Sounds like either the system's over-filled or the master cylinder piston isn't releasing properly hence not letting the pistons retract. If there was debris in the brake line then I guess it could stop fluid returning to the master cylinder but it sounds a bit far fetched. Was there any debris in the old fluid or caliper when you stripped it?
Another common cause is the caliper seal grooves being corroded enough that the seals don't sit properly so the pistons don't retract. If that was the case I'd expect them to move when pushed, but if you don't push exactly squarely they jam easily anyway.
They should be tight enough to withstand the pressures a heavily depressed brake lever can produce.
Your symptoms sound like hydraulic lock which may be that there is brake fluid getting behind the pistons preventing their return. However, in that instance, I'd expect to see evidence of fluid leak in the caliper body. You also say that the pistons return freely until they're bled up which also leads me to the same conclusion.
Is the rear brake pedal adjusted properly? I know it's a long shot but it may be that it is sitting down just enough to 'activate' the brake and cause the lock. You'd find it next to impossible to return the pistons against even gentle lever pressure.
Were these symptoms there before you serviced the caliper?
98% of the time I'm right. Why worry about the other 3%.
fuzzy. wrote (see)
Further to my last, does anyone have access to what the ID and OD measurements should be for the caliper seals? Would be interesting to measure the ones that I have been sent to see if they are correct or not.
Have a look on Gixxerjunkies mate.
If I'd seen this I would have made 2 suggestions.
1 Is the caliper seal in the right way, as they often have a taper, to allow the piston to retract.
2 Check the resevour air hole is not blocked.
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