There is a check / pressure relief valve in the pump which is accessible after removing the pressure switch. You need a pair of long nose pliers with a very precise end in order to grip the valve which protrudes slightly above the body of the casting.
I found that by grinding the end of the pliers slightly on a grindstone to square the ends I was able to get a better grip. It is pressed in with an o ring providing the seal and the internal pressure of the fluid keeps it in place. If you rotate it slowly one way then the other and pull at the same time it comes out fairly easily.
There is a tiny ball which bears against a cup in the valve and I found a minute particle of dirt lodged in mine. I did open the valve but re-assembling it is critical in order to get it to work at the correct pressure so would definitely say dont do it or you may create all sorts of problems eg. too high a pressure which could destroy your system. I replaced mine with one from another pump.