I went to drive my '92 Explorer, which had been sitting for a couple of weeks, and it acted like the brakes were locked. I jacked each wheel up individually and each would turn freely, but the vehicle still wouldn't move without acting like the brakes were on. I then pumped the pedal several times and it freed up and drove normally. The calipers, hoses, pads and rotors have been replaced within the past year. The truck has a tendency to slide the left front tire if the brakes are applied quickly, but not necessarily very hard. That's why the brakes were re-done. It didn't seem to help much.I'm wondering if the master cylinder is bad or if it might be the proportioning valve. Any ideas? L. O. G.