a pressure-valve president is a president that acts as a 'pressure release valve' for government lobby groups to deter pent up resentment of voting citizens towards the direction a country is or has been moving.
Trump supporters are now content enough to allow the powers that be to continue with corruption, just enough pressure has been released from the voting class that any current or future uprisings are quelled, all thanks to PV President Trump.
President Trump is a PV President, just look at all these MAGA hat-wearing supporters their rights are more infringed on now than under Clinton, Bush or Obama yet they refuse to see Trump for the PV President that he is.