Author Topic: Rear brake piston barely moves  (Read 894 times)

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Offline Alfiator

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Rear brake piston barely moves
« on: April 29, 2018, 06:16:18 PM »
I have a problem. When pressing rear brake the cylinder is barely moving. It moves just a fraction of mm up and the same distance back. So basically it moves up and down instead to hit the disc and start to brake the wheel. As I understand piston in caliper supposed to come freely until hit the disc.
I changed both pistons in caliper and master cylinder. I changed also rubber all sealings. New DOT4 fluid. New hose ( the same I used on front and it is OK), no bubbles inside I bled it perfectly. System seals perfectly. Master cylinder seems smooth and shiny inside.
I disassembled it many times and tried everything.
I locked the caliper piston to vice to simulate firm disc. When the master piston is pressed I dont feel any significant resistance. So may ne the problem is seailig inside master cylinder??? Like I said I changed sealing as well and the piston seems shiny from inside. I need to say that through whole system is easy to blow air in and out.
Please help me. Spring is comming and I need to go to MOT :)
« Last Edit: April 29, 2018, 06:31:12 PM by Alfiator »

Offline guylr

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Re: Rear brake piston barely moves
« Reply #1 on: April 29, 2018, 09:20:34 PM »
Because brake fluid is not compressible there are only two explanations for the symptom you are seeing. Either there is still air trapped in the system (most common and likely) or the master cylinder piston is not sealing. If you used the correct kit (1J3-W0042-50-00) with the original master cylinder piston size and new cup seals it should be the correct diameter and push the brake fluid as you step on the lever. Is the cylinder bore smooth without any pitting? Is the cup seal on the center of the piston shaft installed correctly?

Now Retired Yamaha guy. Cut me and I bleed "Cinquasia Red"

Offline CrouchyUK

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Re: Rear brake piston barely moves
« Reply #2 on: May 01, 2018, 08:27:33 AM »
Check all the things Guy says.  When troubleshooting difficult problems it is easy to get stuck in a rut and go round and round without progress.  As a quick check, use the parts you have to absolutely decide where your problem is?  I would use the front MC with the rear caliper and vicy-versa to decide which end of your rear brake is not functioning.  Just on the bench/workshop of course!  It does sound as though you either have air in the system or the MC is assembled wrong/wrong kit though....
1975 Honda CB200T (2) - 1975 Yamaha XS500B (2) - 1976 Yamaha XS500C
1977 Yamaha RD400D - 1977 Yamaha XS500D - 1978 Honda CB550K - 1980 Honda CB400N
1980 Honda CB900FA - 2000 Kawasaki ZX6-RG2 - 2002 Suzuki GSX-R1000K2

Offline Brywil

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Re: Rear brake piston barely moves
« Reply #3 on: May 01, 2018, 08:36:31 PM »

A top tip for brake analysis.
1. Fill your master cylinder with the correct amount of fluid
2. Clamp the brake line with some mole grips or a pipe clamp about 40mm from the master cylinder ( be sure you have compressed the inner core of the line but dont clamp so hard as to damage it)
3. Apply pressure to the brakes and release the clamp ( dont release the brake)
4. Reclamp the brake line and then release the brake, this will pull fluid from the cylinder,
5. Repeat steps 1 to 4 until you get pressure or resistance at the brake lever.

This will detemine if the master cylinder is working 😎

I have done this on new (Dry brakes) many times without fail, if you gain pressure just keep to the procedure but keep moving the clamp closer to the slave cylinder and bingo you with force the trapped air out👍

Just dont be shy with topping up of the Master cylinder as and air drawn in will start the process off from the start

Offline wilberforce

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Re: Rear brake piston barely moves
« Reply #4 on: May 07, 2018, 01:28:43 PM »
I'm curious as to whether the OP has made any progress. 

I stripped my system fully a few years ago, as I actually think what was in was what Yamaha had put in when they built the bike.  The fluid as was horrible.  I stripped the caliper and rebuilt it, the master cylinder seemed fine so left it alone. After lots of pumping of fresh fluid I get a nice usable back brake.

Sadly it is sitting in the garage as I have not been able to get it to run properly, and have various other 'things' going on so it is a low priority.
One of the Boys from the Wall.
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Moto Guzzi Breva V750ie - 2005 vintage - sold.