What year? 1 ton or 3/4?
If it's a 1 ton, is it a dual?
Are you telling the parts place that it has 13" X 3" drums?
You show a 1977 1/2 ton - is this for a different truck?
I haven't had any trouble sourcing wheel cylinders for a 1986 K30 with single axle and 13 X 3 drums.
Try this at NAPA - Part Number: UP 37290:
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Fits these 1977 applications:
Chevrolet Truck C30 1 Ton - Pickup 1974 - 1986
Chevrolet Truck C30 1 Ton - Pickup 1974 - 1986
Chevrolet Truck K20 3/4 Ton 4WD - Pickup 1974 - 1986
Chevrolet Truck K20 3/4 Ton 4WD - Pickup 1974 - 1986
Chevrolet Truck Suburban K20 3/4 Ton 4WD 1974 - 1986
Chevrolet Truck Suburban K20 3/4 Ton 4WD 1974 - 1986
GMC Truck K2500 3/4 Ton 4WD - Pickup 1974 - 1986
GMC Truck K2500 3/4 Ton 4WD - Pickup 1974 - 1986
GMC Truck Suburban K2500 3/4 Ton 4WD 1974 - 1986
GMC Truck Suburban K2500 3/4 Ton 4WD 1974 - 1986
Chevrolet Truck P20 3/4 Ton - Step Van 1974 - 1989
Chevrolet Truck P20 3/4 Ton - Step Van 1974 - 1989
GMC Truck P2500 3/4 Ton - Step Van 1974 - 1989
GMC Truck P2500 3/4 Ton - Step Van 1974 - 1989
Chevrolet Truck C20 3/4 Ton - Pickup 1976 - 1986
Chevrolet Truck C20 3/4 Ton - Pickup 1976 - 1986
Chevrolet Truck Suburban C20 3/4 Ton 1976 - 1986
Chevrolet Truck Suburban C20 3/4 Ton 1976 - 1986
GMC Truck C2500 3/4 Ton - Pickup 1976 - 1986
GMC Truck C2500 3/4 Ton - Pickup 1976 - 1986
GMC Truck C3500 1 Ton - Pickup 1976 - 1986
GMC Truck C3500 1 Ton - Pickup 1976 - 1986
GMC Truck Suburban C2500 3/4 Ton 1976 - 1986
GMC Truck Suburban C2500 3/4 Ton 1976 - 1986
Chevrolet Truck G30/3500 1 Ton - Van 1976 - 1996
Chevrolet Truck G30/3500 1 Ton - Van 1976 - 1996
GMC Truck G3500 1 Ton - Van 1976 - 1996
GMC Truck G3500 1 Ton - Van 1976 - 1996
Chevrolet Truck P30 1 Ton - Step Van 1976 - 1999
Chevrolet Truck P30 1 Ton - Step Van 1976 - 1999
GMC Truck P3500 1 Ton - Step Van 1976 - 1999
GMC Truck P3500 1 Ton - Step Van 1976 - 1999
Chevrolet Truck K30 1 Ton 4WD - Pickup 1977 - 1986
Chevrolet Truck K30 1 Ton 4WD - Pickup 1977 - 1986
GMC Truck K3500 1 Ton 4WD - Pickup 1977 - 1986
GMC Truck K3500 1 Ton 4WD - Pickup 1977 - 1986