I have a 1995 Ford Explorer with a strange brake problem. When the vehicle is cold and I deliberately do a panic stop the brake pedal is much harder than it should be to the point where the truck will slow down but keep rolling to a stop, if I apply light pressure as in approaching a stop sign or traffic light it works perfectly, it only has this behavior when the vehicle is cold. I did the normal check to see if the booster was working properly by holding pressure on the brake pedal while the engine was off starting the engine and feeling the pedal go down slightly. I replaced the check valve and it seems to be working a little better but not absolutely perfect. I just did a brake job on a 99 Ford Explorer for friend and since I had the vehicle I did the same panic stop with it and it works as its supposed to. I hear the air suction at the brake booster inside the car on both cars so I know that's normal.
After making a few stops I can then do a panic stop which will almost lock up the wheels but is prevented because of the ABS system which is what it's supposed to do.
I found this problem out purely by accident not as in having an accident LOL, but I had just left my house and while going down the street a few people needed to cross so I slow down and stopped to let them cross as they cleared the car and I begin to accelerate then I happened to notice a little dog running behind them which caused me to slam on the pedal extremely hard (panic stop) but the truck just slow down and was still rolling fortunately I wasn't going fast enough to hit the dog and the funny thing is the person crossing the street smiled and waved and commented that I have good brakes hahaha.