@Robert F Jarvis A company in Stratford, Ontario; they've been mentioned a number of times here on Control Booth within the past year. A search of Control Booth should find them. In the next 45 minutes I'll look them up and post back.
That was actually my original idea as well, but springs are trickier to get right. The amount of force applied depends on how far the spring is compressed, so it's a lot more touchy about the exact distance between the brake and the floor. With the brake being pneumatically applied, I can control the amount of brake pressure just by adding a regulator to the line and that pressure will always stay the same throughout the whole stroke of the cylinder. That seemed like a worthwhile benefit for being able to adapt to different applications in the future.
That's pretty much how I felt about it: I liked the simplicity and elegance of the idea, but wasn't satisfied with the actual result.