3GSi209

If you have access to rockauto.com, they show to have boosters for both OE '86 Civic and '85 Prelude.

I was able to purchase the 85 Prelude MC from RockAuto, but since I couldn't find the 85 carb prelude booster on that site, I ended up ordering it from Autozone new.

I will try that 85 base prelude Booster setup on my daily red 86 Civic Si. I will try the AP1 S2K setup on the white one

I've just stuck with OE booster. I also have the Prelude MC on mine with 87 Integra PV, 94 Del Sol S front calipers and 87 Integra rear calipers.
 
The main reason for going to the Prelude MC is that it worked with the OE booster while having the larger piston size like the Integra MC.

Thanks for the advice. I will definitely make use of the 85 prelude Master Cylinder on my Red Civic Si Hatch that has a OE Prop Valve along with 1G Integra Rear Disc Setup. I've been having difficulty finding am OE Booster though.

On another note, I did get an AP1 S2k Booster and MC. It's also been suggested that it is a direct bolt on and that the Cap with the wire from the OE MC be used to get the Fluid level reading. This will be tested on my White Civic Si hatch with 1G Integra PV and rear Discs and Calipers, and Font JDM 1G Balade CR-X Si front Hubs, Discs and Calipers.

What would be a good Brake Booster upgrade? BTW, I also bought the 85 prelude master cylinder and yet to install it.

Thank GeezRX for the specific part. I'm going to search for that on RockAuto now!