I want to say that your first mistake was probably assuming that everything was in good running order when you purchased it. Without bench testing, or at least a partial tear-down to inspect for leaks, wear or other problems, you are taking a big chance. There may have been a good reason why the PO was parting it out!
Without a physical inspection, I don't believe anyone is going to be able to tell you where your fluid is going. No puddles? What kind of vacuum are you pulling at the manifold? Are the rotors glazed? Are the pads oil soaked and need to be replaced? There are so many possible causes for the poor braking.
A little more information on the inspection and installation procedures you followed during your retrofit would be helpful along with some information on your plumbing.

First:  Welcome to the best place on the web.  I hope it's a place you want to visit often.

​Your brakes:  You need to change the pedal to have the proper PB ratio.  Using a MB pedal on PB will normally create hyper-sensitive brakes.  Since yours did not, I have to think that the booster isn't doing anyting...at least nothing good.  You can get the parts to convert your pedal to PB from the MS website for very few $$.  If you can well, or know a good welder, this is a simple conversion.  For my money, I'd also convert the pedal support to use the Fox 4-Cyl booster.  Again, the parts/instructions come from the MS website. 
​I have no knowledge about the 68 suspension parts but would not be surprised if the spindles are not recommended for use on a 66 due to "bump-steer" problems.  Other smarter folks will have to chime in.

I would also recommend that you, at the least, buy new (or rebuilt) calipers.  As rhutt says, messing with unknown brake parts is pretty risky.

​You can leave the rear drum brakes and do just fine.  Lots of cars are still being built as disc/drum and the system works well.  70% of the braking is done up front anyway.  Just make sure the rears, and the fronts, are in good order and working properly.

​Fluid loss:  It has to be going someplace...maybe into the booster.  Or, you may find a "starburst" of fluid on one or more tires which would indicate a leak at the caliper of wheel cyl.

​Bleeding:  New MC MUST be properly bench bled before installation.  Calipers MUST be installed with the bleeder at the highest point.  You'd be surprised how many calipers get installed on the wrong side and the bleeders end up below the top of the fluid.

​Good luck, you'll get it, just don't give up.  And come to the Bash, with or without your Mustang.  You'll be glad you did.

​BB
 

Did you install the booster on your car?  Pics would help. The stock 68 booster won't bolt up, so I am wondering what exactly you have.
Let's get more details and I will help you make them right.

I've been trying to post pictures for 10 minutes now, I keep getting messages saying:

• To help control spam, please complete the verification image to complete this posting.
• To help control spam, please complete a verification image to complete this posting. This step will only be required until you have made 10 postings.

So then I do the captcha verification image thing below it, it checks that I'm not a robot, and then I still get another fail message saying the image couldn't be verified. I swear this isn't my first forum rodeo

I've tried repeatedly to post pictures but it's not working, but here's a link to the photobucket album with my pics of my setup:

http://s962.photobucket.com/user/tmcginleyjr/slideshow/Mustang%20brakes

I took pictures of the front suspension, brakes, booster, MC, and how I have the lines routed.  Large bowl goes to thte brass T fitting and then to each front caliper.  Smaller bowl goes to the adjustable prop valve and then to the rear.  In the one picture I tried to take a picture of the spindle so that maybe we could identify which year this is from.  

Last edited by t.mcginley.jr (3/27/2017 10:22 AM)

Those are 65-67 Kelsey Hayes discs.  They are good brakes, but they can have issues.  The first issue is they have four pistons.  Over time, one piston will seize up, then another, and then another until they only have one piston moving.  They do not brake very well when that happens!  You will need to make sure all the pistons are not seized.

Second issue is that power booster.  They typically do not work very well, but may provide "some" pedal pushing assistance.  They do not change the pedal ratio like a good system would do.

Everything you could possibly want & need....

http://www.mustangsteve.com/BRAKES.html
 

t.mcginley.jr wrote:

Hi all, I've looked over all the info and for now I think I'm going to upgrade to the fox body 4-cyl booster and 2000 mustang v6 master cylinder and rebuild my current calipers.  I'm going to buy the PB-2 kit and weld the support/pin myself.

For the booster, does this look like the correct one?
http://i962.photobucket.com/albums/ae101/tmcginleyjr/Mustang%20brakes/5474000-LEF.jpg

 

 
That looks like it. I believe mine is from an 89 fox body 4 cylinder.

There are three different versions of the booster, all serviced as rebuilt under Cardone part number 54-74000, or brand new from MustangSteve.

t.mcginley.jr wrote:

For the MC, are there any Ford ones set up for disc/drum with 1" bore besides the 92 ranger one?  I'm trying to avoid the silly slanted reservoir without having to purchase another $20 reservoir off a mustang or something.  If not no big deal.

When I had rear drums I used a 67 Disc/Drum with left ports.  Worked great with a new Fox combination valve.  Got the MC from MS but they may be available elsewhere.  Went local for the Combi valve...pricey, about $100 new but worked well on our coupe and my added on brake trouble light worked as it should.

​BB