Unloader valve on rigid tri stack how to replace it

Pop off valve?
If it is the unloader valve that is venting the air, the discharge check valve is leaking and has to be replaced.

Hubert, The unloader valve is like a by pass for the water and regulates/controls water pressure and redirects water when gun trigger is released. Check the attached links,instruction and guides to learn more about Pumps/unloader, Good luck
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Pressure Washer Unloader Valve
How to replace an unloader valve on your pressure washer pump
All About Unloader Valves ePowerWash
How to Replace an Unloader Valve on Pressure Washer Repair Guide Help...

ASSUMING THIS IS AN ELECTRIC MOTOR DRIVEN SINGLE STAGE COMPRESSOR, IF THE COMPRESSOR USES OIL IN THE CRANKCASE, THE PRESSURE SWITCH SHOULD CUT OFF AT 125 PSI AS THE ACT OF COMPRESSION HEATS THE AIR QUITE HOT. THERE IS ALWAYS SOME OIL THAT PASSED THE PISTON RINGS IN THE COMPRESSED AIR. IF THE COMPRESSED AIR GETS HOT ENOUGH, THE OIL VAPOR CAN IGNITE AND THE FLAME CAN CARRY TO THE TANK FOR AN IMPRESSIVE EXPLOSION. THIS ORDINARILY DOES NOT HAPPEN BELOW 175PSI OR MORE.

OILLESS COMPRESSORS CAN BE SET TO 150 PSI. WITH EITHER TYPE OF COMPRESSOR, THE HOSE SHOULD BE ATTACHED TO THE REGULATOR OR THE REGULATOR STACK, NEVER TO THE COMPRESSOR, AS THE COMPRESSED AIR LOSES MUCH OF ITS HEAT WHILE IN THE RECEIVER. IF THE WHOLE TANK IS GETTING HOT ENOUGH TO MELT A PROPERLY ATTACHED HOSE, YOU HAVE A DANGEROUS SITUATION, CHECK THE OPERATION OF THE PRESSURE SWITCH, UNLOADER VALVE AND RELIEF VALVE. THE RELIEF VALVE SHOULD OPEN AT NO MORE THAN 150 PSI, THE UNLOADER VALVE OPENS WHEN THE PRESSURE SWITCH SHUTS THE MOTOR OFF, AND SHOULD ONLY HISS FOR A FEW SECONDS AS IT DUMPS THE PRESSURE ONLY IN THE COMPRESSOR AND THE LINE FROM THE COMPRESSOR TO THE TANK INLET CHECK VALVE. IF THE UNLOADER VALVE HISSES MORE THAN A FEW SECONDS, THE TANK INLET CHECK VALVE IS LEAKING AND NEEDS TO BE SERVICED, CLEANED OR REPLACED. IF THERE IS CARBON BUILD UP IN THE VALVE, THERE IS EXCESS OIL PASSING THE COMPRESSOR PISTON RINGS, REPLACING THEM MIGHT FIX THAT IF THE CYLINDER OR RING LANDS ON THE PISTON ARE NOT TOO TAPERED OR WORN.

the unloader is made into the pressure switch. When unit shuts down you should hear air blow out at the bottom of the pressure switch for a short period and stop.If it continually blows until the unit tries to start the tank check valve needs to be replaced. the check valve is located in the top of the tank where the discharge line goes into the tank. Also if you check the bottom of the pressure switch with the cover off you will be able to see the unloader. It's a schrader valve that is hit by a small arm on the pressure switch. you can replace the schrader valve. if it's something on the pressure switch the switch should be replaced. I've seen check valves bad but very seldom do you see the unloader bad. hope this helps you.

This problem usually occurs when the in-tank check valve is leaking air back to the pump and the unloader is not working properly. Follow air discharge line from the compressor to the tank. This line is attached to the check valve (usually brass or chrome). A second smaller line also attaches to the checkvalve and leads to the pressure switch (unloader valve). After filling and shut off, you should hear a discharge of air at the pressure switch. If not, examine valve for possible mis-alignment with contact arm. If air continues to leak from the unloader valve, then checkvalve is leaking. Use universal replacement check valve if possible, check thread size. Parts are on ebay or www.toolpartsdirect.com. Good luck

This is an indication that the unloader not working properly. When you release the trigger the unloader senses the increase in pressure and bypasses the water into the water inlet. When the unloader is not working and you release the trigger, the water pressure builds quickly and overcomes the power of the engine shutting it down. Here is the manual for your machine, http://www.pressurewashersdirect.com/manuals/Simpson_WaterBlaster%20manual_2.pdf

and some information about troubleshooting from the manual.
"Not Unloading. Leak in Gunvalve or Hose. Repair gun leak . Replace leaking or broken hoses. Debris or wear in Check Valve. Remove Check Valve parts and clean. Check O-Ring. Reface with emery cloth, or replace checkvalve. Damaged Piston U-Cup. Rebuild Unloader Valve (Service Center)."
Usually when the unloader fails the pressure regulator will most likely need to be replaced as the unloader is usually incorporated within. For now try lowering the pressure some and just tie the trigger open untill you can service. Good luck.

Unloader valve probably needs to be adjusted. Should be two jam nuts holding the unloader to the pressure switch. Loosen the jam nuts and adjust the unloader slightly away from the lever. Tighten the jam nuts and try it again.

You stated that the compressor started up fine. I'll assume it reached it's max psi then cut out. Then upon reaching it's cut-in psi the motor ran slowly for a few seconds, then kicked off. I also assume you mean check valve when you refer to the non return valve.
If this sounds about right then. The first thing I would do is bleed out all the air from both the tank and the pump supply line leading from the pump head to the tank's check valve. You can do this by simply loosening the fitting that attaches the line at the head or the check valve. Once done try running the compressor again.
The purpose of this is to check the unloader valve. This valve bleeds off the pressure that remains in the compression cylinder and the supply line up to the check valve. When functioning normaly it will bleed off this pressure once the motor shuts off. If it is not working correctly then when your compressor starts back up (once it is already been filled) the piston will be working against a psi close to the max psi.

If the problem reoccurs only after the compressor has reached it's max psi, then the unloader valve is most likely the problem. If however it will not start up at all even after a complete bleed then it has to be something internal to the pump and\or motor.

Please try this and let me know (or repost) your findings.
If it's the unloader valve I should be able to explain how to repair this valve.
Good luck.

Hello:
Here is a schematic. On page 2.1, part #1 is the unloader valve. You need to remove it and lube it with a very good waterproof grease. Also insure the O-rings are in good shape and the spring action is working.

The unloader valve usually is replaced. I dont know if you can get o-rings for it. But if you want to take it apart, there will be a crown or cap style nut just below the spring. Unscrew it completely and you will be able to remove the spool. There is just a series of o-rings on it.