front brake piston won't retract.

[longhorn717]

Hey guys I've tried bleeding the brakes. and apparently that needed to be done anyway but everytime I squeeze my brakes the piston just keeps pushing harder and harder instead of coming back off the brake. A friend of mine said something about a valve in the master cylinder that let's the fluid back into the cylinder after you're done pulling. This is supposed to relieve pressure in the line....? could this be a factor? the caliper definitely needs a rebuild but I'm not sure about the master cylinder. this is on a 1980 honda cb400t

[mscuddy]

Yes, check that relief port that's in the rod that pushes fluid into the wheel cylinder, and back to the brake cylinder on the handlebars after you release the brake lever.

It's been the biggest overlooked item when diagnosing disc brake sticking issues. You can test it by using the bleed port on the wheel cylinder. If it has pressure after you release the lever, bleed the system's presure off, than disassemble the master cylinder's rod to see if any white corrosion has plugged the relief port.

[seruzawa]

I had the same problem on a Magna. The bleed hole, which you can see in the bottom of the reservoir, can be very very tiny. If you remove the cap from the reservoir, squeeze the brake lever and release it gently you should see the fluid in the reservoir become disturbed by the flow of fluid through the relief hole. If not, it's plugged. If it's plugged it's best to pull the reservoir, clean thoroughly and install a new piston.

[longhorn717]

Well I'm starting with the caliper but sounds like I'll be either rebuilding the master also or maybe I'll go with an aftermarket. who knows.