CAT pump not working well

My CAT pump model 310 seems to be acting up. When the customer is using the gun and has the trigger depressed, the water gauge reads 1000 psi.
When the customer releases the trigger, the water gauge jumps up to almost 1,500 psi. This is not normal since the other pumps dont do this. Is this damaging the pump? And why is it doing this?
Any help is greatly appreciated.

Bad regulator, or the regulator setting is turned up higher than the pump will allow with the trigger pulled.

As MEP indicated it could just need adjusting. You might want to install a new spray nozzle and try adjusting pressure downward to see if you can reduce the difference between the two.

Its a brand new regulator. In fact its the 2nd one because i thought the first new regulator was defective. You really think by changing the nozzle this might fix it. I will try anything though.

Any other ideas. Thanks again !

I would opt for the regulator as MEP has indicated. Move the regulator and nozzle to another pump to see what happens.

One thing to remember about the regulators used in a car wash is that they work by relieving excess flow back to the inlet of the pump. If you have a problem that won't let the pump build above 1000 PSI with the trigger pulled, adjusting the regulator higher won't change the pressure because there's no extra flow to bypass. When the trigger is released, the flow is less and the regulator is suddenly working to keep the pump at 1500 PSI. So if the nozzle is eroded and flowing too much with the trigger pulled, it will cause your problem.