The most effective rim brakes are V-brakes, aka linear pull brakes. http://www.sram.com/avid/products/si...it-5-rim-brake But if you do not have bosses to mount the brakes on the fork and frame (and brake levers that pull enough cable) they will not work for you. We have Avid SD5 on our T50 - the handle us (500+lbs team) with no issues. Less than $50- plus cables, and labor.

With the single mounting bolt configuration; dual pivot calipers are the best. http://sheldonbrown.com/harris/brake-calipers.html I have Tektro R559 and R539 on two bikes. The stop as well as linear pull brakes, but require more force on the lever. Read the information on the link carefully, measure your bike, and also determine if you need "nutted" or recessed. $70- plus cables and labor.

Replace your brake cables - make sure to properly lubricate them.

The Avid and Tektro pads are quite good. Kool Stop Salmon are better (best ?); but I can't tell the difference.