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"Betsy" 1978 CX500
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I have been out on a few short runs on my 78 CX500 since her re-build and so far everything has been fine, a few minor issues with the speedo cable going caput and one pot being a little lean, both sorted now. There has also been a squeak from the front wheel, most noticeable at slow speeds, but goes away when the front brake is applied so figured the new pads maybe bedding in ?. Both front callipers were re-built with new pistons, seals, dust covers and pads. Master cylinder cleaned up and new seals fitted. Went out for a run today as it was sunny and warm for a change. Rode for around half an hour and the bike ran great, brakes working well too. Stopped at a junction, then pulled away and bike accelerated well, cruised along at around 60mph for about three miles when the engine note changed and the speedo began to drop. Thinking one pot had packed up, I carried on and the bike kept running albeit sluggishly, but only had a couple of miles to go toget home.

Pulled up at home and turned the engine off, but when I came to push the bike to the garage it would`nt budge. The front wheel was locked solid and the lever too when I tried to pull it in. Both callipers were very hot. Bit pissed off, so went for a cuppa. Back to the bike about an hour later and found now the wheel was free !. Also could pull in the lever and the brakes worked ok and released ok. What is going on here, I am stumped !. My first thoughts were the master cylinder, as I know from other posts there is a small hole in there that can get blocked and lock the brakes on. Could understand that if I had stopped at that junction, then found the wheel locked when I came to pull away. But the bike ran fine for a few miles after. Seems like the brakes somehow came on as I was riding, but I did not touch the lever at all.

Anyone have any idea what could be going on ?. I did wonder if that squeak could be connected. If the pads were dragging slightly, could that heat up the callipers and the fluid in them to the extent it expanded and locked the brakes on ?. Seems unlikely but can`t think of any other cause. Any thoughts or suggestions appreciated.
 

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Have you replaced the old brake lines?? full fluid change? It may be the MC but you will have to probably dig into the entire system. Did you do the calipers or have them done? most of us dont trust mechanics
 

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Corroded/ unlubed pad pins?
(can cause similar issues)....
When you changed the fluid did you see black rubber/dirt bits...if so brake line breakdown has partially blocked some orrifices
 

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Ill add...in really cold weather ice formation at the caliper piston can cause issues as described..but im assuming your rideday wasnt at zero degrees....😁
Rereading your post...this seems unlikely
 

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Sounds like two issues to me. Sticking wheel cylinder pistons causing heat build up, AND tiny hole in master clogged and failing to release heat induced pressure. Then into a failure spiral. Good it didn’t cause a crash. Stiff caliper slide pins could be contributing as was said too. Entirely possible it’s not coincidence but all related to a long term neglected brake system culminating in a “perfect storm”.
 

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If you still have the original rubber brake lines you really should replace them. A stainless/braided set of hose is not very expensive , and is certainly cheaper than dicing with danger from dodgy brakes. Time to do a thorough overhaul of the brake system , sir .
 

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Brake hoses swell internally and close up. This could be the issue?
 

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I'm going to concur with the "two issues" above. Cleaning out the smaller of the two holes in the master cylinder can be difficult. If there is congealed brake fluid in the reservoir it can leave a speck which gets sucked in and blocks this critical hole. Block it and the return of fluid when brake is released is stopped (or slowed). Easy way to check is to look in reservoir when lever is squeezed... do you see fluid movement from the initial piston travel before the hole is blocked? if not you have an issue there as an aside, I had a bike come to me with brake lever slightly pushed in that blocked this return hole, thus brakes never really 'let go'

When cleaning the bore for the caliper piston, great care must be taken with these older bikes to remove ALL the buildup of hardened brake fluid on the inner surfaces... including the inner cut where the seal sits. the worst is usually found above/outside the seal where leaking fluid hits air. This is accomplished with emery cloth, dremel rotary brushes, right angle picks, scotchbrite pads, etc. Try not to score the inner(below the seal) surfaces. The final test of dropping the brake piston back into the caliper bore should result in smooth travel of the piston....not impeded by crumbs of hydraulic fluid scum. Chances are if it won't smoothly push in and easily pull back out (with seal removed) you need to continue cleaning. Cleaning the recess for the seal just means there is no tight spot, the pressure applied to piston wall is symmetric around the circumference. If it looks clean but still hangs on the piston, put hydraulic (brake) fluid on piston during testing. If it still hangs, CLEAN IT AGAIN you missed something

A proper working hydraulic system will seal tightly around caliper piston when lever is squeezed, then release pressure and create a small vacuum to help retract the piston. Thus, when lever is released the piston will be a few thousandths off it's surface and no squealing or scratching noises will occur. Easy to have happen in new stuff, but when seals let brake fluid leak for 20 to 40 years, the cement buildup will invariably cause problems TS
 

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I'm going to concur with the "two issues" above. Cleaning out the smaller of the two holes in the master cylinder can be difficult. If there is congealed brake fluid in the reservoir it can leave a speck which gets sucked in and blocks this critical hole. Block it and the return of fluid when brake is released is stopped (or slowed). Easy way to check is to look in reservoir when lever is squeezed... do you see fluid movement from the initial piston travel before the hole is blocked? if not you have an issue there as an aside, I had a bike come to me with brake lever slightly pushed in that blocked this return hole, thus brakes never really 'let go'

When cleaning the bore for the caliper piston, great care must be taken with these older bikes to remove ALL the buildup of hardened brake fluid on the inner surfaces... including the inner cut where the seal sits. the worst is usually found above/outside the seal where leaking fluid hits air. This is accomplished with emery cloth, dremel rotary brushes, right angle picks, scotchbrite pads, etc. Try not to score the inner(below the seal) surfaces. The final test of dropping the brake piston back into the caliper bore should result in smooth travel of the piston....not impeded by crumbs of hydraulic fluid scum. Chances are if it won't smoothly push in and easily pull back out (with seal removed) you need to continue cleaning. Cleaning the recess for the seal just means there is no tight spot, the pressure applied to piston wall is symmetric around the circumference. If it looks clean but still hangs on the piston, put hydraulic (brake) fluid on piston during testing. If it still hangs, CLEAN IT AGAIN you missed something

A proper working hydraulic system will seal tightly around caliper piston when lever is squeezed, then release pressure and create a small vacuum to help retract the piston. Thus, when lever is released the piston will be a few thousandths off it's surface and no squealing or scratching noises will occur. Easy to have happen in new stuff, but when seals let brake fluid leak for 20 to 40 years, the cement buildup will invariably cause problems TS
In fact, the piston retraction is primarily made by the square piston seal. Hydraulic pressure moves the piston forward and deforms the seal section to a parallelogram shape. When the pressure is released the seal wants to get back to a square section and pulls back the piston a few thousandth.
 
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