I recently replaced my seal with this kit from eBay.
http://www.ebay.com/itm/STOP-the-Dom...pZdPcY&vxp=mtr There are many other options on eBay, just need to know for sure your manuf. and model.
Another thing to try before replacing is to lubricate the seal with a vegetable oil and to make sure it pops back to a fully closed position. I let my foot slide off the pedal to snap the closure back instead of simply raising my foot.