Ive been doing some research on Avid Dot 5.1 vs Dot 4 for a bit (as per Avid X0 brakes manual you can use either Dot 4 or Dot 5.1). I like others have suffered with poor consistency with my X0 brakes. Ive done the whole bleeding procedure many times making sure there are no air bubbles left in the lines. Sometimes Id spend hours bleeding them just to give me a piece of mind and this is what I found out. First Dot fluid being dot4 or dot 5.1 absorbs water from the air. That being said that bottle of avid brake fluid you opened 2 months ago has a higher percentage of water in it than when you opened it the first time factory sealed. Being realistic we have all bled our brakes and have stored the bottle for 6-12 months and then we use it again. Now you opened that old bottle and degassed it really good. Bled your brakes flawlessly and after a week of riding your levers get softer, spongy and you no longer have that braking power you did before. You can almost remember that the brakes felt better when you bled them the first time you opened that bottle. This is what happened. You degassed the air bubbles out of the fluid but you cant get that water/humidity the fluid obsorbed out. That water goes into your lines. After a couple of good downhills that water evaporates (creating gas/air bubbles)with the heat produced by the friction from your pads and rotors in return heats up the fluid. That evaporated water is the air bubbles you now have in your lines. If your the guy that stores his bike upside down youll notice that your lever will now squeez all the way to the grip. Crap right. That being said I think the only way to get a perfect bleed is opening a new bottle of brake fluid everytime but that would cost a bit. Dot 4 is cheaper than Dot 5.1 by about half the price (if you dont care about the price go for the 5.1) and can be found anywhere. The idea is you can completly flush the system of the old oil and put new oil in it thats not contaminated. There is a a boiling point difference and there is a viscosity difference that should be mentioned http://www.reddogracing.co.uk/brakebleedfaq.php:
Q: How much better is DOT 5.1 than DOT 4 fluid?
DOT 4 Dry Boiling point, 230°C min. Wet Boiling point, 155°C min.
DOT 5.1 Dry Boiling point, 260°C min. Wet Boiling point, 180°C min.
Q: What are wet and dry boiling points?
Dry boiling point is defined in the DOT standard as the boiling temperature of the fluid as new, with no moisture present. The Wet boiling point is specified as when there is 3.7% moisture by weight in the fluid, the typical situation after about a year of use.
This can vary from manufacturer to manufacturer but should never be lowere than these numbers.
Prestone Dot 4 Synthetic meets the 260 Dry Boiling Point of 5.1 but does not meet the Wet Boiling Point at 155: Prestone® Synthetic DOT 4 | Print View | Products | Prestone®
Avids 5.1 meets the minumum (sorry couldnt find the website).
What im interested in is the Dry Boiling point because Im going to be bleeding them regulary (hopefully every 6 months) and discarding the bottle because by the time of my next bleed the fluid will be contaminated. That means Avids 5.1 has the same Dry Boiling Point as Prestones Dot4 Synthetic. I personally wont care for the Wet Boiling Point because I wont be waiting that long to change the fluid. As far as for viscocity there is a difference but very minimal. I cant feel the difference.If you dont mind paying double the price go for 5.1. I like the fact that I can get Dot 4 anywhere and is cheaper. I dont have enough miles on the new fluid to 100% recommend it yet. I will be posting my findings though. My brakes are 2012 X0's. I ride All Mountain in Phoenix Az's hot freakin weather. If the fluids going to fail itll be here!!!! If anyonevhas done this already please post.
I just want to clarify Im not telling anyone to do this. Its your risk!!!!
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