Maybe you had some incompatability problems, i.e. the post was just a tad too short. Did you tighten the cap down without the spring? that'll normally tell you how freely the arm is moving on the post, if the arm is stiff it's common for people to up the spring tension, then you have to tighten the bolt further, which can mushroom the post which makes the arm stiffer etc.

If you're going to buy a new brake you should be aware that the fly limits the amount you can adjust the pads by, as the slot is a fair bit shorter than the evolver one.