There are 3 common problems with your front brakes not working well. The brake brake caliper out of adjustment or the cable is old & worn.
 
CABLE OPTIONS:
1976-1980 SHORT - 619mm/24.3"
1976-1980MEDIUM - 659mm/26"
1976-1980 LONG - 679mm/26.75"
1974-1975 - 540mm/21.25"
 
TIPS:
Make sure to lube with a light grease all pivot points so the cable will move smoothly.