Updating brakes on a vintage tandem
Here's the sole Colnago road tandem picture I could find anywhere.

Is your bike anything like this ?



From http://www.ebay.com/itm/Colnago-Tandem-Bike-X-Campagnolo-Groupset-Pantographed-Vintage-Very-Rare-/151292492125?hash=item2339bc395d&amp;pt=LH_DefaultDomain_0&amp;nma=true&amp;si=3DCdggTFjbiHTJYmI3jOxvB%252FJrE%253D&amp;orig_cvip=true&amp;rt=nc&amp;_trksid=p2047675.l2557
and there are more photos there.

If so - it has single pivot caliper brakes, and definitely steel rims.

Your first bet would be to swap out the brake pads.  Then change all the brake cables, both inners and outers.  These are both maintenance tasks and do not detract from the bike.

Replacing the single pivot with dual pivots would help, but finding something period might be difficult.  IMO safety is more important than originality, but others disagree.

Finally, steel rims tend to be poor at braking anytime.  If you can find some 700c alloy rims, they will brake much better.  Downside is they're fractionally smaller than 27" wheels, so you'll need new tyres and tubes, and the brake calipers will need to reach further, which can compromise their effectiveness.