Well, 300mA is not sufficient to run more than 1 valve. Each valve draws about 180mA to 200mA current (and also the inrush current can go as high as 500mA to 600mA). Plus if you are powering RPi from OSPi (using the 3-pin cable), that will require another 100 to 150mA current. So with 300mA transformer, it’s barely enough to drive one valve plus RPi. As the OSPi user manual describes, the transformer should be rated at least 500mA.