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Old 02-02-2010, 08:18 PM   #1
FoMoFo

 
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Vehicle: 1968 Mustang GT Fastback
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Default 1968 brake issue - slow pedal

I have a 1968 fastback that I converted the rear brakes to discs. I have a gm proportioning valve (long story, but its for front and rear discs and its connected correctly with the right fittings). I have a 68 master cylinder and booster.
The brake feels PERFECT when the car is off. When the car is running, the brake pedal is too soft and it takes too long to come back up.
I have bled the lines a thousand times, replaced the master, and the p.valve. And every brake line. No leaks, and the problem won't go away.

I'm convinced that I need a better master cylinder and booster. Can a modern mustang M/C and booster be installed? Is there something at the Ford dealership that I can get quick so I don't have to order online and wait?

Please help! Thanks!
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Old 02-03-2010, 05:55 PM   #2
MustangBradley
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I found this at Summit. Order it with FedEx shipping and you'll have it tomorrow. The dealer is closed now.

http://www.summitracing.com/parts/SSB-A28150/
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Old 02-04-2010, 11:21 AM   #3
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the master that I have on there is a 1" bore. I think I need one with a 1 1/8" bore. Like this one:
http://www.mpbrakes.com/products/pro...product_id=160

I have a turbocharged 331 Dart block with AFR heads and and Eagle Crank and this CAM:
http://www.compperformancegroupstore...Category_Code=
I was told that this cam may be affecting the vacuum of the booster. Can anyone vouch for that statement?????
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Old 02-04-2010, 09:06 PM   #4
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Your cam could very well have an effect on your vacuum. Often on cars with extreme cams, they will have a large vacuum tank to 'store' vacuum to use when needed.

You state that your pedal becomes too soft with the motor running. That leads me to believe you have plenty of vacuum since a 'low' vacuum situation would have you with less vacuum assist leading to a very firm, unresponsive pedal.

As you increase the bore, you increase the flow. This means that you will have less pressure on the fluid. If you decrease bore, you decrease total flow but increase the pressure on the fluid.

Do a test drive with the vacuum assist disconnected from your booster. See if you like the results.

Also, you replaced all of the parts that work the same whether or not the car is running. The booster is vacuum operated which is provided by the running car. I know this is obvious. You have one more part to replace and, frankly, it should have been the first.

I run my 65 unboosted and have no problems.
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Old 02-05-2010, 10:52 AM   #5
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Bradley, thanks for your advice. I'm going to try it without the booster today. I know the pedal will it raises the question of whether or not I will lose brake power as a result. I'll give it a try.

So with the bigger bore, my pedal may feel even softer?
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Old 02-05-2010, 12:50 PM   #6
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Yes. Think of the hydraulic cylinder as a lever and fulcrum. The farther you move the fulcrum from you, the more force you can exert on the load, but, you increase the travel of your lever. This is what is called mechanical advantage. There are no free lunches so you have to move the lever much farther than you actually move the load. So, back to the MC. With a small bore, you move less fluid with each millimeter of travel but due to mechanical advantage, you get more pressure on the output.

Look at modern brake calipers. The rage is 4 or six SMALL pistons while the old school calipers have one large piston. You can get the same pressure on the rotor from 6 small pistons with much less fluid flow than you can with one large piston.

So, all this boils down to this. Most cars use either a 7/8 inch bore or a one inch bore. You can get larger ones. After you do your test drive, you will be able to make a more informed decision as to whether you want a booster or not and if your current MC is the right size.
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Old 02-06-2010, 10:08 AM   #7
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Bradley, i took the test drive without the booster. Whatever brake power I had before was completely gone.
Now I'm stumped. Any other thoughts?
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Old 02-06-2010, 02:44 PM   #8
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I guess we are back to you replacing the booster. I found this one at summit:

http://www.summitracing.com/parts/TFF-2225NB/

It runs $185. I hesitate to advise the purchase but since you have done everything else, this is all that is left. It is an easy swap.
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Old 02-07-2010, 05:49 AM   #9
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Mustang Bradley is offering good advice. A high overlap cam with less vacuum signal at idle would cause an increase in pedal effort. I suspect the booster is the culprit as well. You may want to try to locate a smaller diameter booster. Power brake M/C has a larger bore than manual brake M/C as well to keep pedal effort normal. Drum brakes do give a slight assist in pedal return that disk brakes dont. Did you use the original booster and M/C? If not, did you adjust pedal and booster pushrod to spec?
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Old 02-07-2010, 11:33 AM   #10
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I did use the original booster and master. I didn't adjust the pushrod. Is there an aftermarket pushrod that is adjustable?

I bought this booster and MC
http://www.mpbrakes.com/products/pro...product_id=160
It's the 1 1/8" bore. It looks now like I won't be able to use it. Plus the ports on the M/C are on the wrong side. However, the booster is brand new. Could I attach that booster to my original M/C? Or would there be some compatibility issue?
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Old 02-07-2010, 11:33 AM
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