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

 

You probably have air in your lines.
Bleed the brakes to check for that first. Doesn't cost anything.
If that doesn't work then it's probably the master cylinder.
How many miles on the truck?

 

The truck is 232000 km