ratru,There are two valve "stoppers", in the control valve. To gain access,loosen the hose clamps,and remove all 5. Remove the two bolts that secure it to the SM420. You'll need to split the casing to access the cable-anchor point,to remove the unit.Once you have the unit on the bench,inspect the bores for dust/debris,that may be causing the problem. My "stoppers" were fine,but enough "junk got into the bore,to cause a leak. You may be able to replace just the "stoppers", (if worn),which would be much cheaper than buying a new unit. Be careful when splitting the housing,to try and save the gasket. I had to use "Permatex. It split in 3 places. L.o.L. Hope this helps.