Hopefully someone can help me with my magna brakes! My front driverside caliper keeps sticking! I have rebuilt the master cylinder, replaced the brakelines with SS, and rebuilt both calipers. After the initial rebuild the brakes worked great then one day as I was riding I noticed slowing down faster than normal. When I went to touch the brake lever it was rock hard like I was on the brakes. I had to pull over and release the pressure at the caliper so I could ride it home. Its only the drivers side doing this. So I again broke the caliper down made sure the seals weren't pinch or the was any burrs in the piston/sleeve. Nothing was found unusual, put back together and bleed sysytem again. (vacuum pump) Went for a ride brakes were great for about 10mins then would do it again? At that point I took it to a repair shop to have them look at. The caliper was moving freely and we disassembled the master cylinder and found nothing wrong. But again on the way home it did again. So I then bought another set of used calipers figuring there was something going on in that caliper. (was a complete front system that was never opened) I took apart and rebuilt the used one and made sure the pistons would slide using just my fingers. Everything was good and I reassembled the system. After that they worked fine. I have been riding the bike for 2 weeks no problem until today. I was riding and thought it didn't seemed right, as I was exiting the highway I went for the brakes to find the handle rock hard again. Luckily everytime I have been able to feel the brakes starting to grab before its too late while Im riding down the road at 60mph basically everywhere I ride. (thats the speed limit) So now I have to ride home with this agin which is not fun. (only way to go it to release the pressure which will build again about another 10 of riding) What is causing this problem/ I have inspected and checked everything! The little hole in the mc was clear also. Is that little metal tab abovehole required to be there or is it just to help block large particles from going in the system?
I would think it would be something in the master at the hand lever? I can't see it being the calipers as you have had different ones on it with the same results.
I'm gonna go home and rebuild the mc again even thought I'm not sure how anything would be clogged. Might switch out the housing since I have spares.
Since it is only affecting one caliper on one side, I would suggest that you remove the assembly where the two front brake lines "T" together and check it for an obstruction on the "Driver's side" as you call it.
Something is acting as a one way check valve in your system on that particular circuit. You stated that you changed the brake lines so there may be something allowing the fluid to travel to the caliper but when the fluid backs up slightly upon release of the front brake, the obstruction (check ball effect) will not allow the fluid to "back up" in the direction of the master cylinder somewhere between the "T" and your caliper.
Eventually the pressure will build high enough to leave that caliper/pad set rock hard against the rotor. Again the fluid will flow towards the caliper when you bleed it down because you are only getting a stoppage when the fluid tries to flow in the reverse direction.
Get the assemblies (brake line and "T") off and blow air through them backwards onto a clean cloth and see if something suspicious doesn't show up.
In the master cylinder there is a tiny pressure relief hole, should be pretty much in the middle of the cylinder bottom. Get a very small piece of wire and gently clear it out. Seen this before on a V65 doing the exact same thing you describe.
I believe Greg hit the nail on the head; I too have seen this caused by a clogged return port in the Master Cylinder.