It is normal for your Variac brushes to wear with time, especially if frequent adjustments to the output are made.  Using the Variac in an overload state may also cause excessive wear on the brush.

Description	Variac Type	Cat No.
VB-1	M2,W2,W5H	3200-5901
VB-2	M5,W5,W5L	3200-5900
VB-3	W8,W8L	3200-5923
VBT-6	W50	3200-5906
VBT-7	W50H	3200-5907
VBT-8	M20,W20	3200-5908
VBT-10	M10,W10	3200-5910
VBT-11	W10H	3200-5911
VBT-12	W20H	3200-5912
VBT-13	W30	3200-5913
VBT-14	W30H	3200-5914