clean the area with brake cleaner s;ip the drum back on pump the breaks lots and lots on times the remove the drump again and check for leaks, have you had a low level of brake fld ? if not I would not think they are leaking, they could be sticking, in that case I would just buy new ones , replace both sides not just one

2001 5sp. K&N stock filter 215/70/15
If your check engine light is on at least you know the light still works