This sounds like a sticky caliper. I am surprised that a service centre could not diagnose the cause if you described the symtom as above to them. The calipers should be checked at every service, at least that is what I do. Take them out of the mountings and give the pistons a good clean with old brake fluid and check the seals and make sure the piston moves in the caliper.
The brake fluid should be changed about every two years as it does absorb water during its life which does reduce it effectivness. Old fluid seems to be darker than new fluid and might make the brakes feel spongey. Good tip to always use new fluid of the right type, normally DOT 4 or 5 and make sure its from a new sealed bottle not one you've had on the shelf for a few years.
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