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Old 10-07-2012, 03:08 AM   #1
Hodgo OP
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Front Brake 950SE locks up when hot but not all the time

I have hit the search but found nothing exactly like what I'm experiencing.

Before the last big ride I swapped out the brake pads for new ones (F & R).

Pistons all looked good & moved freely. When I had done about 3000kms I started to complain about having to pull the front lever in too much to get the brakes to come on properly.

So swapped out the lever. Centre pin was a bit flogged out. I bled the brakes and all was sweet.

Hunting along the dirt at a high rate of speed and noticed the motor labouring, So I dropped a gear, thinking it was heavy bulldust that was causing the load. Then thought.. mm not good, so I slowed down to about 20Kph and the front wheel just locked up.

mmmm Front disc very hot, not much free play on lever. Drained some oil out and was off again after 6 good pumps on the lever.

Bike has sat for a few weeks, go for a ride today and front starts to drag again once it gets hot. I pulled on the lever quite hard whilst stopped and managed to brake the lever.

So I drained oil from bleeder, noticed that res was very low and rode home with out a front brake.

So .. long winded I know... but The question is...

Is the master cylinder rooted and not allowing the oil to return to the res. Or is there water in the line causing it to expand on heat ?

Plus got knocked for failing to display a registration sticker for $88 and lost my sunnys worth $300 so a shit day has been had.
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Old 10-07-2012, 05:06 AM   #2
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I think there may be no free play between master cylinder piston and the rod that the lever operates. This is keeping the port inside the master cylinder closed, so as the fluid expands it has no where to go and locks up the brake.

It could be a poorly made lever.
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Old 10-07-2012, 05:29 AM   #3
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The return orifice @ the master might be obstructed, or the caliper pistons sticking. If the pads were replaced w/o removing the pistons to freshen/clean the seals, I'd look at that first. If the pads aren't "homing" back into the caliper, the drag will cause the fluid to heat & expand.
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Old 10-07-2012, 08:18 AM   #4
mousitsas
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I think it is a two-fold problem, for one the brake fluid reservoir holds too little oil which might boil with repeated hard use and secondly, the disc itself is low quality. You can switch to a stock size or 320mm good quality rotor with the stock or even an adv master.
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Old 10-07-2012, 01:27 PM   #5
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So leaning towards the master cylinder rod not allowing return oil.

And.. Possibly brake pistons, So will clean them again and check the rubbers.

So are the master cylinders a serviceable item ? Would expect there to be only a couple of o-rings etc in it to fix ?
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Old 10-07-2012, 03:25 PM   #6
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Part Number:50313008100
Description:REP.SET PISTON EXC 2000
Retail:$24.77
Your Price:$21.06 (15% discount)
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Old 10-07-2012, 07:37 PM   #7
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Simple to check the bleed hole, pry back the brake pads and see if the fluid returns to the reservoir.

A common problem with the Brembo master cylinders is that the air vent closes off. The diaphragm under the cap must have air move in/out on top of it as the fluid heats up or you go up a mountain, etc. The cap has one or two small slits that should allow air to enter on top of the diaphragm. Often, the cap is over tightened and the rubber diaphragm swells to close off the air gaps.

Simple to fix. Get a very sharp razor and shave off the bumps on the rubber diaphragm, don't over tighten the cap.
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Old 10-09-2012, 02:37 AM   #8
isplat2
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Quote:
Originally Posted by Peanuts View Post
I think there may be no free play between master cylinder piston and the rod that the lever operates. This is keeping the port inside the master cylinder closed, so as the fluid expands it has no where to go and locks up the brake.

It could be a poorly made lever.

....what he said...
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Old 10-13-2012, 03:46 PM   #9
Hodgo OP
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So I purchased a new master cylinder repair kit for $30AUD.

Basically consists of a plunger, spring, circlip and rubber boot.

Old one out and it appeared the spring had lost its sprung.

Removed and cleaned the caliper pistons, were clean anyway, cleaner now.

BUT "FMD" bleeding the brake lines was an absolute prick. Had to master cylinder next to the caliper with syringe and tounge held the right way.

So the culprit was the little return spring that was preventing the oil from returning to the res.

BTW i used motrex 5.1 oil too.

Thanks guys.
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Old 10-13-2012, 04:01 PM   #10
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I pulled on the lever quite hard whilst stopped and managed to brake the lever.
holy fuck dude...
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Old 10-13-2012, 06:16 PM   #11
Navin
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I've called my SE brakes "KLR like" in their weakness but I've never thought I could literally break the lever!
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