4. Press on the control and observe the pressure gauges (M2) and (M3). The pressure should rise first on pressure gauge (M2) and should be constantly greater than that on pressure gauge (M3) by approximately 5 bars (73 psi.) Stop pressing on the control, the pressure should return to zero. NOTE : After a certain period of use, the pres- sure gauges (M2) and (M3) may lose their sensitivity. It is advisable to take a second reading after interchanging the tubes on pressure gauges (M2) and (M3). Take the average of the two readings.