My check valve is slightly different than Puffie40 describes but the same theory, and of course my truck is a Low Cab Forward and not as heavy rated as the 9600. Hopefully with oldieoldie's truck, the original piping is still in place, the check valve will be pretty apparent. I'll try to post a picture.

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