Rear disc brake calliper sticking

[andymc744]

Hi a friend of mine has bought a new mountain bike from tesco as a go around. He has assembled it but when he applies strong force to the brake lever the rear disc brake the caliper jams and the wheel skids. The only way to release it is to stop and fiddle with the lever or stick an Allen key in the jammed arm and it will immediately loosen. Can anyone help with a solution please? Is it just a stiff arm or is it more technical? Many thanks

[Painkiller]

I would help to know what brake system you have, I assume they are mechanical. It sounds like a fray in the cable inside the housing.
take some pics and post them
also inspect for any burrs in the moving parts of the system