We have a 2012 Dodge Durango., Recently the brake pedal goes to the floor or almost to the floor. There is no leak detectable. Is it the master cylinder, back brakes need adjusting ? Any comments/ thoughts would be helpful .

 

Checked the fluid level? Is the brake warning light on?

 

I'll add to the above that if you do not notice a leak and the fluid level is low, don't just top it off. It's low for a reason, and if there is not leak, it's low because you are in need of brake pads. There's really no adjusting the rear since they are disc brakes for the primary system. The drum system back there is only for the parking brake and therefore would have no effect on your brake pedal.

When the vehicle is off if you pump the brake pedal does it firm up quickly?

-Rod

 

Here is something to consider, the Brake Booster that is hanging on the firewall drivers side. Mine failed under a recall notice due to corrosion and i experienced the same symptoms you are with a soft pedal and stopping distances increasing. Mine was bad enough that instead of a repair (which was the bulletin) they replaced it.

2011 Durango Express 166k, v6, usa

 

My guess it's the brake booster. I have had the same issue in the past on another vehicle.