As John E encourages, try these steps in this order until you are more pleased with stopping power. Also do you have steel or aluminum rims?

1. Replace the original cables with modern Jagwire brand and replace the pads with Koolstops.

2. If still not happy, replace the calipers with '60s-'70s centerpulls (if they can be made to work). Try to pick these up used. Go with Weinmann or DiaComp. I know I could provide you with at least one set. Use the new cables and brake pads with the centerpulls. Post pictures of your original calipers, seatpost clamp area, and front stem at the top of the headset.

3. Replace the original levers with Tectro or Cane Creek areo levers.

Best of luck. If all of the above doesn't help much, then you could add new dual pivot calipers and use everything I've listed except the centerpulls.