I had this exact same problem about 1 month ago. Here is what I did to fix the lack of braking power:
1. 134 oz./torque hi-tech steering/brake servo $34
2. Replaced fuel tubing with spring on brake linkage arm: gives a more positive and progressive feel to the brakes and can be trimmed down to give really hard braking suited to your tastes.
3. I doubled up the brake plates on the brake disc to prevent bending of these plates (when they are not doubled up)
The above listed modifications improved breaking considerably, so much so that I could lock up the brakes and endo the truck if I wanted to on demand. Took about 1 hour of work and about $40 in parts to transform the brake system.
Some may argue that why not just upgrade the brake pad/disc itself, but for the $15+ for a brake pad/kit, it is still going to wear out, the above modifications will not wear out. I am using a stock brake pad/disc that is probably 5 years old and can still lock the brakes up no problem.
to go further... I also purchased the following items to make the braking even more powerful:
4. Aluminum servo mount $14 on ebay for gpm racing products - highly recommend this brand, very cheap, very durable
hope this helps!