Weak rear brake, why?



20 replies to this topic
  • fdme

Posted May 21, 2013 - 07:14 AM

#1

So I have a 2000 yamaha wr400, ever since I bought it second hand, the rear brake has always felt weak. What I mean is you have to really mash it down to lock up the rear even on dirt. So I inspected the whole rear brake system it is not leaking from any seals etc... its got all new fluid and all air is bled from system. I also replaced the pads even though the old ones still had 50 percent life.

The only other thing I can think of is the master cylinder? It doesnt appear to leak or anything but could it cause poor braking performance if its old? It looks original. It also feels like the rear brake drags excessively, especially after putting in the thicker new pads.

Edited by fdme, May 21, 2013 - 07:15 AM.


  • yamahakid88

Posted May 21, 2013 - 07:47 AM

#2

You could try rebuilding the master cylinder. Did you yourself replace the brake fluid? If so, did you do it correctly?

  • fdme

Posted May 21, 2013 - 08:00 AM

#3

I did indeed replace the fluid. I drained the old stuff, filled it with new and then flushed the system with the new fluid. Drained it again, filled with new, bled system. Im confident there is no air in the system... am I missing something? I did it the same way you would do a car etc... stick a tube from bleed screw in a resevoir of fluid and keep the brake resevoir topped off and pump till no more bubbles appear in tube.

Ive read in several places that wr and yz rear brakes are kinda iffy? Ive never rode another besides mine . I cant compare. But my friends xr400 and cr125 are like night and day compared to mine. As far as braking power is concerned.

Edited by fdme, May 21, 2013 - 08:02 AM.


  • maboomboom

Posted May 21, 2013 - 08:28 AM

#4

OK so I needed to rebuild my rear master cylinder on my Kawi... I could not for the life of me get it to bleed correctly to get pressure. I can bleed car brakes with ease so I thought this would be simple. It wasnt simple for about a week with off and on tinkering and I refused to take it somewhere.

I ended up figuring out that I needed to bleed it in sections then as one unit. SO..I bled the master cylinder on its own first, then the caliper and then the entire thing, It worked great and got it perfect! Hope this helps a little.. dont give up, you'll be a brake pro by the time your done.

Edited by maboomboom, May 21, 2013 - 08:29 AM.


  • k.g

Posted May 21, 2013 - 08:36 AM

#5

I changed fluid in my yz rear brake not long ago. I ended up taking the unit off and bleeding everything separately. On the bike I pumped and bled for an hour, and didn't accomplish anything but spilling brake fluid everywhere.

  • ahaussman123

Posted May 21, 2013 - 08:48 AM

#6

for the record, it may be normal, just that you're not used to dirt bike brakes.

I just added fluid, and replaced the rear brake pads on my yz125, who's pads were down to the metal. NO idea on the condition of the brakes, but didn't have the time to bleed or rebuild the master. I rode homaston dam on sunday, and thought to myself "man these rear brakes suck why do they suck?!" however, after meeting up with some friends and trading bikes, he was complimenting my 125 pads to his ttr230 drum brakes. I think you're expecting too much out of your rear brake, they do kind of need to be mashed to provide stopping power, the point is to mash the rear and tap the front if you need to stop short. Even before I trusted my rear brakes I had surprisingly good stopping ability around a blind curve at 65-70 mph, in the rain, on pavement, when I mashed my rear brake.

  • YZrider2112

Posted May 21, 2013 - 09:12 AM

#7

for the record, it may be normal, just that you're not used to dirt bike brakes.

I just added fluid, and replaced the rear brake pads on my yz125, who's pads were down to the metal. NO idea on the condition of the brakes, but didn't have the time to bleed or rebuild the master. I rode homaston dam on sunday, and thought to myself "man these rear brakes suck why do they suck?!" however, after meeting up with some friends and trading bikes, he was complimenting my 125 pads to his ttr230 drum brakes. I think you're expecting too much out of your rear brake, they do kind of need to be mashed to provide stopping power, the point is to mash the rear and tap the front if you need to stop short. Even before I trusted my rear brakes I had surprisingly good stopping ability around a blind curve at 65-70 mph, in the rain, on pavement, when I mashed my rear brake.


This is true; particularly on an older bike like his. If I remember correctly, the brakes on that bike are the ones with the separate master cylinder? You might be able to buy a later model rear brake set-up on eBay pretty cheap.

  • Holeshot1982

Posted May 21, 2013 - 09:32 AM

#8

I'm going through the same thing on my 2010 RM-z 250... Bought a rebuild kit for the master cylinder... Hope that helps...

  • fdme

Posted May 21, 2013 - 10:30 AM

#9

Thanks for all the replies guys! It does indeed have a separate master cyl. I am definitely pretty new to dirt bikes so I probably am expecting something it shouldnt be. I guess I was kind of expecting more of a mountain bike feel where you could lock em up pretty easy, I just thought it would at least feel up to par with my friends xr lol. I might rebuild my master cyl later... anyways in the mean time I guess I will try to adjust to it. I might also try to bleed it in sections like several mentioned... Thanks again good advice!

Out of curiosity do any of you guys know if the wr450 brake master cyl will fit the wr400? The 450 one seems to all be one unit, maybe it would work a bit better.

Edited by fdme, May 21, 2013 - 11:02 AM.


  • Duderino

Posted May 21, 2013 - 03:27 PM

#10

Check your rubber lines. They sometimes will delaminate on the inside reducing the volume of fluid going to the brakes. I've seen it several times on cars. Just a though...

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  • Duderino

Posted May 21, 2013 - 03:37 PM

#11

Oops reread your post. I noticed you mentioning brake feels like they drag. I also didn't notice if you checked that the caliper pistons move freely. The caliper piston can get rusted/dirty and sticky causing reduced braking force and dragging.

BTW: the rear brake on any modern disc brake system should lock up EASILY. As a braking system it provides a minority share when compared to the front...you should never have to mash it. My kids KTM 65, tiny little rotors and all will effortlessly lock up with my 210 lbs butt over the back tire.

  • fdme

Posted May 24, 2013 - 07:50 AM

#12

Oops reread your post. I noticed you mentioning brake feels like they drag. I also didn't notice if you checked that the caliper pistons move freely. The caliper piston can get rusted/dirty and sticky causing reduced braking force and dragging.

BTW: the rear brake on any modern disc brake system should lock up EASILY. As a braking system it provides a minority share when compared to the front...you should never have to mash it. My kids KTM 65, tiny little rotors and all will effortlessly lock up with my 210 lbs butt over the back tire.


Hmmm yeah I was wondering about the dragging... the piston moves when the brake pedal is pressed and it does appear to retract a little when let off. It definitely doesn't feel as smooth as the front though. However I did completely rebuild the front brakes, im wondering if the back system is also gonna need the same treatment. I could just start with a rear brake rebuild kit since its like 20 bucks... the only reason I didnt rebuild the back at the same time is because it wasnt leaking like the front.

And I did feel like the brake should be easy to lock! I did think maybe I wasnt used to it like others suggested but I have felt much better braking from even lower performance bikes so I was confused.

  • Do I Care?

Posted May 24, 2013 - 11:37 PM

#13

If your brakes are working but not correctly then I would suggest that your caliper piston seals need replacing. If the master cylinder needed replacing it would leak (already been there).

If you want suggestions for replacement ie. eBay- YZ125 00-03, YZ250 00-03, YZ400, YZ426, WR250 00-03, WR426.
If you look on eBay, you should be able to find a good deal on a full system- I found a complete rear brake system that was near new and payed AU $100.00 delivered!

Enjoy!

  • GlennR

Posted May 25, 2013 - 07:34 PM

#14

I doubt this is your problem, but my Honda Rancher's rear brake pedal's pivot shaft got sticky and needed to cleaned & greased. It had the same symptoms that your bike has, hard pedal & brakes drag.

It was a simple fix that wasn't even part of the hydraulics. (I wish everything was that simple to fix...)

  • Do I Care?

Posted May 25, 2013 - 07:39 PM

#15

I doubt this is your problem, but my Honda Rancher's rear brake pedal's pivot shaft got sticky and needed to cleaned & greased. It had the same symptoms that your bike has, hard pedal & brakes drag.

It was a simple fix that wasn't even part of the hydraulics. (I wish everything was that simple to fix...)


Good point- they are always needing cleaning and regreasing with lithium based grease.

My 1999 WR400 (one year older) needed a full replacement as the pistons were even worn and not worth the extra cash to fix. Hope your problem is not that bad.

Edited by Barra8, May 25, 2013 - 07:42 PM.


  • behive

Posted May 25, 2013 - 08:58 PM

#16

Ensure the caliper can slide on the guide pins, if pins seized the piston is acting on 1 pad only.

  • fdme

Posted May 26, 2013 - 06:25 PM

#17

Ensure the caliper can slide on the guide pins, if pins seized the piston is acting on 1 pad only.


So I took apart the caliper, I dont know why I didnt think of it before but the guide pin is pretty messed up. It has a very rough groove worn into it, (from the pads sliding accross it im assuming.) Im hoping the rebuild kit will be just what the dr ordered...

Thanks for all the replies everyone this forum has been immensley helpful and everyone has had really solid advice.

  • Road Rage

Posted May 27, 2013 - 06:53 PM

#18

Had a similar problem with my 2007, for me it was the creation of a vacuum in my master cylinder reservoir. Take the cover off the master cylinder and in the cap there is a plastic cover under the metal cover, make sure all the vent holes are clear in the plastic insert and metal cover (2 slots cut in ends). That did the trick for me. When the cap was off my brakes were fine, with cap on I had little or no rear brake.

  • GTSMS

Posted May 27, 2013 - 09:30 PM

#19

From what you're describing your brakes are not normal as some have suggested. It's possible that the caliper pins are the culprit but I'd check the seals in the master and maybe rebuild them as if they wear they may not leak externally but internally and make the system mush. Also the rubber lines from the factory get old and will start to flex more. I'm guessing this is probably where the majority of your sponginess is coming from. Also when you're bleeding the way you're doing it is OK but it's best to pump the pedal up and hold it then crack the bleeder. This will help keep air from coming back in when you let the pedal up. If you want to just crack the bleeder and start pumping I'd at least finish by using the method I just mentioned to make sure it's all out. Also as some have suggested doing this bleeding it at the master cylinder as well as sometimes you'll get a bubble hung up in the banjo bolt. And finally it's not a "separate master cylinder" it's a separate reservoir. They went to the combined master cylinder/reservoir to reduce weight and by all means you can switch to this set up but it's not part of your problem.

  • behive

Posted May 28, 2013 - 02:27 AM

#20

When I've bled brakes I just just pressurize with a syringe at bleeder on caliper, saves a lot of pissing about because you are moving any bubbles higher towards reservoir. :cool:




 
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