Changing brake line size will change nothing. Pressure on a hydraulic system is the same throughout the system regardless of line size.
Next time the brakes lock, open a bleeder in front. If they release the problem is the fluid is being held in the front, rather than released back to the master cylinder. The cause can be a misadjusted brake pedal, misadjusted booster pushrod, wrong master cylinder, something in the line acting as a check valve, or defective brake hoses.