Here are a few things to check.
are the rear calipers working( stuck or out of adjustment) do they slide alittle when you grab them with the wheel off.
are they out of adjustment ( use the park brake a couple of times to adjust them ) hopefully you use it ocasionally so it dont up your brakes.
Here was the problem with mine :
Check the vacuum line to the booster. Ocasionally they get oil soaked and will calapse when you let off the gas to brake and have a high vacuum signal. I noticed this on mine after I changed my intake gasket. Brakes are as good as new.
$11.00 hose at the dealer.
Mark