The front brake on my 08 WR250 will almost lock the wheel from rolling after using it only a few times on a ride. This just popped up from nowhere a couple days back. I've verified the brake floats on its pins as expected, verified both pistons move in and out when not on the bike, and changed out the fluid ... No real dirt on the outside around the pistons and I sprayed them really well with WD40 ... No real help really ... Do these things have some sort of relieve valve that allows the pressure to be release when the lever is released?
I can think of 3 possibilities- 1-too much fluid in reservoir 2- brake pistons binding in caliper 3- kink in brake line- creating somewhat of a check valve effect there is no relief valve.
#1 - checked OK #2 - maybe but I can move both pistons and when I press them all the way inwards, the lever can be used to move both pistons outward. #3 - might have some traction ... My cable is a little bent in a couple places... Is there an easy way to check to verify that the brake line is not the issue?
I don't know of a way to check the brake lines for this, short of pressure sensors to measure pressure in the lines, but if such a tester existed it would be money. I can't think of a home test. Best I have is the probability- that physical kinks you can see visually can produce this problem. When you pull the lever you have the strength of your hand, leverage (mechanical and hydrological) to your advantage in creating positive pressure to flow through the brake lines (and kinks/problem areas/ and overcome a sticky piston in the caliper). The reverse of this is simply releasing the lever- which largely is a vacuum (negative pressure) to return to normal. A Kink can create enough resistance (like a check valve) to not allow the system to go back to "zero".
I think I will look at the entire system off the bike next .... I'll check if I can see pressure being retained on the pads after the lever is released...
Does this make any sense: If the brake line is causing the issue and I have the problem re-produced on my bike ... Will wiggling the brake line (at the kink) remove the additional pressure and make the brakes release?
Still could be warn and hanging where the high wear position is. You sure you don't have to much fluid int he resi?
I've ran the fluid level up and down and no real change ... I might try changing the pads tomorrow also ...They aren't worn out but look all gnarled up ... After I took the caliper off the bike and was just compressing the pads together and watching the action of the pistons, I think I could see the pistons releasing slightly when I released the lever... So the brake line is still on the table along with the pads as a cause ...
I checked this also .. No help After playing with the brake line small kinks, I noticed the master cylinder fluid was all blurry looking again so I pumped more fluid out the bleed hole and the brake started working as expected again . I managed to get some air in my brake lines also but I'll deal with that later .. Thanks for help here ...
