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Geeratedink
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Discussion Starter · #1 ·
So i recently bought an 02 f4i. I've slowly doing all the maintenance. I just changed the rear brake pads and bled the system with a bleeder and all. While riding the next day i was going about 60 down the street while my bike started slowing down suddenly then to a complete stop. Turns out my rear brake somehow self pressurized. I couldn't pump it or get it to unstick until i released pressure through the bleeder valve which then freed it what can be causing this?
 

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CAPTAIN AWESOME®
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did you change the fluid or just bleed it down? Bad fluid, water in the fluid,.......... causing it to heat up and pressurize the piston?
 

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Geeratedink
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Discussion Starter · #3 ·
did you change the fluid or just bleed it down? Bad fluid, water in the fluid,.......... causing it to heat up and pressurize the piston?
Yes i did also change the fluid to motul 5.1 as the old fluid was trash...
 

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I know there was a big deal with 18 wheelers running DOT 5 brake fluid especially when they were above or below sea level the system would either over pressurize or not pressurize at all so you could try going back to DOT 4 fluid (honestly if you're not racing the bike on a weekly basis DOT 4 fluid is more then you'll ever need)
 

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5.1 fluid is just less viscous than the 3/4 specs, it was designed for newer bikes with ABS pumps that perform better with a thinner fluid. That said, in itself it shouldn't cause that issue.

The times I've seen brake lock it's been from
-aftermarket levers/pedals that do not fit properly, and always slightly drag the brake. The heat expands the fluid and pushes harder. I have heard of this suddenly and abruptly locking the wheel. Not good. This can also be the case if the stock ones were bent in a crash. Lift each wheel off the ground and make sure it spins easily. If you're suspicious, take it for a short ride, use the front brake only, and immediately afterward check the rear rotor for heat.

-Spacers improperly installed on the rear wheel/ caliper bracket. Some bikes the L/R spacers are similar enough to look the same, but slightly different lengths, so the caliper sits cockeyed and rubs if they are swapped, causing the same heat issue.
 

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Sick of being frozen
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Sometimes dirt gets into the return hole for the master cylinder check to make sure both are clean and free
This was my first thought too, if the return hole in the master is gummed up with old fluid you will get that exact behavior
 
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Geeratedink
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Discussion Starter · #10 ·
Alot of good input Thanks guys .. Got a big ride to go to this sunday so ill be taking off inspecting and bleeding and checking for all that
 
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Dot 3/4 is hydroscopic Dot 5.1 isn't, but if you didn't fully clean the system before changing over the 5.1 is contaminated & this can cause problems
 

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Dot 3/4 is hydroscopic Dot 5.1 isn't, but if you didn't fully clean the system before changing over the 5.1 is contaminated & this can cause problems
I believe you're thinking of DOT 5, which is silicone based, unlike 3/4/5.1

DOT 5.1 is the same chemistry as 3 and 4.

If DOT3 is "basic", then DOT4 is the same thing with a higher boil temperature. DOT 5.1 is essentially DOT 4, but with a lower viscosity so ABS pumps can pulse the brakes faster.

Short of track days or extremely aggressive street riding, dot 3 is fine for anyone. Most new bikes come with 4 or 5.1, just because it really doesn't cost any more and is technically "better", though 99% of riders will never experience the difference from 3.
 

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Feckin Eejit!
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5.1 has half the viscosity (I think... I'm not sure of the scale used on the study I read. so having 5.1 be 'half' of 4 could be something else) I don't really see this being much of a help in any way, except in a computer controlled braking system like ABS as mentioned above. Viscosity of fluid in a closed hydraulic system is beyond me.
5.1 has only a 50degree higher boiling point.
Source: http://www.stoptech.com/technical-support/technical-white-papers/brake-fluid



I have boiled dot 4 in a CRX once. Pretty sure it would have boiled dot 5.1 too... It wasn't an autocross event, but I was sure pretending it was :p
 
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