what size master cylinder diameter are you using? you need a smaller bore to raise pressure in the lines.

what pedal ratio do you have ?

The master cylinder is a 15/16".
Regarding pedal ratio, my brake pedal actually has a higher ratio than stock. I drilled a hole above the original pushrod hole to increase the leverage. The A body booster and linkage are all on the engine side as opposed to the B body setup where the booster bolts to the firewall and the reduction linkage is under the dash.
The increase in ratio at the pedal is minimized by the reduction linkage but in short, the ratio is still greater than stock, even if only by a small amount.

i believe at some point there was actually a 7/8 bore diameter master cylinder available too. may want to look into swapping to that.

I don't want increased travel.
The A body brake booster is a single diaphragm unit. The B body version is a dual. It provides double the boost. I suspect that the difference in the two will make the biggest improvement.

I went an looked up the booster my old 96 Mustang had, V6 car 4 wheel discs.

8.732" dual diagphram.

When you say " The B body version" what does that mean, specifically. There are many years of "B body" out there.

Here's a link to an excel spreadsheet that will let you play with the booster numbers. It was in Feet's brake thread pinned at the top of this forum.

manual vs vacuum - uhcoog1.xlss the one you want

http://www.3gduster.com/brakes.html

Sorry for the confusion.
This A body booster is a single diaphragm, the 1966-70 B body booster is a same diameter but a dual diaphragm, effectively providing double the boost.

Man oh man I was so excited to see this thread. I started reading eagerly hoping that you found the fix for this…. Sadly we are in the same boat…. Well sort of. I have a 68 barracuda with 4 wheel disc brakes from wilwood. Their master cylinder, proportioning valve, and calipers etc…. Complete wilwood system meant for my car. Or so they claimed.
The car doesn’t stop well. I cannot lock the tires up either. I re-drilled my brake pedal to create a better pedal ratio and no change in braking performance. Wilwood was no help in trouble shooting. I went from a 15/16 manual master to a 7/8 manual master both from wilwood. I have 1100 psi at the calipers with 100lbs of pressure on the pedal.
The pedal feels great, not hard, not soggy or soft, but car just don’t stop like it should. I am waiting on a set of race pads from a friend to try out.
I have replaced every hard and soft line in the car, all couplings and tees have been replaced and no change, I even replaced my proportioning valve hoping maybe I had a defective one?!?!?

I feel you on this. My car has tripled in power but the brakes have not come along with the rest of the car. It makes me hesitant to take it on the road or even the track.

I think a booster would help but it won’t fit with the big block!!!!

I have been sidetracked with home repairs lately so I haven't been able to follow through with this.
My intention is to switch to a 66-70 B body dual diaphragm brake booster and see how it feels with that. I just don't have the time right now.

Mark Williams recommends a MINIMUM of 1200psi for a 4 wheel disc setup. 1000 is for disc/drum

https://www.markwilliams.com/brake-system-pressure.html

that's what i was thinking, 1200 or more.
then, what kind of bias do you have for front and rear tire sizes ? [or not]
when [if ?] i get my hotrod going, i have tiny diameter front tires and large by huge rear tires, so this is going to be a challenge for me to get it right, master/caliper/pedal ratio wise.

I registered 1200-1250 momentarily but the pressure dropped off with the pedal being held firmly. That shouldn't happen, in my opinion. The pressure should remain steady or you're not keeping consistent pressure to the pads and rotors. Either my booster is leaking or the pump just craps out.

Or no good master

To bad you gave up on the hydro boost. Hydro boost is a great solution. My brakes in a Challenger = Hydroboost, 11.75 dia stock slider calipers front, Ford calipers out back, 4 wheel disk GM prop valve (straight thru). Rear parking brake hooked up or not, either way. Easily skids the tires. And that's with a large bore master cylinder.

I should check the psi at the calipers with hydro boost now. Never did because it solved the problem. I'm sure its way more than 1200. When I checked my manual brakes setup with 7/8 bore it was 700 psi. When i switched back to stock power booster (single but large diameter), engine vacuum was 14 and the psi was 1200 at front caliper.

My opinion about stock booster is you need to get the vacuum up near 19-20 to get impressive brake performance.

The vacuum pump I have puts out 22 inches of vacuum.

Hydroboost = 1800psi., but that's pushing the pedal harder than normal.

I tried a HB system and it was an epic failure. I couldn't get the HB to build pressure and damaged 3 power steering pumps in the process.

What kind of pump are you using? Does it have any shims in it to reduce pressure? I'm asking because I'm thinking about going to a hydroboost setup on my car.

Its the stock power steering pump for a 73 challenger. I have some poorly done video's of the install on my youtube page.

https://youtu.be/xFVBDHaSEtQ

FWIW I believe max tire braking effect is when tire is turning at approx 70% of road speed. Other than maybe F1 skilled/talented drivers that is an unattainable goal for us mere mortals. Even Standard ABS does not achieve that on an instantaneous basis, but might get close on average, in that brake pressure is cycled/pulsed to a large degree. The key is not letting the tire stop or slow long enough to allow the rubber to turn effectively to liquid grease at the road contact point. I am sure there are many variables that effect the ideal limited slippage for any given situation to achieve maximum braking.