pistons won't retract

[donny]

:unsure: the front disc brake, the  piston just will not retract. Neither does the inside pad.  I have cleaned and cleaned the innards, but the rotor gets mucho hot very quickly. The adjustment screw doesn't appear to do anything. Clymers isn't much help on this. Any ideas??  thanks,  Donny

[SteveD CB500K0]

You are definitely doing the right thing.  The piston is retracted when you release the brake lever by the pressure reduction in the system (ie the fluid goes back to the master cylinder)

To work smoothely, the piston and pad must move freely in the caliper.  If the pads are new, they sometimes need sanding down as the painted edges always seem to stick...  Also, when you have it apart, remove the square-section seal and clean the groove with a toothpick (or similar) as any gunge in there (and there always is) will press the seal onto the piston and make it stick.

Finally, the piston itself may be corroded so smoothe it down with some wet and dry.

When reassembling, make sure that the piston moves freely in the caliper and that the pad does likewise before refilling and bleeding the system.

In my experience, adjusting the screw on the caliper pivot does very little.

Oh, and take the pivot apart while you are there as they sieze up too and cause brake squeal.

[donny]

:unsure: mucho gracais..i recently bought the 76 f  (looked for a smaller f)...and I have had more brain ache with brakes than tuning the engine.  I did not know how freely the pistons should move in the unit while off the bike. At least the front brakes work, I installed a rear MC kit, cleaned the rear pistons, but they will not move, fluid leaks from the hose connection (new).  The threads are somewhat stripped, even with threads taped, fluid leaks heavily, the piston still will not move. What do you think is the best way to fix the threads??  Would new bushings fix this? ?:whistle:  thanks,  donny

[JonnyB]

If you've given the Pistobs and Seals a good de-grease, they will be difficult to push back. I normally smear the piston with break fluid, only lightly 'cos you don't want brake fluid everywhere and then push the piston back.

Leak at the pipe fittings are normally due to the washers being corroded, there should be two, one each side of the fitting.

To be honest it may be worth looking for another caliper.

[donny]

Just how easy should the pistons move in and out while disassembled? ? I can't remove the pistons with my pinkys, it takes vice grips or pliars....   wellll helllll...I put on new bushings, polished everything some.  It Still leaks fluid.  The oil bolt looks a little crooked, it must be cross threaded worst than I thought. ... Anyway, is it possible to drill out the threads, and plug the hole and then rethread for the oil bolt? ?.  Has this been done on a caliper??  I am fearful of an ebay caliper being defective, and I would be in the same boat.  thanks,  Donny

[donny]

Just how easy should the pistons move in and out while disassembled? ? I can't remove the pistons with my pinkys, it takes vice grips or pliars....   wellll helllll...I put on new bushings, polished everything some.  It Still leaks fluid.  The oil bolt looks a little crooked, it must be cross threaded worst than I thought. ... Anyway, is it possible to drill out the threads, and plug the hole and then rethread for the oil bolt? ?.  Has this been done on a caliper??  I am fearful of an ebay caliper being defective, and I would be in the same boat.  thanks,  Donny

[JonnyB]

Vice grips is a standard procedure for removing a brake piston, as long as your only gripping the top flange bit. I've never had a set I could just pull straight out anyway,

If the bleed nipple is cross threaded you may get it out with a stud or screw extractor, soak the screw in penetrating oil over night.

Dilling out is not easy as the screw is harder than the soft alloy around it.

Sorry it took so long to reply been on holiday, (Well enjoying the English Sunshine, but it's gone now)