Brake problem help please.
 

I have a 1950 Mercury with stock brake system and master cylinder. My problem is that when shifting into drive the brake pedal drops and the car lurches forward about 10 or 12 inches.

I have had the brakes bled and fluid replaced and brakes adjusted. The pedal is high and firm. The issue above persists. I do not see indication of wheel cylinder leakage on the inside of the tires. If anyone has an answer to this issue it would be much appreciated. Thank you.