Hey Bud, the factory service manual for '96 trucks calls for a minimum of 25 psi, with no more than a 5psi drop across the filter/separator. The 25 psi reading was to be taken at the inlet to the filter. I assume the info. would be the same for your '98 12 valve. If you believe you have a problem, check your lines from the tank and your overflow valve. Someone else may have some more suggestions as well. (I assume you meant 24 - 26 psi at 2,000 rpm's. )