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Discussion Starter #1
Hi everybody,
I have a 1986 GL1200 sei,
since i bought earlier in the year there never was a good front brake on it.
I have read all of the previous posts with similar problems , and suggestions of master cylinder rebuilds, caliper rebuilds and braded lines , so i have set out to identify the problem.
In do know that the front brake master cylinder only operates the front left caliper,
The first job i done was to rebuild the master cylinder, but problem still persists.
next thing i done was to check the brake liner delivery of fluid to the caliper, so i removed the caliper, bled fluid down to the banjo bolt where it connects to the caliper , and then put a clamp on to the end of the pipe ,brake lever is now solid as a rock , bingo, i now know the the brake master cylinder and brake lines are good.
now its down to the caliper, connected all up to the caliper, bled it and brakes are now back to square one.
got a caliper repair kit fitted it and bled it ,and the problem still persists, so i removed the pads and pins and placed a solid metal bar between the pistons and caliper, and when the lever is pressed, i now can see what is happening.
it is the caliper is flexing as the pistons presses against the pads and forces the caliper apart.
i never see this before, so its a replacement caliper to cure the problem , has anybody ever seen this before??
 

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Have you replaced the pads, sounds like you are having to put too much pressure for adequate braking.
 

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, i now can see what is happening.
it is the caliper is flexing as the pistons presses against the pads and forces the caliper apart.
i never see this before, so its a replacement caliper to cure the problem , has anybody ever seen this before??
Never seen that myself. Can you see if there is a crack in the caliper when it spreads?
 

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Discussion Starter #4
Never seen that myself. Can you see if there is a crack in the caliper when it spreads?
No, not that i can see, but it can clearly be seen pushing the pads away from the pistons.
 

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You put a solid bar between the piston and caliper and you're surprised the caliper flexed?
If it developed enough pressure to flex the caliper I would tend to suspect something else.

Did you see a leak?

Rayjoe
 

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Your bike has linked brakes. The right front and rear are connected together, the front left is activated by the handlebar lever. I take it your issue is with the front left brake.

You mention that you can see the caliper flex. Don't believe this can happen so my thought is the pivot pin called a sleeve in the parts list #16. If this is seized and will not move the caliper cannot move correctly.

Thinking you see the caliper flex could be it twisting because it is not free to move - both pistons are not moving out together. It will affect the braking as well. I'd check all three sleeves. Small part generally missed during a caliper clean/rebuild. You can tell that this part is not free to move by the brake pad wear.

Upgrade to SS brake lines is good. After some 35 years, the OEM hoses will tend to wear out, especially from the inside, and there will be significant flex in the brake hose that absorbs a lot of the pressure from the MC.

Good luck.
 

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Discussion Starter #8
You put a solid bar between the piston and caliper and you're surprised the caliper flexed?

My guess would be the rebuild kit is defective.


Rayjoe
Your bike has linked brakes. The right front and rear are connected together, the front left is activated by the handlebar lever. I take it your issue is with the front left brake.

You mention that you can see the caliper flex. Don't believe this can happen so my thought is the pivot pin called a sleeve in the parts list #16. If this is seized and will not move the caliper cannot move correctly. Thinking you see the caliper flex could be it twisting because it is not free to move - both pistons are not moving out together. It will affect the breaking as well. I'd check all three sleeves. Small part generally missed during a caliper clean/rebuild. You can tell that this part is not free to move by the brake pad wear.

Upgrade to SS brake lines is good. After some 35 years, the OEM hoses will tend to wear out, especially from the inside, and there will be significant flex in the brake hose that absorbs a lot of the pressure from the MC.

Good luck.
Thanks for the reply.
yes i am aware of the link brake system, ( had three Blackbirds all with linked brakes)
with the caliper removed from the bike , but still connected to the brake line, and to simulate the disc , i placed a flat piece of steel between the pads, ,i then using a Vernier caliper to record the travel of the caliper body. today i recorded 2.6mm. and can clearly see both pistons moving. and a sponge feeling at the lever. with the brake line pinched at the caliper, rock solid lever, which shows that the m/s and fuel line are fine. bear in mind that the caliper and m/s have been rebuilt.
 

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Keep in mind you are stopping close to 1000 pounds with the rider,I think you are putting too much pressure on the caliper,I suspect if you replace it you will have the same issue,normall just 2 fingers will be enough pressure to stop ( with help of the rear brake),
 

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Keep in mind you are stopping close to 1000 pounds with the rider,I think you are putting too much pressure on the caliper,I suspect if you replace it you will have the same issue,normall just 2 fingers will be enough pressure to stop ( with help of the rear brake),
But the caliper should stand all the pressure the master cylinder can apply without spreading apart.
 

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The system is like a hydraulic jack,it can make tremndous pressure,the smaller the supply line is ,the greater the pressure,the launch system on aircraft carrier use a really small pressure line for that great power,with the caliper only being bolted on one side I can see where it would flex On the unbolted side by .100 that he is seeing.
I’m courious now,tomorrow I’m checking my caliper for distortions
 

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Copy from google.,motorcycle may not be that high but just an example,I was thinking 1000 psi
How much pressure is in a brake system?
Typical brake line pressures during a stop range from less than 800psi under "normal" conditions, to as much as 2000psi in a maximum effort. 3)
 

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From ducati information.
the pressure of motorcycle lines . 800 to 1000 ps
 

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The system is like a hydraulic jack,it can make tremndous pressure,the smaller the supply line is ,the greater the pressure,the launch system on aircraft carrier use a really small pressure line for that great power,with the caliper only being bolted on one side I can see where it would flex On the unbolted side by .100 that he is seeing.
I’m courious now,tomorrow I’m checking my caliper for distortions
A principal of hydraulics, the size of the line has no effect on the pressure, just like the size of the wire has no effect on voltage in electrical systems, size = flow volume. A smaller pipe can generally handle more pressure simply because there is less area.
 

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There’s still the possibilities of huge pressure on the caliper that would cause it to flex
 

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There’s still the possibilities of huge pressure on the caliper that would cause it to flex
From what? The only thing that could add any pressure would be a smaller bore master cylinder.
 

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Ducati says their brakes pressure to 800 psi,would you consider 800 psi was low pressure?
 

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Paste from a forum,seems like .100 flex is standard for production brake calipers.


So, with a 10mm bore, and say a 20mm stroke, the MC displaces 1.57 cc/mL of fluid.

There are 2 pistons moving in the caliper, so that's a rough baseline for acting area. Let's say those are 25mm in diameter and there are 2.

With that volume being displaced, the deflection should be 1.6mm, or roughly 0.100". This was a while ago, but I think I was measuring about 0.020"-0.030" of flex at the caliper (from the outside of the caliper to the opposite side pad backing. So it's in a reasonable range.
 

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Paste from a forum,seems like .100 flex is standard for production brake calipers.


So, with a 10mm bore, and say a 20mm stroke, the MC displaces 1.57 cc/mL of fluid.

There are 2 pistons moving in the caliper, so that's a rough baseline for acting area. Let's say those are 25mm in diameter and there are 2.

With that volume being displaced, the deflection should be 1.6mm, or roughly 0.100". This was a while ago, but I think I was measuring about 0.020"-0.030" of flex at the caliper (from the outside of the caliper to the opposite side pad backing. So it's in a reasonable range.
All that being said he is getting what I would consider more than normal flex and a soft brake. If you measured 20-30% of what he is measuring then his caliper appears to have a weakness.
 

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Maybe but I bet all the calipers would flex,I haven measured mine but I will
 
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