It's part of the brake mechanism and holds the brake in the OFF position. Without it the brake flops down and continually rubs against the tyre. I have been holding it in the OFF position with a rubber band.
A 5mm drill bit fits through the hole in the middle of the spring. If you look closely you can see one of the spring arms has snapped off
I had to drill the rivets out to access the mechanism.
There's nothing wrong with the brake apart from the spring and a new brake mechanism is http://www.epc-wheelchairs.co.uk/wheels ... rs%5B95%5D