brake light flashes on 1991 Chevrolet Suburban 2500

brake light flashes 3 short and 1 long what does this mean

Asked by for the 1991 Chevrolet Suburban 2500
Sounds like your ABS system has a fault code stored in memory. A code 1 is a problem with the ABS ECU thus the 1flash.
A code 3 is an open dump valve code, thus the 3 flashes.

Best to see a shop that knows how to deal with an ABS problem:
its a non abs truck