The size of the piston of the rear metric caliper is most likely around 2.5" in diameter. This is about the same area as the front LS1 calipers you have. I am only guessing you are running metric calipers on the rear.
A 7/8" bore master cylinder cannot supply the volume of fluid needed to fill the volume required by four calipers that require roughly the same volume. I am sure if you pump you pedal a few times you will start to build pressure in the system and be able to get a pedal and stop the car. This is not a realistic way to drive your car though.
A larger 15/16" bore master cylinder is a better option for your brake setup currently on your car or you need to reduce the piston size of the rear caliper to run the 7/8" bore master cylinder.
You MAY be able to use a Wilwood 2.0" diameter piston metric caliper that will bolt in and keep your 7/8" bore master cylinder, but even at 2.0" piston size, you still may be at the max limit of the master cylinder to supply the volume needed for the calipers.
You will have a long pedal stroke with the smaller bore master cylinder, but you will have higher line pressures and greater clamping forces at the calipers. Going to a larger bore master cylinder will increase your volume, but your line pressure decreases and you will have less clamping forces at each calipers. Hope that made sense.
Let me know if you have additional questions.
Dave
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