re:the OP I had these disc brakes once and this problem could usually be cured either by:
1) loosening the QR skewer on the relevant wheel and making sure the wheel is properly aligned in the fork and holding the brake tightly closed while you refasten the QR or if this doesnt work then;
2) realign the brake caliper as described in the service instructions on the Shimano website (loosening the caliper bolts etc).
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only the dead have seen the end of mass motorized stupidity
Plato
(well if he was alive today he would have written it)