I don’t think that will work.  Do you have an adjustable proportioning block?  If so, add more pressure to rears. Turn knob clockwise. Test to be sure rears don’t lock up before fronts. This will firm up the pedal

Rather than inducing an artificially high brake pedal, I would prefer to find out why the brakes are not functioning as they should.

MS wrote:

Ok, I apologize for a short answer when the question deserves the long version.
Here is the longer version. The MustangSteve 65-66 power brake conversion is designed so the static position of the brake pedal is 1” lower than stock. The reason is this: the new pedal has a different ratio. Originally it was 6:1 and now it is approximately 4:1. That makes the travel required by the pedal to be only 2/3 of what it originally was to get the same master cylinder travel. So, I designed it with the same LOWER travel position as a stock pedal but the 1” lower static position.

The fact that your pedal has some free travel where nothing occurs is concerning, and there are a couple of possibilities that can cause that.
First is, of course, air in the system. Be darned sure there is NO AIR still in the system. I’m sure you have bled them, but be sure there are no high spots in the system, no fittings tuned sideways where an internal pocket could trap air and the master cylinder has been properly bench bled.
Second, but probably should have been first, is you MUST have a properly installed and adjusted parking brake cable connected for those rear brakes to work. You must use the parking brake repeatedly before you bleed the rear brakes or there will be excessive pedal travel. Pull it multiple times until it cannot be pulled any tighter. You can do this under the car by pitting vise-grips on the parking brake lever to make a hand-operated lever. Move the lever as if applying the parking brakes until it won’t tighten the pads any more. Then release the lever. That takes all the slack out of that caliper/pad assembly. THEN release the parking brake and you can bleed all the air out of it. Not tightening the parking system leaves the piston too far away for a 1” bore master cylinder to fully force it full of fluid. Ok, that should take care of the pedal slop.
My 66 has a similar setup as yours, but I have Cobra fronts. I have s total of 2” of pedal travel until a hard pedal is achieved.

Now, for the clutch and brake pedal being equal height. When a MustangSteve power brake system assembly with a cable clutch system is sold new, it comes with the static height of the clutch pedal set at exactly the same height as the power brake pedal. This is achieved by copying Ford’s original design for an adjustable clutch pedal stop that was installed on all 1964-1/2 Mustangs and some early 65’s. Why they quit installing it could only have been to save money. All it is, is a slot where the clutch pedal stop bolts onto the pedal support instead of just a bolt hole. If you have the MustangSteve pedal support, that slot is there. You can adjust your clutch pedal static height to any position you want. If you did the MustangSteve conversion on an original pedal support, and you don’t have the factory slot, then you can remove the pedal support and slot the hole downward about 3/4” x 5/16” wide snd then you can adjust your pedal.
Ok, that begs the question about not having as much pedal travel. To me, having to move a mechanical part a set distance over and over and over is something worth addressing. Do you have mechanical linkage z-bar original setup or the more modern MustangSteve clutch system?
For cable systems, simply adjust the cable looser and enjoy having less overall pedal travel. You won’t have to lift your leg as high to apply the clutch. And the same goes for the brake. It sits lower to make your leg not have to be lifted so high every time you use it. Later factory power brake Mustangs all enjoyed this feature. The pedal sat about 1” lower than the clutch pedal, from the factory.
For the antiquated z-bar system that typically required 3/4” of free travel before anything happens, you can enjoy the fact that modern diaphragm type clutches do not require that free travel as long as you use a modern throwout bearing that is capable of turning constantly with the pressure plate. They do require modification of the release lever so they can clip in place. Or, you can just reduce that free travel until you have just a tiny bit instead of alot, and adjust your clutch pedal where it sits a bit lower while still actually engaging the clutch at the same part of its stroke where it used to engage. It just has less air gap than before. Or you can adjust the lower rod to help reposition the actuation point.
Hopefully this helps explain why the cable system is a much better system.

I hope this helps you get your brake working properly, and your clutch pedal even to the brake pedal. Adding a longer rod is not an option since the rod is part of the booster. It was always my rule to NOT MODIFY a part that you might have to one day replace due to wear. You want replacement Ford parts that bolt right up. You do not want to have to custom-modify a replacement part in the future.

Let me know if you need more help with this. I am sure there is another typing project involved somewhere!

Wow! Now we’re getting somewhere. I understand everything you said except the part about the clutch pedal stop slot thingy. Any chance you have an illustration that would point that out? I like the idea of a lower clutch pedal because at 6’3” and my advancing years the clutch is a little too high. I do have a very effective hump mounted e brake lever and I use it a lot. That said I’ll readdress the rear caliper adjustment as well as the prop valve. I’ve always thought extending the rod would work but I’ve been reluctant to do that because, as you noted, modifying a part can become a real PITA. Thanks for the help.

MS wrote:

Do you have the MustangSteve pedal assembly? If so, the slot is there. A stock 66 just has a bolt hole, but it can be elongated to provide adjustment. Works best with cable clutch actuation.

I have the MS cable and bearings but stock 66 pedal assembly. Pretty sure I know what to do. I’m sure it will make sense once I can see it out of the car. Thanks.
 

To be able to have the clutch pedal static position be lower to match the brake pedal, remove the bolt-on stop from the pedal support and slot the mounting hole about 3/4” downward.  Then you can bolt it back on and adjust the stop so the pedal sits lower.  Be sure the rubber stop is parallel to the pedal piece so they mate together flat.  “A” and “B” need to be flat against each other once adjusted.  You can hold the pedal stop with a crescent wrench to keep it from rotating as you tighten the nut/bolt.

Yep
That works and is certainly alot easier, just not adjustable.  But, the thickness needed could easily be calculated, and it works perfect on your car.

Good thinking!