Brakes leaking from driver side rear

Hello, I'm Danny.

Losing brake fluid from what you've mentioned is an obvious leak in the system. You will need to remove the tire to see if a brake line or the brake caliper is loose/broken/defective. I would suggest having it repaired as soon as possible. Your truck should not be driven until repaired for fear of losing brakes entirely. Here is a tutorial to view to get you started on what to look for:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Once you figure where the leak/defect is the whole brake system will need to be bled.

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

Hope this helps and thanks for using 2CarPros.

Danny-