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Hi all. Hoping someone can maybe offer some advice here.

I've had a front brake issue ever since I owned the bike (1998 KDX220 I'm the 3rd owner but the bike hasn't had much use, I just put on the 2nd replacement rear tire)

The front brake has only about 50% power. I rode a KDX200 and it was a lot better. Last year I bled the brakes, no change. Pulled the front caliper off. When pressing the front lever the rearmost piston would move smoothly outwards (and eventually would have popped out of the bore) but the front piston would advance and then pull back each time, kind of a stutter.

With the caliper removed from the bike I used compressed air to remove the pistons, the rear came out easy, the front required almost 40psi to extract it. The pistons, seals and bore looked ok so I reinstalled everything but the piston continued to do the same thing when I pressed the brake lever.

I was thinking of swapping the pistons around to see if the issue stayed with a piston or remained with the front bore. Never did. Instead I asked Jeff Fredette (actually acked if he sold a rebuild kit).

He thought it was the master cylinder issue based on my description and said "apply the brakes. if it does not hold and gets weeker stopping power over time then i would say the master cylinder is leaking. pull the piston out and look for grooves in the bore or a cut/tear in the rubber cup.".

They do seem to fade so I just today pulled the master cylinder off.

Is the rubber cup "49016 COVER-SEAL,MASTER CYLINDER"? It seems ok.

I think I can see a groove inside the master cylinder bore and there is a definite groove in the piston, about 4mm long and just deep enough that it is visible to the eye and I can feel it with my fingernail. It's at the right end of the piston assembly (next to the brass end that the brake lever presses against) so it matches up with where I think I can see a groove in the master cylinder bore.

But it's not a massive groove in the piston. It's $ka-ching$ for a new assembly, I imagine used would be cheaper but since the bike doens't have many hours on it, a bit curious how it happened. Also, not convinced I don't have a caliper issue given the compressed air issue.

Any advice?

It is simply a matter of bleeding the brakes properly.

You really need a vacuum pump like a Mighty Vac to do it correctly. And even at that... you may need to do it a couple of times. Most of us old-timers have been through this time and time again.


It is a pain in the @ss to say the least!

Like this... http://www.tool-home.com/products/catal ... ts_id=5216

That just happened to my kid's KX65. The bike fell over on the front brake lever. It fell hard enough to bend the lever. We put on a new lever, but the brake wouldn't firm up. We changed the fluid and bled it several times, but it just wouldn't work.

I finally broke down and took it to the shop to have them look at it. They found a small nick or scratch on the master cylinder piston. That was keeping it from working properly. They replaced that part and it worked perfectly. That could be your problem since you have actually seen a scratch with your eyes.

If your bore/master housing is damaged you may need to buy another one. 95-06 KDX parts are pretty cheap on Ebay.

Not sure it's a bleeding problem. Here is a pic of the piston and bore. Click for hi-res version.





Even if it is this, I'm still a bit baffled by the fact that using compressed air, the front piston took 40psi before it would come out. Of course the rear had already come out so air was leaking out of that rear bore.

It honestly doesn't look like wear... I could be wrong since I have not disassembled my master cylinder.

Indawoods wrote:It honestly doesn't look like wear... I could be wrong since I have not disassembled my master cylinder.

I'm in uncharted territory also.

Maybe someone else who's done it will chip in ..... in the meantime, so is one piston stuttering a symptom of air in the lines? I've seen some threads on KDX brake bleeding being tricky but need to search again (if anyone knows of a canonical thread, please PM me).

I have a Mitivac but IMO it's not effective. It's hard to get a good seal between the hose and the brake bleeding nipple so air tends to get in.

(I guess when I go searching old posts) I'll find out why the normal bleed valve closed, pump brake, hold lever, release bleed valve, fluid escapes, tighten bleed valve, repeat procedure doesn't work.

The best way IMO is to use a syringe and push the fluid up thru the caliper bleed valve and syphon excess out of the master cylinder. I just don't have a good syringe. I need to get one for bleeding the clutch on my KTM so maybe ......

Mityvac always seems to work well for me

Maybe need to try different size hoses? One I use fits nice and tight on the tit. Tight enough that it'll suck air bubbles right through the threads. Cure that with teflon tape

try replacing the adjusting screw for lever play for a longer one. Mine was worn out, did the trick

Re: 'When pressing the front lever the rearmost piston would move smoothly outwards (and eventually would have popped out of the bore) but the front piston would advance and then pull back each time, kind of a stutter.'

Watching piston movement in a multiple piston caliper with the caliper in your hand doesn't tell you a whole lot. It's a matter of 'path of least resistance'..and the 'fact' that one piston moves such-and-so..and the other doesn't do the same thing is pretty much meaningless.

There are tools that measure pressure of individual pistons in such a case. THAT tells you something. This isn't a tool you're going to buy at your local AutoZone.

There are some things you can try to test piston pressure individually..like placing a piece of soft wood in the disc slot..measure the resulting depressions made when you squeeze the lever. That's not at all 'scientific' testing..but gives you an idea of what's happening.

Re: 'I have a Mitivac but IMO it's not effective. It's hard to get a good seal between the hose and the brake bleeding nipple so air tends to get in.'

Fix it. As Mr. Wibbens said, if the bleeder leaks, put teflon tape on its threads. If the hose/nipple joint leaks, get some better or use the correct hose. The hose I use is a tight enough fit that it's a grunt to get it ON the nipple...and it is NOT going to leak or come off. A MityVac or similar pressure/vacuum bleeder is an excellent brake system tool. That you find it ineffective isn't a problem with the tool, but the application of the tool.


Frankly, I don't know what the problem is from the description. 50% power? That is not explanatory. Is the lever firm with no stopping power? Maybe...but why focus on the MC, then?

Re: 'if it does not hold and gets weeker stopping power over time then i would say the master cylinder is leaking.' One can get the gist of that if it's tweaked some. IF braking is poor due to MC leakage (past the piston), you will FEEL it in the lever. The lever WILL NOT remain hard, braking effectiveness decline, and the problem be the MC. That in reference to leaking fluid. The MC could have a physical obstruction that would prevent movement, give a hard lever feel..and not brake for diddle.

..the subject is leakage past the piston seal..

Yes...I've had MC failure resulting in unpredictable slave activity. There was no evidence of piston/bore/seal failure. I replaced the piston assy (piston, spring, seal) in the MC...problem solved. That was obviously a leaking MC...I could feel it.

HEREis some good reading on vacuum bleeder tools and sources.

I could think of better things to do than suck brake fluid into a glass jar..and the author seems ignorant of how to drill a clean hole with a drill bit...but the tubing comments are good..