Excess brake pedal travel with new power front discs - Vintage Mustang Forums
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post #1 of 64 (permalink) Old 04-10-2019, 02:31 PM Thread Starter
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Excess brake pedal travel with new power front discs

Hey guys, I know this topic has been covered before, but my searching has not led me to a reliable answer to my issue. I installed this kit last year as part of a full restoration of my first car from my high-school years:

https://www.cjponyparts.com/front-po...1966/p/DBF193/

Now that I'm on the tail end of the project, and have the car running, I have had trouble getting any kind of firm brake pedal. There are several inches of travel before the brakes feel like they engage at all. I have bled them multiple times, adjusted my rear drums to the point of dragging, but I still don't think things feel right. I tried to engage the self-adjusters further with some reverse braking, but this didn't seem to change things either. The pushrod is adjustable under the dash, but this only sets the pedal height up, doesn't change the free play. With the car off, the pedal feels almost normal, but once vacuum is applied, it basically drops to the floor before it grabs at all. I sure wouldn't trust it in a hard braking scenario. The booster pushrod into the master cylinder has some adjustment as well that I set up per the original instructions. There is an intermediate pin that slides into the master cylinder and the instructions specified a measurement for the adjustable rod to be. I have also confirmed that my calipers are installed with the bleeders facing up. Is there any way to troubleshoot what the issue is before I pull the MC and bring out the rod more? I know this will likely make it "feel" like less throw, but I don't want to cause any problems if the brakes are in a state of partial engagement. Any advice would be very much appreciated. I have 3 other 1st gen mustangs, but everything is stock drums. The mods I've done to this car have taken me out of my knowledge base a little and this is yet another example. Thanks for your help!
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post #2 of 64 (permalink) Old 04-10-2019, 02:46 PM
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You did bench-bleed the master cylinder, to eliminate this aspect? Did the bleeding sequence go as expected, that is RR, LR, RF, and LF and no bubbles? Just seeking for others to eliminate the bleeding operation.

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post #3 of 64 (permalink) Old 04-10-2019, 02:59 PM Thread Starter
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You did bench-bleed the master cylinder, to eliminate this aspect? Did the bleeding sequence go as expected, that is RR, LR, RF, and LF and no bubbles? Just seeking for others to eliminate the bleeding operation.
Yes, sorry...I did bench bleed per the instructions as well. No obvious issues at the time, although when I initially bled the brakes, I wasn't pushing enough fluid to express any out. I had to lengthen the pushrod to the pedal under the dash to increase the throw to get it to start bleeding. That put the pedal up to an unreasonable height, but once they were bled, the pedal could be brought down closer to normal height. Still, it's higher than I want it, but it needs to be due the travel issue, the braking point is near the floor as it is.
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post #4 of 64 (permalink) Old 04-10-2019, 03:41 PM
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Assuming your brakes calipers are holding the pads right up against the rotors...there's no way a properly bled and functioning master cylinder and brake line set can have as much travel as you describe. That leads me to believe, despite your efforts, you still have air in your system. If I were you, I'd either take it to a mechanic...OR if you want to DIY it...get a vacuum bleeder. You can get one for 60 to 100 dollars. They work a lot better than the old "2 man...one guy push down the pedal, the other guy loosen and tighten the bleeder" method.

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post #5 of 64 (permalink) Old 04-10-2019, 03:46 PM
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My bone stock 67 with original equipped power disk brakes has about 2 inches of travel before anything happens. Engine off the pedal is hard. I think your experience is pretty normal for factory 60's power disk brakes.
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Last edited by RV6; 04-10-2019 at 03:49 PM.
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post #6 of 64 (permalink) Old 04-10-2019, 03:53 PM
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Originally Posted by kellysedars View Post
There is an intermediate pin that slides into the master cylinder and the instructions specified a measurement for the adjustable rod to be.
Take the booster off and make sure the measurements you got are correct. Use a depth caliper and measure how far the pin should go into the MC and how far it should go into the booster. Then make sure you're pin is almost exactly that length.

I had the same problem once on an unrelated build and the intermediate pin was too short.

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post #7 of 64 (permalink) Old 04-10-2019, 03:55 PM
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Quote:
Originally Posted by kellysedars View Post
Yes, sorry...I did bench bleed per the instructions as well. No obvious issues at the time, although when I initially bled the brakes, I wasn't pushing enough fluid to express any out. I had to lengthen the pushrod to the pedal under the dash to increase the throw to get it to start bleeding. That put the pedal up to an unreasonable height, but once they were bled, the pedal could be brought down closer to normal height. Still, it's higher than I want it, but it needs to be due the travel issue, the braking point is near the floor as it is.
Your brake pedal, at rest, should be right up with the rubber bumper just about touching its bracket. You shouldn't be "lowering" it.
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post #8 of 64 (permalink) Old 04-10-2019, 04:08 PM
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Or buy this tool to get the pin set properly...what's another $26, right?

https://www.macsautoparts.com/ford_m...E&gclsrc=aw.ds
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post #9 of 64 (permalink) Old 04-10-2019, 05:06 PM Thread Starter
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Originally Posted by RV6 View Post
My bone stock 67 with original equipped power disk brakes has about 2 inches of travel before anything happens. Engine off the pedal is hard. I think your experience is pretty normal for factory 60's power disk brakes.
I have factory power drums on two other '66 cars. The pedal is much more responsive than what I'm dealing with here.

I do have a vacuum bleeder, though it seems like it doesn't work the best. I'll have to play with it some more.

My car is automatic and the brake pedal does not have a rubber stop, only the pushrod stops the backwards travel. I was thinking those were only on the manual trans pedal assemblies? Could I be missing something?
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post #10 of 64 (permalink) Old 04-10-2019, 06:20 PM
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You wil find many people here with the same issue, and many other people here that will tell you that you must be doing something wrong. I have the same issue. I used to be ASE certified in brakes. I couldn't make it better. I had a buddy who is an ASE Master Tech check it out. Still not better. What I have found was that when I was bleeding the MC on the bench, I was carefully watching to see how far I had to push to get fluid to move. The front bowl (rear brakes) start to move fluid as soon as you push in the plunger. The rear bowl (front brakes) doesn't move any fluid until almost half throw. This means that the front brakes are doing nothing until I get to that point. It's not broken. It's not anything to do with adjusting the rod. It's a crappy MC. It's a feature, not a flaw. I'm very happy with the parts of the kit at the wheel . I'm not impressed with the MC/booster combo. I plan to find something better later.

Last edited by jgrote; 04-10-2019 at 06:21 PM. Reason: typo
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post #11 of 64 (permalink) Old 04-10-2019, 06:21 PM
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I had my front power booster drum brakes converted to disk brakes 2 years ago. This was a bolt on conversion kit very similar to yours. I was warned by the mechanic that the brake pedal would feel different than before and that the brake pedal would travel for approximately 2-3 inches before I would feel the brakes would respond . He said that this is very normal and that I would adjust to the new pedal movement. He was correct. I donít know if this matters, but my car already had the power booster and has a manual transmission. Btw, I love the brakes.

1969 Mustang Convertible
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post #12 of 64 (permalink) Old 04-10-2019, 06:46 PM
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You also need to ascertain if the master cylinder has an integral residual valve. The rear drum brake require 10 lbs of pressure on them to keep the springs from fully retracting the brakes. Without this the preassure to preload the brakes pedal will behave exactly as you describe. The pedal will drop a couple inches as the brakes expand to reach the drums.

A residual valve can be added on the main line to the rear brakes to provide the necessary preload.

https://www.npdlink.com/product/resi...0001%26year%3D
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post #13 of 64 (permalink) Old 04-10-2019, 07:07 PM Thread Starter
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You also need to ascertain if the master cylinder has an integral residual valve. The rear drum brake require 10 lbs of pressure on them to keep the springs from fully retracting the brakes. Without this the preassure to preload the brakes pedal will behave exactly as you describe. The pedal will drop a couple inches as the brakes expand to reach the drums.

A residual valve can be added on the main line to the rear brakes to provide the necessary preload.

https://www.npdlink.com/product/resi...0001%26year%3D
Is there a way to tell?
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post #14 of 64 (permalink) Old 04-10-2019, 07:08 PM
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You also need to ascertain if the master cylinder has an integral residual valve. The rear drum brake require 10 lbs of pressure on them to keep the springs from fully retracting the brakes. Without this the pressure to preload the brakes pedal will behave exactly as you describe. The pedal will drop a couple inches as the brakes expand to reach the drums.

A residual valve can be added on the main line to the rear brakes to provide the necessary preload.

https://www.npdlink.com/product/resi...0001%26year%3D
This is a great point. I was told that the kit I bought came with an internal valve, but I don't know for certain that it does. @kellysedars, this might be worth a phone call. It is very important to have this.
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post #15 of 64 (permalink) Old 04-10-2019, 07:29 PM
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Originally Posted by kellysedars View Post
Is there a way to tell?
Easily--Straight out a paper clip, insert into the port. If you feel a obstruction close to rear of seat, it has a residual valve (Rubber plug with spring pushing on it).
If clip slides feely in port, you feel nothing, NO residual.

Nearly, closet to NONE of aftermarket, will fit, if masters have residuals.
Residuals need to be placed in the line, on low pedal, POOR rear brakes.---Yes adjustment are major factor, . Most do not know how to setup the rear drum brakes adjustments.
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