Vehicle is a 2006 frontier with 220k miles. When I’m on the gas cruising and need to quickly brake, the pedal will feel very stiff for a half second before depressing and slowing the truck. Very pronounced if I have cruise control on the highway and hit the brakes hard without cancelling cruise first. When I’m driving normally and leave a bit more time between coming off the throttle and going to the brake it feels normal. I’m assuming this a master cylinder issue, can someone explain what is happening? Can it be repaired or should the whole cylinder be replaced? Thanks for your help!
Without doing any diagnosis, I would replace the brake booster vacuum check valve. It is most likely part of the hose/plastic pipe that plugs into the booster. In some cases you will find that the hard plastic pieces that get pressed into the plastic line, where the press together they crack and leak at those joints. Sometimes those can be sealed up with epoxy. One test you could do is check the effort it takes to press the pedal after you shut the engine off. In a proper operating system (not leaking vacuum), the booster retains enough vacuum to have a normal pedal feel for at least the first push. If the pedal is hard, then you would have a vacuum leak or bad check valve that is not keeping vacuum in the booster. In some cases it could be a bad booster, but that would be very uncommon… good luck, hope this helps.