There has been a warning for newer hayes brakes ( hfx-9 ) that dot 5.0 and 5.1 fluid is NOT recommended for use in their brakes. hayes's reason is that 5.0 and 5.1 tend to leach gas into the system and create a small pocket of gas that makes the brake fell like shit. Hayes is saying that Dot 3 or 4 will prevent this from happening and therefor recommend those fluids only.
All i know is that the Hfx-9 brakes have had nothing but problems and i dont think that the brake fluid has much to do with this. let me know what you think.
Cheers