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Rear brake dead

M

Moabxjeeper

Active Member
Location
SLC, UT
My rear brake has felt really weak since I got my bike (07 KX250F). I basically had to stand on it to get it to do anything and even then it still wouldn't lock up, so the other day I decided I would try and bleed it.

I started by just topping off the master cylinder, then replacing the cap, pumping it up, and opening the bleeder valve. I really only got 1 good stream of oil that came out, after that it was either just a trickle or nothing at all.

I decided there must be air trapped in the line somewhere, so I got a big syringe and some 1/4" tubing and tried to do the whole "reverse bleeding" thing. I hooked it up to the bleeder nipple and pumped about 30ML through it. There were a few times when some really good sized bubbles would come out of the MC. I cleaned some of the oil out and continued to pump more oil through until no more bubbles came up. I was always careful to close the bleeder valve every time I would clean the oil out of the MC as to not let any more air in.

Well, I was feeling good about all that, especially after seeing all the air that came up and through the MC, but only one problem.. My brakes are still limp as could be. I can spin my rear tire while it's sitting on the stand and hit the brake lever and it will just continue to spin freely.

Anyone have any suggestions on this? I was hoping to get out riding this weekend but not sure that's going to happen now, unless I really want to learn all about engine braking :D
 
Greg

Greg

I run a tight ship... wreck
Admin
Location
Grand Junction, CO
Moabxjeeper said:
Could be, but I'm not really sure how to diagnose it?
Click to expand...

Does the pedal build any pressure? If so, I would think the caliper could be seized. If not, I would suspect the master may have a o-ring that's shot, not allowing it to build any pressure and push the caliper closed.

I'm fairly certain the master cyls are rebuildable, shouldn't be too hard IMO.
 
M

Moabxjeeper

Active Member
Location
SLC, UT
Greg said:
Does the pedal build any pressure? If so, I would think the caliper could be seized. If not, I would suspect the master may have a o-ring that's shot, not allowing it to build any pressure and push the caliper closed.

I'm fairly certain the master cyls are rebuildable, shouldn't be too hard IMO.
Click to expand...

It honestly doesn't feel like it's building any pressure at all. The only resistance I feel when I depress the brake lever is probably from the spring on the underside. I noticed every once in a while, I would get a drop of oil at the bottom of the MC, but I was never sure if that was just oil I had forgotten to wipe up, or if it was leaking from somewhere.
 
Greg

Greg

I run a tight ship... wreck
Admin
Location
Grand Junction, CO
Moabxjeeper said:
It honestly doesn't feel like it's building any pressure at all. The only resistance I feel when I depress the brake lever is probably from the spring on the underside. I noticed every once in a while, I would get a drop of oil at the bottom of the MC, but I was never sure if that was just oil I had forgotten to wipe up, or if it was leaking from somewhere.
Click to expand...

If you're loosing brake fluid at the master, than I would say that's a good sign that it's your problem. Either rebuild or replace.
 
Rot Box

Rot Box

Diesel and Dust
Supporting Member
Location
Smithfield Utah
Vacuum bleeding is the only method I've ever had luck with on motorcycles. Give that method a try if you can. Be sure to add thread tape to the bleeder screw threads to get a good seal.
 
Caleb

Caleb

Well-Known Member
Location
Riverton
I'd pull the rear master and see if it's seized. You can get pretty much every part, spring, clip, o-ring, etc for your master cylinder or buy a complete new unit for $190. Unless you line is completely full of air, pushing on the pedal should get you a good squirt out even if there is some air in the lines. Also make sure the pistons in your rear caliper are free to move. You should be able to push on your caliper with you hand have compress it in.
 
Jared

Jared

Formerly DeadEye J
Location
Ogden, UT
Check the brake line at both ends for looseness. My DRZ did that to me at the dunes and I later found that I just had a loose line at the caliper which was allowing air in.

Jared
 
M

Moabxjeeper

Active Member
Location
SLC, UT
UPDATE: I made sure I mopped up all of the excess oil off the master cylinder and made sure the bolts were on tight and there was nothing leaking out the top. I came home the other day and there was a drop of oil on the bottom of it. There's definitely not a steady drip but something isn't sealed off right.

I called Wright's and just ordered a whole master cylinder rebuild kit. I figure I'll go ahead and rebuild the whole thing while I'm at it. I'm about 77.6% sure that's what it is, and those odds are good enough for me. We'll see what happens on Thursday when it comes in.

Wish me luck guys! I swear if I miss another beautiful day of riding like Saturday, I'm gonna scrap the whole thing and buy something else. :D
 
M

Moabxjeeper

Active Member
Location
SLC, UT
UPDATE: I got home from work yesterday and tore the MC apart. The seals looked like they were in pretty rough shape, but I couldn't find any tears or anything. One thing I noticed though was how dirty it was. I don't know what got in there but everything above the washer was black.

I spent probably an hour cleaning out all the internals with some WD40. When it looked nice and shiny again, I put it all back together and tried to reverse bleed the brakes again, but my stupid syringe kept building up pressure in the wrong spots and popping off.

I decided I was trying to use the wrong tools for the job, so I stopped by good ol' Harbor Freight and got a Mityvac 8000. It took forever for it to start pulling brake fluid through the lines so there must have been a ton of air trapped in there.

The results, however... Success! This is the first time since I bought my bike that my rear brake fully locks up without having to absolutely stand on it.

Thanks a ton for your suggestions guys, it's really nice having a source online that knows what they're talking about. I only have one problem now.. It's freakin raining outside :rolleyes:
 
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