JayEstes

Try unbolting the master cylinder & pull it forward a little & make the back seal in the master cylinder is not leaking fluid into the brake booster. I have had this happen to me & it drove me nuts until I found it.

Are your rear drums adjusted?  If there's a lot of space between the pads and drum, your brake pedal will feel much as you described.
The brake fluid must be going somewhere without affecting the braking ability.

Thanks ridge. I ended up installing without painting it. It was shiny aluminum after stripping the paint but I think the anodizing stayed on. 

Mine was cracked and it wasn't all obvious it was until I tried to pull it out of the booster, and it broke.  Make sure you get the right size hoses to go with it.  It should go from the booster into the main vac port on the back/top of the engine.  This hose is very important to overall vacuum, as it is a larger size, and leaks have a big effect.  It won't cost you but a little more to get the hose replaced.

The first thing I would check is the check valve where the rubber hose connects to booster. Also I had a vacuum booster once that leaked around the push rod shaft (the one between the booster and M/C} that caused the booster to function erratically sometime it sealed off and other times when you pushed the petal and released it wouldn't .

1 1/8in sway bar upgrade on the front is absolutely the best handling/safety upgrade I have made tot he car.  very little body roll for normal driving, much better handling

your sensor of the fan activity should be located at the exit of the radiator/entrance to the engine (lower hose).  basically, if the radiator (without fans) is able to adequately cool the fluid before it enters the engine, then there is no point in turning on the fans or have them turned up.    locating that sensor in the intake manifold will basically have you running the fans more necessary.  which of course won't hurt beyond consuming a little more energy/power at certain times.

65 66 is to narrow ,i have found the 1 1/8 front bar to be the best ,start going bigger and it can rip the sway  bar mount from the body 

A lot of people, present company included, based their less than stellar car's performance on worn out parts. Mach1 Driver is correct.  With proper functioning stock components, your brakes should do a good job. Anytime you change from stock, you're opening a can of worms that will bring in a few extra parts that will need modded. 
A guy in the Mustang world who really knows stock brakes, and steering, is Dan @ Chockostang.com.  He can be believed and trusted. 
 

This will be an unpopular opinion- they raced these cars with front discs and rear drums, and the stock brakes were good enough for that. Friction materials have improved in the 50+ years. For the street, barnett468 recommends EBC green pads and shoes, Zray uses EBC Redstuff 3000 Series Sport Brake Pads, and says that the popular Porterfield pads are inconsistent from batch to batch.
The advantage of going stock is that you don't have to guess how to set it up as its all in the Ford shop manual, the parts are readily available and the part numbers are known. Why reinvent the wheel when something works? Yes you can get aftermarket brakes with gigantic pistons and air cooled slotted rotors, but chances are you're going to be messing around with the system to get it right, and you won't be driving it. In my opinion, unless you're married to Shirley Muldowney, the stock brakes will be fine. Just my 2 cents.
 

Check the plug on the back of the ignition switch. It is common to have a burnt up connector on the plug on cars with a/c due to the higher current load. Back in the day when you bought a new Motorcraft / Ford ignition switch it came with a new pigtail to make a proper repair.

Remind us, is yours a 69 or 70? Fusible links started on the 70s, but the only one I'm aware of is at the solenoid, and would cut the main power. Once it is gone, power wouldn't be restored without replacing the link. This sounds like a loose connection somewhere.

Sounds like something going on at the fuse box, as radio, backup lights, and blower motor/AC are powered by ACC and fused by 2 fuses.  Might be a bad ignition switch or bad/nearly broken wire for ACC at fuse box.  Usually, it is the BAT line that breaks, not the ACC line.  Something is going on that when more power is pulled through those lines, continuity is lost due to higher resistance.  I'd check your fuse clips first.

Finally got almost 53 years of greasy grimy gopher guts scraped off, new aprons on, prepped, primed and painted the engine bay.  Now to get the Opentracker suspension and Chocko rebuilt steering back in. 

Did you increase your referral bonus? I received $15 from you last week.

Thanks, Jay.  You'll be getting your $20 in the mail soon, so watch for it! 
Just kidding folks, but thank you for the kind words.

Jay, fyi, before you print something, you need to think about the environment the parts is going to be used also. Like heat, UV, wear etc and choose the correct flament kind. Also on items with small details or critical dimensions, you need to create the 3D with your printers specs/capabilities etc, print a sample, measure and modify the model most of the time. So if I gave you the models I created/printed parts with, the part you printed from that on your printer may not work. For example the ears/tabs of the window roller, I had to modify a few times to get the ears/tabs to have the spring action when printed. I also made the rubber piece that is part of the horn switch of standard 70 steering wheel from flexible flament and had to modify the model 4-5 times to have my printer print a functioning piece.

One disadvantage of doing tank parts in plastic is danger of breakage in case of collision.  I think ductile steel with rubber connections is going to behave much better in that case.

No, it is for a side/quarter panel mounting but it wouldn't take much to modify to work on the taillight panel.

The seat is based on an existing design I found some basic dimensions for then created a model.
The badge I created in SWX from scratch. The Ford emblem is created using an available text font I had previously downloaded then doing an extruded cut like the 351 & cleveland. I have not had any made as of yet.
This is the type of things I design at work. I created this image for the manual for this container. This is a model.
 

A shipping container for a gas turbine engine for electric power generation. That one is actually just for the core engine. size is approx 10' H x 9' W x 16' L give or take.
Back to the OT, more parts that could be 3D printed. Fuel cap & flange.
 

PICS....

They come back perfect, and with a documented wire-list specifically for your car.  Its a Waaaay better deal than replacing all the wiring IMHO.

Anyone interested in starting a 3D printing thread?  I've got a decent printer at home, and since I got it I have taught myself some CAD.  I think it would be nice to have a library of parts for the mustang, and those of us that can make them, can print stuff in our spare time, and provide to other users.  I recently wanted a box to contain a fuel-gauge calibration device, and I created this in about an hours worth of CAD and then some longer hours printing:

It's not perfect, but it allowed me to use an existing bolt to hold down the device, and store the wires/connections in the trunk.  Here it is installed:

It seems to me there is a need for 3D printing stuff for ourselves, and as we design the parts we can perhaps keep a library of them somewhere, and if people need one or several, those of us with time and printers can help out.
What do you all think?
Jay

MikeStang, it's hard to tell, but from the top view photo it looks like something is bent on the seat back.  He mentioned he is going to try to adjust the brackets.
 
Yeah, he did a nice job fitting the upholstery.