh seals.
So will I notice any different with the 4? Am not concerned about overheating.
Isn't 5.1 the 'better' fluid (higher boiling point etc)?
I thought 5.1 absorbs moisture more readily than 4?