Pressure build up in a pipe system containing solvent

Ok, having a bit of trouble here. there seems to be a gremlin in a system at work that is causing a pressure build up but we cannot find the cause.

in essence, the system is run off a huge vertical storage vessel, the solvent is pumped out via a pump at approximately 40psig. This then feeds two dispensing guns, 4 large mixers, 5 washers and a dispensing room with three outlets. The pump is only activated when a dispensing point is operational, and stops as soon as operations cease. The issue at hand is that there is a large pressure build up, intermittently, of up 150psig. Enough so that it can blow through a dispensing guns trigger operation.

Im looking for any help or ideas at what could cause such a build up within this kind of system and to this magnitude. Any ideas are welcome and happy to explain further details if you need more information.

thanks