No spray At low pressure have a slight stream of water. When I pull the nozzle back to go high pressure it cuts off completely. Cranks/runs ok.

pump
Craftsman Pressure Washer Model 580 752190 580752190 580 752190 Parts

You have to use a low pressure nozzle ( usually black with very large spray opening) With the low pressure nozzle siphons your detergent in with the spray water . If you are using a high pressure nozzle the soap will not prime.

nozzle stuck with rust spray WD40 or something like it, in it.

Could be low water pressure I have found.
Also, I purchased a can of "stuff" at Lowes today which allows you to circulate a liquid substance which lubricates the pressure pump....the pump could be frozen up...
I have a rotating nozzle which I bought...and it had frozen up...this stuff works

that is sure signal of insufficient water supply!

try easy solution first:
make sure that your mains is fully open, that tap on whihc hose is hook up is fully open and hose is of "decent" diameter to allow sufficient flow.

if above solution do not work then you need to make another, more difficult arrangement:
prepare 20litres pressure tank with inlet connected to mains and outlet by short hose to power washer, fill it with water from mains, keep it under mains pressure and use karcher power washer normaly. this will solve water supply problem. twenty litres will last about 2min, because it is re-filled, under standard pressure therefore it will last about 3min...

third possibility is to stop washing...

if you find my advice helpfull, please do not hesitate to left the comment and vote for me, thanx!

NorthStar Pressure Washer Turbo Nozzle --> --> Turbo nozzle sprays up to 3.1 GPM at 2300 PSI and 194°F. Unique design delivers 0° high pressure impact with wide angle spray coverage.
The point of the Turbo is that it is a solid ( high pressure stream of water )

drain your system of pressure. remove your air cap and nozzle from the gun. slowly increase fluid pressure till it comes out of gun. this will clean out debri from gun. reduce pressure so you don't wast anymore fluid.
clean nozzle properly / completely
clean aircap. properly / completely
inspect needle tip for abnormal wear.
if cleaned properly you will find your gun works much better.

HELLO THERE:
Here is some troubleshooting tips for you and a web link for you ok this should help
best regards michael
Troubleshooting Your Pressure Washer General points to check if a problem occurs

• Are the engine and fuel switches in the “ON” position?
• When troubleshooting, always check oil and gas levels.
• The oil for the pump is a 30W non-detergent mineral oil. Do Not fill over halfway on the sight glass.
• Is the water tap open?
• Check the choke and throttle levers.
• Is the correct nozzle in the lance? Is the nozzle plugged?
• Only the black nozzle is for soaping at low pressure.
• Is the water-inlet screen clogged?
• Hold the gun trigger open while pull starting the engine.
• Check for O-rings in the lance and quick couplers.
• Has the unit tipped over? Close the fuel switch and keep pull starting the engine until the excess fuel is burnt up, then open the fuel valve and try again. Some engines will need to be primed every time they are started.
• Make sure the engine is turned off after each use. It should not be left running without use for longer than 2 minutes as the pump can overheat.

Are you experiencing low pressure?

• Is there an air leak?
• Dirty pump valves?
• Check for worn pump valves or broken valve springs.
• Check for a worn out spray nozzle.
• Check for leaks in your plumping or hoses.
• Troubleshoot your nozzle, is it turned to the highest pressure setting?
• Is the unloader improperly set?
• Are the engine RPMs too low?
• Check if your water supply is sufficient.
• Check if your engine is getting enough power.

Trouble with high vibrations

• Dirty pump valves?
• Do you have a sufficient water supply?
• Is there are in your water supply?
• Is the inlet water filter clogged?
• Clogged or damaged spray nozzle?
• Make sure it has the correct fuel
• Has the choke been left on after the engine has warmed up?

If the engine won’t start

• When troubleshooting the engine, check if it's low on fuel or oil?
• Worn or dirty spark plugs?
• Is the switch in the off position?
• Is the fuel valve in the off position?
• Is the engine flooded from over choking?
• When cold starting, was the engine properly choked?

If the starter is hard to pull

• Low oil in the crankcase or pump assembly.
• Oil or fuel in cylinders (hydro locked).
• Pressure built up in the system, relieve this by squeezing the trigger.

I have seen this problem alot and it can normally be rectified by clean out your unloader/bypass valve the orings on this valve get damaged as jerr said by leaving hoses and trigger pressurised, the easiest way to test if it is a faulty unloader valve is to do as you say it does and when it is on its weak stream turn your nozzle to detergent or low pressure mode then back to high pressure it should kick the pump back in...

If you do not have low pressure(chemical applicator) on your nozzle you may be able to remove the front half of your lance to achive the same result...

Kind Regards
Brad

In general, pressure washers will draw detergent from the reservoir only when the spray nozzle is set to low pressure. At high pressure a non-return valve closes to prevent water flowing back in to the reservoir. If you've got the spray nozzle set to low pressure and still have the problem, it may be that the spray nozzle is partially blocked - you can try clearing it with a length of wire.  Failing that, the non-return valve is probably stuck (and that may need professional attention).