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Intermittently losing rear brake. 2014 250

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The last two races I have been losing my rear brake.  The pedal will just go all the way down and have no resistance.  I found if I pump the pedal a bunch it will come back, but then it'll happen again a while later.  There is no leak.  The first race the fluid was slightly low with small bubble seen on sight glass.  Topped up and it happened again.  Brake pads are great, replaced after the first time because they were due but it happened again.

I figure this must be a master cylinder problem.  But possibly the fluid is boiling?? It's a little older now.  I will completely flush out the fluid before replacing the master.  

Just wondering if this happened to anyone else and if they found anything.  

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You have leaking seals in your master cylinder.  It lets the fluid move from the "front side" of the master cylinder piston which pushes on the calipers to apply the brakes to the "back side" of the master cylinder.  There is no net loss of fluid to see in the master cylinder sight glass.

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How often do you replace the fluid? I usually only went 3/4 races rear 1 season front.

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I have had the exact same problem. I am going to try rebuilding the master cylinder. Let me know what you find out with yours. My bike is a 12 xc300.

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6 hours ago, markit said:

How often do you replace the fluid? I usually only went 3/4 races rear 1 season front.

It's probably been 2 years on this bike and 100 hours.  And alot of races.  So probably way overdue, it'll get flushed when this rebuild kit comes in.

mxracer I'll let you know but won't be til mid July because I got a cast on my arm for now.

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56 minutes ago, SilentAssassin II said:

It's probably been 2 years on this bike and 100 hours.  And alot of races.  So probably way overdue, it'll get flushed when this rebuild kit comes in.

mxracer I'll let you know but won't be til mid July because I got a cast on my arm for now.

Whoooo, you must be light on the rear breaks. I would say after 4 races i would have no breaks at all. I use the Valvoline synthetic from auto stores for years with very good results. Ones you get a good back bleeding set up (looped hose and plastic bottle) it only takes minuets to change fluid, literally much easier than changing a filter. 

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17 minutes ago, markit said:

Whoooo, you must be light on the rear breaks. I would say after 4 races i would have no breaks at all. I use the Valvoline synthetic from auto stores for years with very good results. Ones you get a good back bleeding set up (looped hose and plastic bottle) it only takes minuets to change fluid, literally much easier than changing a filter. 

I'm not that hard with the rear brake as others can be, mostly just use it for english.  I am harder on the front. Depends on the terrain though we have a wide spectrum so it can change depending on that.

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Yea I'm in the deep south and most our races are on the tight side so a lot of break turns. When i had the Recluse i would just hold the throttle wide open and use the rear break to turn, That was some good times almost made the 4T (250 xcf-w w/ht 310 bb) fun. LOL Damn, now i'm wanting another 250F.

Edited by markit

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Check the rubber bladder in the master cylinder under the cap.  I have had weird brake problems in the past that would show up under race conditions, and it turned out to be pin hole leaks in the bladder.  Unfortunately, it is a Brembo part.  I don't remember what the price was, but it was not cheap.  

Edited by rpt50

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