Typically these pressure washer have an "unloader valve" that permits the engine to be "unloaded" when there is not any FLOW of water through the outlet circuit. I would suspect a clogged or defective unloader as the culprit here.

After everything that you have done, to keep an eleven year old machine running--maybe it is time to convert it to a boat anchor, and buy a new one! What you describe is akin to pouring sand down a rat-hole!
Gee-there are so many things that could also be wrong with it, i'd say to scrap it.
Most folks don't run lawn tractors that many years!

Mownie, your assessment was right on the money.
Pulled the unloader apart it was stuck. Cleaned everything up and put it back together.

Test ran the unit and the first time I let off the trigger, the pressure would not build up again. Tapped the unloader valve with a ball peen hammer and the valve opened up and we have full pressure again.

Orderd an unloader valve rebuild kit from my local B&S dealer. I'll install it and let you know the outcome of the rebuild.

Rustyj, believe me, I have considered that but at least I need to know what is wrong before I toss it overboard.

If the $16 kit fails, then it may be last rights for the old pressure washer.

Glad it's going your way and I hope it continues to do so. :^)

Update.....

I ordered an unloader valve rebuild kit and a full set of seals for the pump. The unloader valve had quite a bit of calcium deposit which kept it from moving smoothly in the bore.

Cleaned it out, polished the bore with emory paper and installed the kit as well as installed all the seals in the pump.

Test fired it up tonight and it works just like it should.
Total cost was $16 for the valve kit and $65 for the seal kit.

Thanks Mownie for your accurate assessment of the problem.

My old unit lives to wash another day!!!!!!