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Old June 22nd, 2012, 01:47 AM   #1
Solunatic
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front brake issues. master cylinder?

So I've noticed slowly my brakes have been getting spongy. So i did the "tie the lever to the handlebar" bleed method, which seemed to help, but just slightly. I figured maybe it was my pads which DESPERATELY needed replacing anyways so i replaced my pads and cleaned up the pistons, man those things were gunked up. It also had stock pads on it that were about .25-.5 mm thick when removed. I decided this was probably my problem so after riding a few miles with no improvement over old pads i figured I'd wait until they were bedded in to see improvement. well 300 miles later, and multiple agitation and overnight brake lever zip-tieings still no improvement. I can pull it back about 2/3 of the way before I get any noticeable braking, and at full grab can't lock it up over 20-30 mph. Checked line for damage, none. Brake fluid looks alright. caliper pins well greased and look dandy.

So unless i effed up the pistons/dust seals at the caliper, It's pretty much narrowed down to the master cylinder, right? well i inspected visually, and found that the dust cover on it was torn. i'll post up pics tomorrow. The problem is, i'm broke as ****. Oh the joys of being a 19 year old college student. it's ~$45 (+a two week wait)for the replacement master cylinder parts, and if thats not the problem then that's wasted. I've never dealt with a master cylinder before, but i understand the basics of it.

So my question is, should i take this puppy apart right now and inspect and change the brake fluid with a proper bleed? or is this most definately a master cylinder issue and i should just search ebay or order up the replacement internals and change and bleed then?

is there a way to determine whether the problem is at the master cylinder or the caliper? It's possible there was piston seal damage i overlooked/caused, but would there be a way to diagnose where the problems at?
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Old June 22nd, 2012, 05:30 AM   #2
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Did you replace the old pads?; I believe the limit is 1 mm.

Are the pads contaminated?

Bad seals produce leakage; did you notice any, up or down?
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Old June 22nd, 2012, 12:14 PM   #3
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Bad seals produce leakage; did you notice any, up or down?
Yes the pads were replaced, have fresh EBC HH's on now. I was very careful not to contaminate the pads during install.

That's the weird thing, is I'm not seeing leakage anywhere. Today I'm going to give my brakes a good workout to make any leaks more apparent, then reexamine the caliper pistons/dust seals, and if all is well there drain fluid, remove and inspect/clean master cylinder and replace fluid.

If it's still not fixed after this, I guess i'll just replace the entire front brake assembly. It's cheaper to buy a lightly used caliper and master cylinder on ebay or a forum than it is just for the OEM master cylinder rebuild parts.
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Old June 22nd, 2012, 12:17 PM   #4
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Try cracking the banjo bolts and bleed any air that may be trapped there. I would also try bleeding the front lines with the caliper off the bike and raised above the master cylinder. This will help get any trapped air to rise towards the caliper and out the bleed nipple. Make sure you put a block of wood between the pads if you do this.
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Old June 22nd, 2012, 03:57 PM   #5
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The master cylinder is so far fine, some dirt had gotten in from the dust seal which is completely shredded. i cleaned it out and am ordering a new one.

once i got most of the fluid out of the reservoir I noticed a fine black film that was accumulating in the corners and in both the supply and relief ports. I cleaned this out and in the process accidentally squirted a drop of brake fluid in my eye- i don't recommend following my lead here once my eye was better i put new fluid in and bled an entire bottle of brake fluid through just to be thorough. Each time i stopped to refill the reservoir i pumped the lever vigorously and tapped on the junctions with a soft mallet, which usually yielded a couple bubbles.

conclusion-brakes are as firm as the day i bought her, which still means there's something to be desired, but i think i'd have to make the jump to SS lines to get the feel i want. When i determine it needs lines that will happen, but for now I'm happy with it as long as it stays this way.
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Old June 22nd, 2012, 04:03 PM   #6
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edit: deleted. should stop just reading the first post then replying.
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Old June 22nd, 2012, 04:09 PM   #7
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$0.01 says it's the master cylinder. If the calipers aren't leaking, they are fine. If you didn't open the system, it isn't air. It's a 1990 bike. It's probably the master cyl.
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Old June 25th, 2012, 09:48 AM   #8
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Try cracking the banjo bolts and bleed any air that may be trapped there.
Probably not a good idea. Once the washer has been torqued down, you shouldn't loosen and re-torque on the same washer. In a pinch, I've re-used a washer but generally its safer to always use a new one.
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Old June 25th, 2012, 10:48 AM   #9
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$0.01 says it's the master cylinder. If the calipers aren't leaking, they are fine. If you didn't open the system, it isn't air. It's a 1990 bike. It's probably the master cyl.
brake fluid is hygroscopic. all brake fluid turns into a mix of brake fluid + water after only a few years. the longer it goes, the worse it gets.
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Old June 25th, 2012, 10:56 AM   #10
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brake fluid is hygroscopic. all brake fluid turns into a mix of brake fluid + water after only a few years. the longer it goes, the worse it gets.
?
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Old June 25th, 2012, 11:02 AM   #11
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water compresses a lot more than brake fluid. what i meant to say was it might not be air, it could be water compressing.
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Old June 25th, 2012, 11:06 AM   #12
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?
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The master cylinder is so far fine, some dirt had gotten in from the dust seal which is completely shredded.
That is the reason for Alex post.

The dust seal is more a humidity seal that isolates the brake fluid from the atmosphere.

Otherwise, the brake fluid sucks humidity (water) from the air.

That is bad for two main reasons:

1) Water corrodes internal metal parts, which damage seals.

2) Brake pads transfer a great deal of heat into the brake fluid. The water inside boils and becomes steam, which is compressible and acts like air in the lines.
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Old June 25th, 2012, 11:10 AM   #13
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^-- hernan does a much better job explaining things
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Old June 25th, 2012, 11:34 AM   #14
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conclusion-brakes are as firm as the day i bought her, which still means there's something to be desired, but i think i'd have to make the jump to SS lines to get the feel i want.
If the rubber brake lines are original then some of the loss of braking power could be attributed to them. You might want to get new lines (SS if that's the route you want to take) first and see if they cure your problem before tearing the master cylinder apart. You probably don't want 20+ year old brake lines on in any case....
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