Magnetic Brakes Losing Effectivness.

I have an application where we are using magnetic brakes to hold spools of wire. The wire is then would around a mandrel to make coils. The brakes areter so long seem to loose there effectiveness (can't think of another way to put it) and need to be adjusted to get the same results. Our brakes have a % mark on the side and we can start out at 25% lets say and it could be a couple years and we need to change them to maybe 30-40% to get the same amount of drag desired. To fix this we basically take a power drill hook it up to the brake shaft and spin it at a high RPM for a bit and it seems to build the magnets back to full strength.

Here is the website of the brakes and we are using model number 527-016.

http://www.magnetictech.com

Three things:

Can someone possible explain to be what is happening?

Is there a way to hook these up to a torsion tester or something where you would turn the motor on to a certain speed and you can turn the brake dial until you get the desired drag you are looking for?

Also is there a better way to fix these brakes by re-tunning them?