If I change my brake from single to  a dual diaphragm with brake booster why are they showing 7" or 8". Both items show tat they are compatible. Which one do I use.

Your question is not clear.  Do you mean that you want to replace a single reservoir master cylinder with a dual reservoir master cylinder?  Your second question is about the booster,  Are you asking if you should use a 7" or 8" vacuum brake booster?

You are saying you want to go to a dual circuit (reservoir) MC and are confused by either 7" or 8" DUALING DIAPHRAGM BOOSTER?

The DUALING DIAPHRAMS are used for tight spaces (as you will have in the 1956 with FYB exhaust manifolds) as opposed to one large diaphragm.

https://www.speedwaymotors.com/the-toolbox/best-brake-boosters-for-better-braking/31313

https://techtalk.mpbrakes.com/single-and-dual-diaphragm-boosters

Please be very careful of the kit you choose as there is a lot of junk kits (CHI-COM and GM) and vendors out there.

DISC/DRUM or DRUM/DRUM?

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