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Discussion Starter · #1 ·
I replaced my front brake master cylinder on my 2001 GSXR 750 with a radial master cylinder off a used 2006 GSXR 1000 (purchased from ebay) for a braking upgrade. I bled it all out and got a good lever. Now, when I ride for a short period of time, the lever gets firmer and firmer and as it does, my brakes begin to constantly drag / grab harder and harder until I can no longer move forward (completely locked up). I can't determine the source of the problem.

Things I've done / checked
1) reservoir supply hose seems to be fine / not kinked
2) system seems to be free of air / bled
3) disassembled completely and cleaned calipers / pistons / etc

Any ideas what I'm missing?

Thanks for your help in advance!
 

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old fart
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have you got aftermarket levers fitted!? or badly fitted brake lever, sometimes the actuation piston gets held in by them that causes the brake to drag, if it drags for too long heat builds up which transfers to the brake fluid and causes the brake to lock up.
 

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Discussion Starter · #3 ·
Yup.....just replace the lever on that side with an ebay special. I didn't even think about that, but now that you say it, that makes sense. Thanks man, I'll see what I can do to get that straightened out.
 

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Old, bold rider
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More than likely the problem is an overadjusted master cylinder pushrod that is not allowing the primary seal to travel back far enough to uncover the compensating port. As the fluid heats with use it expands but has nowhere to go, so the brakes progressively drag worse and worse until they lock up.

To test this, try to push the pads back -- likely you won't be able to. Now remove the lever pivot bolt and try again. If they will retract the problem is with the lever / pushrod / brake light switch or a combination of them. If they still won't retract the probability is there is a blockage in the compensating port preventing fluid flow back into the reservoir. This will require a complete teardown the correct.

Many think if it is possible to bleed the system there is not a problem with the master cylinder -- t'aint necessarily so. While bleeding the lever is pulled inward a substantial amount. Once the bleed fitting is tightened down and the lever released the negative pressure creates a pressure differential behind the seal sufficient to draw fluid from the reserve chamber past the seal, filling the cylinder. It may seem to be normal but is not.

Rob
 

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Discussion Starter · #6 ·
Yeah. I could attack it a few different ways. I could either make the brass bushing hole a little deeper that the pushrod seats into, or shave the actual master cylinder pushrod a little shorter. I guess that's what I get for ordering el-cheapo levers. :)
 

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old fart
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Yup.....just replace the lever on that side with an ebay special. I didn't even think about that, but now that you say it, that makes sense. Thanks man, I'll see what I can do to get that straightened out.
If its a cheap Chinese copy lever im not surprised! i have heard of a few doing this, coz they are badly machined, or machined to the wrong dimensions :(
glad to have helped :knucks:
 

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Sounds like a bad master cylinder but I'd start with the simple by getting brand new brake fluid in there if you used some that you'd had sitting around. Water, contaminants, or air in the brake fluid will expand when heated up and create drag but the cylinder has a mechanism which should compensate for that and drain off. You could've bought a bad cylinder.
 

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Rob has it spot on. Fluid warms up with use, expands, has nowhere to go so it slowly applies the brakes for you. Saw it once before on a customer's bike at the dealer, the culprit was a cheap aftermarket lever that didn't fit right.

You shouldn't have to depress the master's piston at all to install the lever. If you have to squish it in to get the pivot bolt in place, it's no good.

It's really dangerous to drive it like that, the lightest touch on the lever could just lock the front wheel.
 

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have you got aftermarket levers fitted!? or badly fitted brake lever, sometimes the actuation piston gets held in by them that causes the brake to drag, if it drags for too long heat builds up which transfers to the brake fluid and causes the brake to lock up.
It's this.

Remember, any time you change something, that's always first suspect.

Why would changing the master cylinder then the hand lever cause problems with the lines or calipers like some folk have suggested?
 
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