While slowing to a stop and fully depressing only the brake with my right foot my vehicle rapidly accelerated into the back of another truck. It felt like the front brakes engaged, but the rear wheels continued to accelerate as the vehicle transmission worked to downshift. Serious safety issue-I am 100% confident that I was standing on the brake and not the accelerator. The road was dry with no gravel and I was gradually slowing to a stop when this occurred.