2012 F150 XLT Brakes

I only put around 2,000 miles annually on this truck so it sits for maybe 4 or 5 days at a time. Lately when I put it in reverse and start to back up I don’t have brakes so I have to push down with force to stop, also there is a loud pulsating noise when I have to do this. When I put it in forward gear and pump the brakes all is good and I can use reverse with nothing wrong. Can someone please give me an answer for this situation? Many thanks.

Just sitting with the truck running. When you step on the brakes does your idle change?

No it does not, steady at around 950.