I had the same problem with an LTD that I had just replaced the Brake booster on. The newer brake booster wasn't as thick as the original and therefore let the shaft push in the master cyclinder. (Causing the brakes to be on all the time). They will work for a short time, but will then boil the brake fluid in the lines and cause the brakes to lock up.

Fortunately I had some tools with me when this happened and I had it fixed within about 10 minutes.

Just make sure that there is a little freeplay in the brake pedal before taking the car out for a test drive.