I've been upgrading my 70 Coronet (originally 10" power drums) to a disc setup. I've got the 73 A body spindles, 10.87" discs with slider calipers, and the 1 1/32" aluminum master using the correct adapter. Braking is pretty good, but I can't really get any wheels to lock up. I don't have the prop valve installed yet. Brakes are properly bled. I'm thinking now its the single diaphragm drum booster causing the weak braking.

What are my options for a dual diaphragm booster? I'm looking for direct bolt-on as correct appearance is not important.

I think the '69 Charger disc/drum Bendix booster (Cardone or NAPA 54-73603)would work, but I am seeing if there are any other options. Maybe a direct bolt-on that accepts the 2 bolt master and doesn't need the 2 to 4 adapter? What are my other disc brake booster options?

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I used the Master Power brakes booster/master setup... it's way overpriced! I believe the exact same setup is on a 69 Newport...
You can check MPBrakes website and then go to Rock Auto for pics/matching...
The rock auto equivalent will be much cheaper.

The MPBrakes setup comes with a 1" master cylinder.though not a 1 1/32....

Also, did you happen to get the metering valve off
of your A-Body donor car?
if not, you need to.
The power 4 drum metering valve will not give you
the right pressure at the calipers....
More pedal effort slower braking times...

Best of luck....