15 sounds WAY to high. I never got my electric brakes to lock, Over 6 volts it felt the same. At 25 mph, I would coast (giant empty parking lot in back of a commercial building for lease) and aplly the trailer manual brake control all by itself. I would gauge how long it would take to stop just using the trailer brakes. It seems that over 6 volts it didn't brake any harder. So I put it at 5.9 volts.
I adjusted my P3 until the "pull" of the trailer when braking was almost nil. I left it there (5.9 volts). I tried the boost levels, and either nothing or B1 seemed fine. B2 I could feel the Pod yank when touching the brakes. I ended up setting it to nothing after a year of towing with the P3.
If you firmly step on the tow vehicle brakes, and it feels like the trailer is pushing, you can feel it, like a clunk. Set the voltage higher. When stepping on my brakes, I almost never feel the Pod pushing or tugging, which I think is perfect.
You also may want to check to see if your electric brakes are adjusted properly, depends on the brand and type on their. Most have a mechanical level pulled by solenoid, which is controlled by the voltage level sent by the controller.
I also know what it feels like when it's not connected, which happened once when I didn't have a good connection and the cable just barely pulled out enough to cause the P3 to register NO CONNECTION while driving. I had no idea until I actually saw that when parking. My tow vehicle was stopping everything for maybe two hours (all highway right lane no traffic) and I didn't know. I learned to look down once in a while to check, and also make damn sure I push the cable in all the way and snap the clasp shut. Once I pushed it back in and started driving again, I could tell the brakes were again doing their thing.