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1966 GT Fastback, 289, TKO 5-spd, EFI, 4-discs, TCP coilovers
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I'm very impressed that you researched other threads for things I wrote - thank you. And maybe I should have listened to myself. The fact that I had very touchy brakes before was likely a symptom of power brakes combined with the standard pedal pin position (that was the whole reason why pins were relocated in the first place). I also said that the only things that are physically different between then and now are the new master cylinder and booster, so I do kinda know where to look.

I have had a lot of encouragement to measure pressures at the calipers, so I will go and do that.
 

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too much travel means you need a larger bore master cylinder to push more fluid
if you have a correctly designed master you should not need a proportioning valve
 

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65 Convertible disc/drum setup ,just had new dual reservoir m/c and booster installed , new rotors and calipers and lines ,new wheel cylinders and shoes on the back.
Struggling with long peddle travel and soft stopping. Have bled several times and adjusted the rear brakes as much as my mechanic is comfortable with. It's better now but I'm still not thrilled with the stopping ability. I don't think its air in the lines because I can't pump the peddle up. I been following a couple of recent threads with great interest.
Should I be able to "lock" these brakes up ? right now that's not possible.
Wondering if I just need to get my head around the fact that it's an old car or is there a problem?
65 Convertible disc/drum setup ,just had new dual reservoir m/c and booster installed , new rotors and calipers and lines ,new wheel cylinders and shoes on the back.
Struggling with long peddle travel and soft stopping. Have bled several times and adjusted the rear brakes as much as my mechanic is comfortable with. It's better now but I'm still not thrilled with the stopping ability. I don't think its air in the lines because I can't pump the peddle up. I been following a couple of recent threads with great interest.
Should I be able to "lock" these brakes up ? right now that's not possible.
Wondering if I just need to get my head around the fact that it's an old car or is there a problem?
There's a problem. U didn't mention a proportioning valve, which is a must when you have a disc/drum brake system mix. And yes, unless U also installed an anti-lock brake system, you should be able to lock 'em up.
When I took automotive technology in the 90s, my teacher told me that with a car as light as an early Mustang, disc brakes are NOT required unless you plan on pylon racing or something of that nature where a lotta brake repetition is gonna take place.
My 1964.5 restoration project utilizes a completely refurbed stock brake system, drum brakes and I have no issues stopping that vehicle under every-day conditions.
When ya get to fancy & complicated, ya run into unforeseen issues!
An electronic Mallory Unilite ignition system is as far a reach as I've gone in upgrades & it's been flawless for 30+ years. Carburetor, no fancy FI system for me either! Just a modified 289, done my way, about 325 horse power, put in a 3.50 to 1,
9" equa-lock rear end and a stock 4-speed top loader. Even the fuel pump is mechanical, a hp Holly unit. No electric pump for me, don't need it! 😎
 

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Discussion Starter #105
@KD Blue , I do have an ssbc proportioning valve and distribution block. You can see them in the photo in post #47
 

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1966 GT Fastback, 289, TKO 5-spd, EFI, 4-discs, TCP coilovers
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I'm very impressed that you researched other threads for things I wrote - thank you. And maybe I should have listened to myself. The fact that I had very touchy brakes before was likely a symptom of power brakes combined with the standard pedal pin position (that was the whole reason why pins were relocated in the first place). I also said that the only things that are physically different between then and now are the new master cylinder and booster, so I do kinda know where to look.

I have had a lot of encouragement to measure pressures at the calipers, so I will go and do that.
So the LEED Brake Pressure Gauge kit is on its way. But here is an excerpt directly from the installation instructions (copyright credit to LEED Brakes) that has some interesting diagnostic information:
  1. A power brake booster requires a minimum of 16 in Hg of engine vacuum at idle in gear to function correctly. Low engine vacuum will cause a hard pedal and low brake pressure. A problem with engine vacuum or a faulty power booster will never cause a spongy brake pedal. I have "spongy", so this case seems less likely (but I'm not ruling anything out yet).
  2. In general, a small power brake booster will provide less assist than a larger one. Running a small brake booster with a stock bore size master cylinder can cause a hard brake pedal and low pressure. I have a 1" bore master cylinder and a 7" (small-ish) booster (and this is the combination sold by SSBC USA for front disc brakes), so this case looks less likely as well.
  3. A master cylinder with too large of a bore size will cause a hard pedal and low pressure (not my case). A master cylinder with too small of a bore size will make good pressure but have a spongy brake pedal (possible - we'll know soon). The proper bore size will depend on whether your brakes are power or manual and what type of calipers or wheel cylinders you are using.
  4. Brake pedal ratio is the mechanical advantage created by the length of your brake pedal and the position of the pivot point and pushrod. In general, power brakes use a pedal ratio around 3:1 and manual brakes use a ratio around 6:1. Too much pedal ratio can cause spongy overly sensitive brake pedal and too little pedal ratio can cause a hard pedal and low pressure. I may have too much pedal ratio since I am using an unmodified brake pedal pin position with power assist. This would correlate with the "spongy" symptom, but not the "overly sensitive" symptom (but it used to be overly sensitive).
I have a little spongy pedal, so once I get some pressure data, we'll see which of these cases applies and what to do about it.
 

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Discussion Starter #107
@drsbmiller , I'm watching with great interest as I have the same problem as you and a similar set up with a 7 inch booster and 1 inch bore m/c
Please keep us updated!
 

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1966 GT Fastback, 289, TKO 5-spd, EFI, 4-discs, TCP coilovers
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The brakes on this car used to be so powerful, the car would stand on its nose with light pedal pressure, and with basically the same setup. There is something very specific that is wrong, and it almost has to be in the master cylinder / booster system because that is the only thing that is different (other than operator installation error). I will figure it out and report.
 

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I may have too much pedal ratio since I am using an unmodified brake pedal pin position with power assist.
It depends on if your kit got the additional linkage behind the booster. That's the orignal Ford way of getting less pedal ratio with power brakes on pre '67 cars.

Back when I still tried to get a pedal feel I liked with manuel brakes, I put the pressure gauge in the drivers side caliper and placed a big mirror so I could see the gauge from the driver seat. It will remove a lot of guessing on what actually happens when the pedal are pushed. With a little thinking you should able to see if pedal ratio, booster or MC bore size is you problem. It the booster are suspicious, then a manuel vacuum pump may help. With that you should also be able to see how the booster reacts on different levels of vacuum. Measuring instead of guessing. 😎
 

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1966 GT Fastback, 289, TKO 5-spd, EFI, 4-discs, TCP coilovers
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I'm a scientist - I like measuring things. Thank you very much for your insights.
 

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Discussion Starter #112
so I just found out that my car makes 13.5 inches vacuum.
Would this be the cause of low braking pressure ?
 

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so I just found out that my car makes 13.5 inches vacuum.
Would this be the cause of low braking pressure ?
Low line pressure? No, just make the pedal harder to push.
 
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