If you have to perform fixes on your own Derbi Dirt Boy 50 then it is always a good idea to have access to a Workshop Manual to use as reference should you need to know the right way to strip off a specific part including Brake Pads and install it again in the correct manner.
Derbi Dirt Boy 50 Workshop Manuals will give specific diagnostic wisdom for your vehicle so if something is wrong it can help you determine what the situation is and ways to make it better. Then you are able to determine if you can do this yourself or in case the job is just too big you can make a reservation for your Dirt Boy 50 in with a suitable friendly mechanic.
Derbi Dirt Boy 50 manual