I took the check valve apart and cleaned it, seems like better vacuum since I did that, it may have sticking partly closed and not allowing full vacuum to the HydroVac. Your pic shows the check valve.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy