There is an adjuster that controls the amount of soap and water that the machine uses. If the adjuster doesn't work either the know or the piston that controls the flow may be broken.
If you try to fix it yourself putting in the piston is a little tricky. There is a groove in the top housing to hold the piston. Use the adjuster knob to hold the piston to the housing. It helps to have it at eye level, and make sure that you put something under the lower housing so that the wheels nest in the housing. You then have to lower the housing making sure that the piston goes into the valve hole.
Ed, December 2007