K100RT asymmetric brake calliper piston action - K-Bikes.com - Excellence in Motion
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post #1 of 6 (permalink) Old Dec 13th, 2010, 4:25 am Thread Starter
Join Date: Apr 2010
Location: Canberra, , Australia
Posts: 35
K100RT asymmetric brake calliper piston action

I noticed my rear brake pads were making a little noise when not braking, so I inspected the pads. One side nearly all gone, the other hardly worn. Decided to replace the pads.

(When trying to press the pistons back in with the calliper on the bike couldn't get at them, tried to lever with a screwdriver, chipped the leading edge of one of the pistons).

After taking the calliper off and pressing the pistons to flush with the calliper body, decided to check their action by working the brake pedal a bit. Sure enough, one wasn't moving.

Kept trying, repressing the moving one back in. The other one started working. Thought maybe a fluid flush might help - no difference. As I kept repeating, sometimes one would stall, sometimes the other. Sometimes neither. One side stalls more often than the other.

After many repeats I over extended one of the pistons and a bit of brake fluid leaked out of it. It then moved more freely, presumably from some lubrication from the fluid. Did the same to the other one. They both now move more freely, but still generally asymmetrically. I've extended and pushed them back at least a hundred times now.

Took them apart to see if I could get in to the innards of the pistons, but can't see how. Reflushed with fluid.

If I leave them both out about 3-4 mm from the calliper body they tend to work symmetrically, but not always. What's going on? Should I pursue stripping them down and cleaning and lubing them? If so, how, and what with? Their action is smooth.

I'm in Australia, where the cheapest second hand ones are $100 (the US ones would be that expensive by the time shipping is added). I'm told I can get a new one for $220 (to be confirmed). Any advice which way there? I'm on a low income at the moment, but I don't want to get a second hand one with seal or other issues.

Any advice welcome.
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post #2 of 6 (permalink) Old Dec 13th, 2010, 8:39 am
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Brake pistons can get cruddy. Extend the piston as far as you can, then patiently scrub off any varnish looking stuff with the finest steel wool you can find. It's not easy to get in there. A popsickle stick may help.

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post #3 of 6 (permalink) Old Dec 13th, 2010, 8:54 am
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that's the brakes

I normally avoid pumping the brakes up until they leak past the seals at all cost, however; considering you've had both pucks leak by now, I'd go all the way to dismantle, clean and inspect for problems.
Your original issue was either corrosion, dirt or air and as you discovered you need to be super gentle with the delicate pistons and seals.
Best practice for spreading the pucks is to put 2 thin pieces of wood between the pucks, then you can use a screwdriver to pry without damage. Anytime I pull my calipers to inspect them, I put a piece of wood in place of the rotor and pads, then pump the brake up against that, it makes it easy to spread the pucks back out and allows you to pump them up solid without a total re-assemble.
When you re-assemble the pucks should be all the way back. Get yourself a large disposable syringe and fit it with a plastic tube in place of a needle, fill the syringe with brake fluid and press fit the tube over the bleed screw. Filling the brake system through the bleed screw makes it very simple to displace any air from the entire system. I can post some photos if you need them.
Unfortunately brakes and tires are not a good place to save money and nothing really compares to replacing an entire brake assembly with new parts.
If you do need to clean the caliper parts with steel wool, make sure you only do so in a axial direction, never in the direction of the piston travel, just like you would hone a cylinder sleeve.
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post #4 of 6 (permalink) Old Dec 15th, 2010, 3:16 am Thread Starter
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Location: Canberra, , Australia
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Well, I dismantled the callipers, took out the pistons (thanks to those who advised how) and guess what! They were spotless. Gleaming.

Popped them back in with the original seals, and now they're working symmetrically. Radios are like that - not working, take them apart, poke around, don't find anything, put them back together and they work. They just want some attention.

Now, a couple more questions:

1. The master cylinder has fluid on it as it pops back out after releasing the brake lever. Is this normal, and if not, should I be overly concerned?

2. The brake lever has about 1 1/4 inches between top and fully depressed. Is this a normal amount? It seems high to me.

Thanks again for all of your help and advice.
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post #5 of 6 (permalink) Old Dec 15th, 2010, 8:45 pm
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Not finding anything obvious suggests the original problem was air trapped behind the lame puck, which should have also produced a mushy feel at the lever and reduced brake effect. The rear brake master cylinder should not leak fluid anywhere. Mine has 1" of travel measured at the levers longest point and there is an adjuster to effect the brake lever travel.
Air in the system will result in increased lever travel before it produces full brake force. Water or air in the fluid can also cause the brakes to seize on when they become extremely hot. ...all stuff I learned the hard way on my Trials bikes.
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post #6 of 6 (permalink) Old Dec 16th, 2010, 10:35 am
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Was the one that moved easiest the one closest to the hydraulic inlet?

My observation would be that due to the dynamics of hydraulic actuation it is the strange phenominon where the hydraulic pressure moves the one closest to it's entry point first. Then it will activate the one farthest from it's entry point. When I rebuilt the calipers on my Triumph Spitfire (Twin opposed piston calipers from Jaguar) and bench blead and pressure tested them they always accuate the supply side piston first This is the reason most opposed brake systems will have about 10% more pad left on the seconardy piston side. Especially if you have a tendency to coast brake to a stop light.
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