Brake Disc Pads problems of the 2004 Mitsubishi Endeavor

We own a 2004 Mitsubishi Endeavor with 90,000 mile s on . The problem we are having is brake related and as occurred 6-7 times the last 5-6 months - its doesn't occur all the time its intermittent and only after car sits for a period of time usually over nite . The problem is the when the brakes are first applied there's a slow response time for them to work and the pedal has to be depressed very hard or pumped . We first noticed this after thanksgiving of last year the problem repeated till in March when I had a local auto service shop look at the problem (ase master certified ) - he couldn't find a problem but machine font rotors thinking that might be the problem - a few weeks later the problem occurred again . Thats when I contacted the local Mitsubishi dealer and explained the problem to him explained it hi in delai - he said it might be brake pads and replaced them - however shortly there after the problem happened again and my wife just missing rear ending the truck in front of her. I again called the Mitsubishi dealer - explained things again to them - this was after I contacted the corp. Customer service center of Mitsubishi - she said to take it back into them - we did - they stated the couldn't find any problem. I am becoming quite concerned about this safety issue and would like to know what my next steps re in solving this situation and my legal rights . Im extremely concerned about injuries here - please advice.