you also will have 2 bleeders on top of the hydrovac unit. Here is a link to an online manual for a 55.2 truck. Brakes should be the same except the hydrovac unit is on the pass side on the 55, driver side on yours.
http://chevy.oldcarmanualproject.com/shop/1955truck2/55ctsm0526.htmI got my cylinder kits from Rock Auto if I remember correctly. There were 2 styles of cups in the Rear wheel cylinders so, need to pull it apart and see what you have. You should also replace all the hard and soft brake lines as they for from the inside out. They may look OK on the outside and still be paper thin! Napa and Raybestos part #s below that I found. Raybestos were bought from RockAuto Napa #s are UPxxx Raybestos are the others, sorry the columns didn't come across properly.
___________________Napa Raybestos
rear brake hose (58) UP4000 BH4000
front brake hose (58) BH36548
front brake hose (58) UP 25664 BH25664
____________________________________ Napa Raybestos
rear wheel cylinder kit 1 1/2 bore, ring type UP 147 WK147
master cylinder kit 1 1/4" UP 33 MK33
front wheel cylinder kit 7/8" UP 129 WK129