what size master cylinder bore

[vardenafil]

i just did a rear disc conversion on my 1972 k20 truck. the truck now has both front and rear disc brakes. the truck has power brakes. i think my master cylinder is shot. it just bubbles air into the reservoir when i apply the brakes. I see there is a 1 1/4 or 1 1/8 master bore available. I went down a rabbit hole trying to figure out which one to order. im more confused now than when i started. do i need to upgrade to the 1 1/4 bore now that i have dual disc brakes or should i stick with the 1 1/8 bore?