The Foot Switch Valve (previously called the Sand Flow Valve) has an inner rubber gasket and metal core. In time this will wear out. It could last 1 to 5 years depending on your usage.
When the abrasive won't stop when the foot pedal is not depressed you may have a hole in the diaphram or the metal core part may be worn. See Trouble shooting for further causes of abrasive flow leak.
It could also be that the entire casing is worn and you may need to replace the entire Foot Switch Valve.
To check and replace make sure you first shut of the abrasive lever under the pressure pot and try to let any residual abrasive flow out before shuitting off the air. Once done you can remove one end of the Foot Switch Valve to get access to the replaceable part. You may need to remove the entire block but hopefully with an emptied valve you can avoid full disasembly.
In this video (at 20:47) Liz shows you how to replace the Foot Switch diaphram and core.