GUNN MAN,
A thermal releif valve functions like a thermostat. It will open if the water temp. in the pump gets above 140 F or temp. specified by valve. It should close once the temp. drops back down.

Some of the valves can be disassembled and cleaned like Mark said. The valve should not open up on start up unless it is defective or you have low flow to the pump.

Check to make sure you have good flow to the pump. No collapsed, pinched hoses, stuck float valve or inadequate water source.

Ater checking the above you can try this:
A quick test of the pressure releif valve would be to remove it and install plug. If the valve was defective you should be back to normal pressure at the gun!

Replace the valve asap if it can't be cleaned. If you purchase a new ($10.00-15.00) one try and get a Hypro valve or one that can be disassembled and cleaned. You can also test a valve after cleaning by placing it in hot water. Place on the stove in a pot
of water. Cut heat on and bring temp. up to valve spec. using a thermometer. The valve should open. Cool down it should close.

Beleive it or not the pump is the most durable part of any system. Low pressure is usually the fault of an external component.

Hope this gives you some insight!

B.E. Young Sr.