Low brake pedal

ok there can be a number of reasons why you have a low brake pedal. one is using the wrong pedal ratio, air in the system, the wrong master cylinder, a bad master cylinder, a leak in the system. there can be an air bubbel in the system due to the way the brake lines are run.

the first thing i would do is pressure bleed the system. and work from there.

"He installed a power front disc brake kit " This is your first clue and where you should start. What pedal did he use? If you are not using the correct pedal ratio it will cause problems. Power brakes are not an upgrade to braking power...they are only a QOL upgrade to make it easier on your leg....but the first place to turn is the power brake kit, what components are in there? What is the bore of the MC? I am going to assume he knows how to bleed brakes and that he has tried vacuum bleeding them to remove any air.

Mustang Steve has a write up showing the differences between power and non-power pedals. You never mentioned which year your car was, so I can't narrow it down any further.

65-66 pedals are all the same, the leverage point does not change on the pedal itself for those years...however that does not tell the full story.

A factory single bowl "fruit jar" manual brake master cylinder has a 1" bore.
According to the Leeds website so does their kit: https://leedbrakes.com/i-20507149-hydraulic-kit-power-brakes-64-5-66-mustang-auto-trans.html

A factory dual bowl manual brakes master cylinder has a 7/8" bore.

Changing the bore size will change the pedal pressure AND the throw. What this means, is that if you have manual brakes and swap from a 7/8" master to a 1" master cylinder while NOT upgrading to power brakes, you will feel DECREASED pedal pressure and a longer throw, . If you add power brakes to the mix, the pedal feel will be even worse.

In your case, you are simply adding power brakes...assuming you started with a 1" single bowl master, your brakes will now feel softer but the throw will remain the same. If you started with a dual bowl master, you will feel decreased pressure and a longer throw.

There are other things to consider though...the master cylinder is only ONE end of the system...the other end is the brake caliper piston size...the larger the caliper piston, the more fluid is needed move it...a larger piston reduces pressure and the brakes will feel softer.

Do you see the problem here? Just because you throw some components together and offer it as a "kit" doesn't mean they are optimized. Do a google search, you will find page after page after page of vintage mustang enthusiasts unhappy with the result of power brake kits....this is because components such as master cylinders and wheel cylinders(or caliper pistons) are so often mismatched.

I hate to diagnose by the internet, you make no mention of what the pedal actually feels like....but if the feel is good and the travel is just too long, the solution would be to increase the size of the piston on either the master cylinder side or the wheel side....but if the brakes are feeling soft and its assumed they are correctly bled, you could reduce the master cylinder size to increase pressure.

In the end...I never recommend swapping to power brakes unless you do the homework and put together the parts yourself...a lighter pedal is just not worth the effort to me.

You would increase the bore by changing the master cylinder to a different larger one(theoretically you could bore out your existing one if you could find a larger piston for it, but since a new one is $100 that isn't very cost effective) If I were you, I would look at trying to find a master with a 1 1/16" bore that fits the standard Ford pattern and try it out. I know Wilwood, Jegs, and Summit sell universal 1 1/16" cylinders...but more research would be required to see if they would work for you.

In the end its all about line pressure though, so me diagnosing from the internet really isn't worth much of anything, I am making a lot of assumptions(like the brakes being 100% bled) that may be incorrect.

wicked93gs said:

You would increase the bore by changing the master cylinder to a different larger one(theoretically you could bore out your existing one if you could find a larger piston for it, but since a new one is $100 that isn't very cost effective) If I were you, I would look at trying to find a master with a 1 1/16" bore that fits the standard Ford pattern and try it out. I know Wilwood, Jegs, and Summit sell universal 1 1/16" cylinders...but more research would be required to see if they would work for you.

In the end its all about line pressure though, so me diagnosing from the internet really isn't worth much of anything, I am making a lot of assumptions(like the brakes being 100% bled) that may be incorrect.

Click to expand...

What line pressures should you be seeing with aftermarket disk brake upgrades (like Wilwood 4 piston 11" rotor set up with 1" master and booster). Vacuum is 15"