jaycoop - According to a 1984 NAPA Brake Parts Catalog I have, it lists the rear 1-7/8" bore wheel cylinders (upper and lower) as NAPA# 19111 for your 1955 GMC Model 630. It also lists the 1-7/8" bore repair kits as NAPA# 176. Granted NAPA is very pricey, but it proved a good starting point for tracking down more info.
NAPA On line shows they are currently listed as:
UP 19111 full wheel cylinder (pic 1 below)
UP 176 wheel cylinder repair kit (pic 2 below)
Here's another link to a different parts site listing the
Raybestos Part number WC19111 (and also listing lower down a cross reference of numerous other sellers part numbers) for replacement cylinders.
I could not find a Raybestos number for the wheel cylinder repair kit, but did find that even Rock Auto carries the
WC19111 wheel cylinders at a much more reasonable price than anybody else.
I think these newer currently available parts should be trustworthy, whether the repair kit or new cylinders, and I think considering your EIS cylinders shelf life time is about 60 years they may be more up to the critical task at hand.
Hope this is helpful.
