A method of checking a deaerating valve for an injector valve, wherein the injector valve with pistons (6,7) and a top cover is placed in a holder and the deaerating valve is mounted in the top cover, said deaerating valve having an oil inlet port communicating with a channel within a piston, an oil outlet port with an outlet connection, a spring controlling the piston for opening the outlet connection thereby providing a fluid communication from the oil inlet port through the channel within the piston and to the oil outlet port, and said top cover having a control oil supply port communicating with an oil supply channel and an oil outlet port communicating with a spray chamber, which oil supply channel further communicates with the oil inlet port of the deareating valve, and wherein control oil is supplied to the deaerating valve, said control oil being accumulated in a hydraulic accumulator, from where the control oil is supplied to the control oil supply port, through the channel to the oil inlet port of the deaerating valve and further into the channel within the piston, characterized in that the pressure of the supplied control oil is increased so as to close the deaerating valve by pushing the piston to overcome the force of the spring thereby closing the outlet connection within the deaerating valve, after which the control oil pressure is slowly decreased until the deaerating valve opens, whereby control oil flow via the control oil supply port, through the channel to the oil inlet port, through the channel of the piston, through the oil outlet port, and through the oil outlet port in the top cover into the spray chamber, and the opening pressure is shown on a display. wherein the injector valve with pistons (6,7) and a top cover is placed in a holder and the deaerating valve is mounted in the top cover, said deaerating valve having an oil inlet port communicating with a channel within a piston, an oil outlet port with an outlet connection, a spring controlling the piston for opening the outlet connection thereby providing a fluid communication from the oil inlet port through the channel within the piston and to the oil outlet port, and said top cover having a control oil supply port communicating with an oil supply channel and an oil outlet port communicating with a spray chamber, which oil supply channel further communicates with the oil inlet port of the deareating valve, and wherein control oil is supplied to the deaerating valve, said control oil being accumulated in a hydraulic accumulator, from where the control oil is supplied to the control oil supply port, through the channel to the oil inlet port of the deaerating valve and further into the channel within the piston, characterized in that the pressure of the supplied control oil is increased so as to close the deaerating valve by pushing the piston to overcome the force of the spring thereby closing the outlet connection within the deaerating valve, after which the control oil pressure is slowly decreased until the deaerating valve opens, whereby control oil flow via the control oil supply port, through the channel to the oil inlet port, through the channel of the piston, through the oil outlet port, and through the oil outlet port in the top cover into the spray chamber, and the opening pressure is shown on a display.