Brake not coming on when trigger is released

Could be one of 3 things - I'll give them in order of most common to least:
1) Brushes - check your brushes, if they are very worn or not contacting the armature well they could be the culprit.
2) Switch - tools with a brake have 1 set of normally open contacts & 1 set of normally closed contacts (the configuration is usually stamped on the switch itself) You will need to disconnect the leads to the switch and test it with a continuity tester - if the switch is your problem the normally closed contacts will show open.
3) Field - lastly (least common but possible) is the field, tools with a brake have 2 separate windings in the field. If the field is the issue there will be one winding in the field that will not have continuity (usually the smaller wires in the field) Again, you will need to disconnect the wiring to the field and test with a continuity tester.