ABS code one long 8 short , short on dump valve how do I find the short on 1991 Chevrolet G Series Van (G30)

I had to by pass valve cause no fluid went to rear brakes

there is no code 18 in all data, seek diag and est to determine issue
I counted the flashes when I did a self diagnostic, it was 1 long flash followed by 8 short flashes, on the internet it said it was a shorted dump valve