i have a dakota sport 98.
When i press the brake pedal and hold it, after fews seconds it goes slowly to the floor.
My question is : Is it the master cylinder or the brake booster.

I forgot to say that i have no leak,. my reservoir is full

Thank's

 

your in the wrong section they will be able to help you in the Dakota section, just a hint

here is a link to the right section 2nd Gen Dakota

 

If it was the booster you would loose power assistance to the pedal - it would get harder to push the pedal and stop the the truck. It looks like you have a internal leak in the master cylinder, do not mess with it as you could get loose the brakes and crash it.