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Author Topic: '77 750F rear caliper heating up and piston not compressing  (Read 1312 times)

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Offline cb750fbomb

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'77 750F rear caliper heating up and piston not compressing
« on: June 28, 2009, 04:03:49 PM »
Did a search and didn't find anything about this. This only seems to be happening when it's real hot outside, only during the day so far that I've noticed at 100+ degrees, but recently my rear break keeps getting real hot and it's seizing up. It engages and stops great until this happens. I have to wait about an hour for it to cool and it unlocks, but until then it's stuck. It's apparently has been happening while riding and I've not known about it because my fairly new break pads are shot. The first day I noticed this happening all the fluid seemingly boiled out of the caliper. I filled it back up and tightened the bleeder valve which didn't seem screwed in enough as fluid seemed to be coming out from the threads. Anyone know what could cause this and how to fix it? I'm going to take the disk off to see if it's warped thinking the pads could be rubbing and causing the over heating. Thanks for any help in advance.
« Last Edit: July 02, 2009, 05:27:59 PM by cb750fbomb »
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'77 CB750F2
“The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair.”

Offline OApb

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Re: '77 750F rear caliper heating up and piston not compressing
« Reply #1 on: June 28, 2009, 04:23:22 PM »
I bet if you were to take the caliper apart there is a bunch of road grime and dirt in there. Maybe think about taking it apart and cleaning and rebuilding it. Also check to see if the piston is damaged or out of shape
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Offline BeSeeingYou

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Re: '77 750F rear caliper heating up and piston not compressing
« Reply #2 on: June 28, 2009, 05:02:04 PM »
Maybe the piston is not retracting all the way and leaving the pads in light contact with the disc.  Depending on how long you are riding then the whole thing heats up and the slight expansion locks it all up.  Seems this would be more likely instead of what time of day or the temperature outside.  Maybe you have just been riding longer during the day or when it's hot out.  Could be crud in there like 0Apb says or could be worn out piston seal.  Most likely both.   Looks like time for a caliper rebuild.  The usual advice would be to clean out the piston seal groove, new seal, polish the piston and check for excessive pitting, and check that the fluid return hole is open in the master cylinder.
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Offline cb750fbomb

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Re: '77 750F rear caliper heating up and piston not compressing
« Reply #3 on: June 28, 2009, 05:26:43 PM »
I will check all this as I just got the caliper off, but I went riding for about 3 hours last night and the night before with no problems,but it happened day riding on those days. I'm not saying the time of day, but the temp outside may contribute. We've had record high heat indexes in the low 100's here lately which can't help. BTW, part of the clip that hold the pads in place at the top of the caliper is melted.
« Last Edit: June 28, 2009, 05:33:11 PM by cb750fbomb »
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“The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair.”

Offline cb750fbomb

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Re: '77 750F rear caliper heating up and piston not compressing
« Reply #4 on: June 28, 2009, 06:00:31 PM »
The piston seal is eaten up pretty bad. Is the the piston seal what I need to worry about if the rest of the pieces look fine? If so the PO left 2 of those, along with two fork seals, in the toolbox.
« Last Edit: June 28, 2009, 06:05:52 PM by cb750fbomb »
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“The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair.”

Offline BeSeeingYou

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Re: '77 750F rear caliper heating up and piston not compressing
« Reply #5 on: June 28, 2009, 06:10:48 PM »
Yeah, the seal is what retracts the piston.  It stretches slightly when you apply the brakes and when you let off it pulls the piston back.  So even though they may look good and don't leak they can lose their elasticity.  And make sure the groove is clean as crud builds up in there and also effects it's operation.

I am going out for a ride along the river now. :D
« Last Edit: June 28, 2009, 06:12:28 PM by srust58 »
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Offline cb750fbomb

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Re: '77 750F rear caliper heating up and piston not compressing
« Reply #6 on: June 28, 2009, 06:15:11 PM »
The piston is soaking in chem-dip right now.

Thanks srust58 and have a nice ride. Keep the rubber side down   :)
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“The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair.”

Offline 750goes

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Re: '77 750F rear caliper heating up and piston not compressing
« Reply #7 on: June 28, 2009, 10:36:41 PM »
I have just finished rebuilding my F1 rear caliper, and believe me it was doing exactly what yours is doing..

Basically the piston seals are too old and need replacing, plus the build up under the seal iteself, is where most of the problems come from.
the brake fluid and moisture build up over a long period of time causes corrosion of that groove where the seal sits, the seal can then no longer do its job of "rolling" with the application of movement from the piston, it will move but then not be able to return to its original spot, releasing the pad at the same time, and then the piston remains forced out of position agains the pad pushing on the disc.

There are also TWO pistons and seals on the rear caliper.... yo gotta do both..

when cleaning out the groove - use a smooth action with whatever tool you use - and move it in a circular motion - you will soon find out where there is some corrosion... and it must be clean before you put a new seal in.

Coat the groove, new seal and cleaned piston with brake fluid, then pop it back in - will now work properly if you have cleaned properly, and bled it properly.

 :)
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Offline 750goes

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Re: '77 750F rear caliper heating up and piston not compressing
« Reply #8 on: June 28, 2009, 10:40:48 PM »
My apologies - you only have one piston - should have put my glasses on..
 :)
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Offline cb750fbomb

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Re: '77 750F rear caliper heating up and piston not compressing
« Reply #9 on: June 28, 2009, 10:48:32 PM »
Awesome. Now all I need are some new brake pads since the problem killed them.
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'77 CB750F2
“The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair.”
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