Air supply unit for a pneumatic system of a commercial vehicle, said air supply unit comprising: a compressor inlet to be connected to a compressor for receiving compressed air in a supply mode (I), a consumption outlet for connecting at least one consumption circuit of said commercial vehicle, an air processing unit connected to said compressor inlet for drying and filtering said compressed air and delivering dried compressed air at its outlet , a supply line extending from said outlet to said consumption outlet of said air supply unit , a purge tank connected to said supply line for receiving and storing said dried compressed air from said supply line in said supply mode (I) and delivering stored dried compressed air as purge air in a purge mode (II) through said supply line to said air processing unit for regenerating said air processing unit , a separating valve provided between a first section and an end section of said supply line , said end section extending to said consumption outlet , wherein said purge tank is connected to said supply line through a throttle for pressure reduction of said purge air characterized in that said purge tank is further connected to said supply line by a filling valve , wherein said filling valve is open in said supply mode (I) and blocking in said purge mode (II), wherein a first section of said supply line extends between said air processing unit and said junction area and a second section of said supply line extends between said junction area and said end section , wherein said dried compressed air flows through said first and second section and said purge air only flows through said first section . a compressor inlet to be connected to a compressor for receiving compressed air in a supply mode (I), a consumption outlet for connecting at least one consumption circuit of said commercial vehicle, an air processing unit connected to said compressor inlet for drying and filtering said compressed air and delivering dried compressed air at its outlet , a supply line extending from said outlet to said consumption outlet of said air supply unit , a purge tank connected to said supply line for receiving and storing said dried compressed air from said supply line in said supply mode (I) and delivering stored dried compressed air as purge air in a purge mode (II) through said supply line to said air processing unit for regenerating said air processing unit , a separating valve provided between a first section and an end section of said supply line , said end section extending to said consumption outlet , wherein said purge tank is connected to said supply line through a throttle for pressure reduction of said purge air characterized in that said purge tank is further connected to said supply line by a filling valve , wherein said filling valve is open in said supply mode (I) and blocking in said purge mode (II), wherein a first section of said supply line extends between said air processing unit and said junction area and a second section of said supply line extends between said junction area and said end section , wherein said dried compressed air flows through said first and second section and said purge air only flows through said first section . <EOS>