One of the main problems that arise when converting to manual brakes using a 7/8" bore master cylinder is LOW drag calipers.
LOW drag calipers require a step bore (quick take up) master cylinder. In my opinion they are too large to operate smaller 2.50" bore calipers with sufficient pressure, they are harder to bleed, and they have a 100lb residual valve that can fail. My opinion also is to change out to new (NOT rebuilt) calipers to make sure you do not get a set of LOW drag calipers.
To learn a little bit about new stock size calipers, I just purchased a set of left and right AFCO 2.5" bore metric calipers that are a bolt in, stock, replacement caliper for g-bodies, S10s, and most 3rd gen f-bodies. I removed the stainless steel piston and square piston seal to make sure it was not a LOW drag caliper.
MFG. Part #: 7241-9003 RH and 7241-9004 LH
Centerline of Holes: 5.50
Caliper Pistons: Single Piston Diameter: 2.50
Inlet fitting: 10mm-1.5 Material Type:
Steel Finish: Natural
Sold in Quantity: Each
The 2 1/2" bore steel GM metric caliper is designed to be a used as a stock replacement caliper. The caliper features a stock appearing remanufactured castings, remanufactured grounded 2 1/2" stainless steel piston, and low drag seals
(see below) . Each caliper is assembled and pressure tested.LOW DRAG SEALS
Though the description says "low drag seals", the seals are square with no noticable taper.
The seal-groove in the bore of the caliper are also square with no noticable taper.
When the seal is installed, it barely clears the top of the bore, and because of this, the piston to bore clearance, it seams, to have fairly tight tolerances.
The small end of the piston is what contacts the back of the brake pad. It measures 2.38".
The large end of the piston is what is inside the bore of the caliper. It measures 2.50"
Inside of piston cup, facing the brake pad.
Backside of piston that is installed inside the caliper bore.
I have bought these same exact part numbers a few years back and these new ones are a different casting with, what looks to be, a stainless steel piston. These calipers DO NOT come with pads, but they come with slider pins and slider pin bushings. At this time, they are around $40, and seem to be an improvement over the previous design.
Bottom line is that these should be a good stock replacement, NON low drag, brake caliper that will work with both strait bore master cylinders and step bore (quick take up) master cylinders.