What was found
RIGHT HYDRAULIC SYSTEM FLUID QUANTITY WENT TO ZERO DURING TAXI OUT. FUMES ENTERED THE CABIN BUT CREW WAS ABLE TO CLEAR THE AIR.CREW STATED THAT THE RED EMERGENCY WARNING LIGHT DID NOT ILLUMINATE. AIRCRAFT WAS TOWED BACK TO THE GATE WITHOUT FURTHER INCIDENT. MAINTENANCE FOUND THE RUDDER POWER SHUTOFF VALVE CRACKED AND REPLACED IT. ALSO REPLACED THE NBR 2 ENGINE DRIVEN AND ELECTRICAL HYDRAULIC PUMPS AND ASSOCIATED FILTERS AS A PRECAUTION. COALESCER BAGS WERE ALSO REPLACED. AIRCRAFT RETURNED TO SERVICE.