Brake free play issue after installing CSRP front disc power brake conversion kit on 1965 Mustang

mustang6518

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Jan 31, 2021
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Upgraded a 65 mustang from drum brakes to front disc power brakes using a CSRP conversion kit. Kit went on fine, but when the car is turned on there's a good 3" of completely free play in the brake pedal before it gets firm and starts actually braking. If I adjust the pushrod length pretty far out then there's no play, but that causes the brake pedal to be up ridiculously high - much, much higher than the gas pedal and practically in your lap. Even with the current situation, the brake pedal is still a little too high at 3.5" higher than the gas pedal.

When the car is off, there's about 1" of free play in the brake pedal, and then once I turn the car on and the booster is being used, the free play is around 3-3.5".

I've tried the below troubleshooting steps already, which have made zero difference:
  • Master cylinder was bench bled
  • Brakes have been bled in order several times
  • Brake pedal does not "pump up", further indicating that there's no air in the lines
  • Rear drum brakes have been adjusted to slightly drag
  • Replaced all brake lines with stainless steel and re-bled everything again
  • I was thinking maybe I need a 10-PSI residual pressure valve for rear drum brakes, but the site where I bought the kit claims the master cylinder already has an "integral residual pressure valve" of 10-PSI
  • I've checked the booster to master cylinder pushrod length with one of these adjustment tools and the rod is dead-on (zero/almost zero gap)
    • 1612127273741.png
  • I thought maybe I need a new brake pedal, but I've read that the 65 and 66 Mustangs used the exact same pedal for manual and power brakes, and only in 67 and newer did they use different pedals... This is the only thing I can think of left - something different needs to be done with the pedal, even though supposedly the 65's used the same pedal across manual and power brakes.... I think Mustang Steve's conversion kit comes with a pin that you're meant to weld in around 1" lower on the brake pedal, and a different generic kit I saw includes what looks like a custom pin that you just tighten down after drilling a new hole, instead of welding in:
    1612139599632.png
  • 1612140211363.png
Very, very, frustrating experience. I've read multiple posts on various sites of people having the same/similar issue, but haven't seen any solution yet. Any ideas on what to do from here?
 
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wicked93gs

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Sep 30, 2006
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What is the master cylinder bore size? A larger bore will result in shorter travel and greater pedal effort(not a big thing with power brakes) while a smaller bore will result in less effort and more travel. The factory "fruit jar" is a 1" bore. I have said it before and will say it again...all swapping to power brakes does is cause it not to work like it should...there are a ton of kits out there and very few are actually designed to work well in a vintage mustang.

From my own personal experience with moving the pedal rod(in my case on a hydraulic clutch conversion), moving the pedal mounting point up the pedal provided shorter travel and marginally increased effort...down was reduced effort and longer throw...however, there is a strict limit to how far up or down the pedal the pushrod can be moved, otherwise it WILL cause the master cylinder to suck air into the system.

The fact of the matter is that if the system is not working correctly, its a badly designed kit...maybe they changed out the calipers and the 1" bore master is all of a sudden not moving enough fluid to actuate them properly, causing a longer throw, etc etc etc. Power brakes do NOT stop better, they are only an advantage if actuating manual brakes is a problem for some reason.
 
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mustang6518

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Jan 31, 2021
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What is the master cylinder bore size? A larger bore will result in shorter travel and greater pedal effort(not a big thing with power brakes) while a smaller bore will result in less effort and more travel. The factory "fruit jar" is a 1" bore. I have said it before and will say it again...all swapping to power brakes does is cause it not to work like it should...there are a ton of kits out there and very few are actually designed to work well in a vintage mustang.

From my own personal experience with moving the pedal rod(in my case on a hydraulic clutch conversion), moving the pedal mounting point up the pedal provided shorter travel and marginally increased effort...down was reduced effort and longer throw...however, there is a strict limit to how far up or down the pedal the pushrod can be moved, otherwise it WILL cause the master cylinder to suck air into the system.

The fact of the matter is that if the system is not working correctly, its a badly designed kit...maybe they changed out the calipers and the 1" bore master is all of a sudden not moving enough fluid to actuate them properly, causing a longer throw, etc etc etc. Power brakes do NOT stop better, they are only an advantage if actuating manual brakes is a problem for some reason.
From the website for the kit I used, it looks like a MC with 1" bore:

Master Cylinder: Iron, 15/16" bore manual (with adjustable push rod) or 1" power, dual reservoir, front disk rear drum, optional 4 wheel disc type available, 3/8"-24 female flare forward outlet, 7/16"-24 female flare aft outlet
 

wicked93gs

10 Year Member
Sep 30, 2006
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If I were to upgrade to power brakes(I wouldn't) I would go with a larger master cylinder bore size than stock...doing so will shorten pedal travel. I think that is the root of the problem with many of these conversions...they add more or larger slave cylinders inside the calipers, then don't account for the needed increase in fluid flow by going to a larger master cylinder. In the end, where you really need to start is measuring your brake line pressure...once you know your pressure you can determine whether the master cylinder is up to the job or not....even if it is marginally ok for the application, that does NOT mean the throw is acceptable.
 
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mustang6518

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Unfortunately still fighting with this. Tried a couple more things:
  • The kit maker sent me a replacement brake booster, thinking maybe it was faulty, but the replacement booster has made no difference
  • Pulled out the master cylinder and "dead headed" it by plugging the brake line holes, and then tried to depress the MC -- it's hard to push, which should indicate the master cylinder is good. If I really put my weight into it it will move maybe an 1/8"
 

wicked93gs

10 Year Member
Sep 30, 2006
1,098
166
93
Nashville TN
Unfortunately still fighting with this. Tried a couple more things:
  • The kit maker sent me a replacement brake booster, thinking maybe it was faulty, but the replacement booster has made no difference
  • Pulled out the master cylinder and "dead headed" it by plugging the brake line holes, and then tried to depress the MC -- it's hard to push, which should indicate the master cylinder is good. If I really put my weight into it it will move maybe an 1/8"
This doesn't tell you much. I tried "bench bleeding" my MC for my clutch...no matter what I did, I couldnt get the piston to move much at all...once installed in the car, it was easy. In other words, without the leverage of the brake pedal, you aren't going to be able to get much movement out of the piston in the MC...at least not without a jig built specifically for that.