I am putting GSR rear discs on a civic, I have everything I need. The last question is-

Do I swap the GSR master cylinder only or the master cylinder and the brake booster from a GSR?

Or

Is the master cylinder from a GSR the same as a master cylinder from a civic DX?

 

Master cylinders are different, just use your stock one.

 

What model civic do you have and specifically what MC, there were two. The CX (and probably DX/VX/LX) has a 13/16" master cylinder (it will be stamped into the side). The EX/Si's have a 15/16". The Integra's with ABS have 1" and the non-ABS are also 15/16". If you have a 13/16" I would recommend you upgrade to an EX 15/16". You'll get a more solid pedal as the MC can push more fluid without having to be compressed as much. I had the 13/16 (with 150k miles) in my hatch and with the GS-R front brakes, my pedal could go to the floor. I installed the 1" ABS MC in my hatch but it involved cutting, reflaring and bending on of the hardlines and was honestly probably more work than it was worth. The 15/16" will be more than sufficent. With the same brake lines (both time I bleed the crap out of the brakes) the pedal was much stronger. With an older car I would definately recommend stainless lines.

If you do upgrade, you will need both the MC and booster as a unit. I know the Integra parts won't bolt to the Civic and Integra booster is actually physically bigger. It will probably be a lot easier to just replace the two as a unit anyway. Seeya.