Brakes Siezing

[Drbrown]

After driving a short distance and first using brakes, the brake system siezes up, slows the car down and when stopping the car it comes to an abrupt halt as if the brake pedal is firmly applied. If the car then sits for a day everything returns to normal. System was flushed and self-bleeders installed about a year ago. I assume I should start by jacking entire car up to check individual wheels and check for pedal free play, which SEEMS absent when the system is siezed-up. Any advise appreciated. If it comes down to the Master Cylinder, any recommendations re good place to buy one ?.