SSBC power front disc vacuum booster issue - Page 2 - Ford Mustang Forum
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Slack in the top of the pedal travel doesn't indicate air in the system. Air in the system results in a spongy feel at the bottom of travel. I suggest that you adjust the pushrod further out (longer). You may have some slack in the vacuum booster itself.

The distribution block in your photo has no moving parts. Pressure goes in one port from the master cylinder then goes to the three other ports. Two should be front right and front left; the plugged port used to go to the rear when the car had a single master. Now that there is a dual one port gets plugged and the rear line gets connected to a proportioning valve. The proportioning valve is typically adjustable to lower pressure in the rear breaks to avoid lock-up.


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post #17 of 27 (permalink) Old 01-11-2018 Thread Starter
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Slack in the top of the pedal travel doesn't indicate air in the system. Air in the system results in a spongy feel at the bottom of travel. I suggest that you adjust the pushrod further out (longer). You may have some slack in the vacuum booster itself.

The distribution block in your photo has no moving parts. Pressure goes in one port from the master cylinder then goes to the three other ports. Two should be front right and front left; the plugged port used to go to the rear when the car had a single master. Now that there is a dual one port gets plugged and the rear line gets connected to a proportioning valve. The proportioning valve is typically adjustable to lower pressure in the rear breaks to avoid lock-up.
I'm struggling how to properly adjust the pushrod. I did set the gap between the MC and the booster rod to zero using a Baer tool so I don't think there is slack. The pushrod is currently adjusted so the brake pedal is about 2" higher than the gas pedal. That position leaves about 1/2" gap at the top of the pedal where it bottoms out. I tried adjusting the pushrod longer/pedal higher, but it didn't feel right having to lift your foot so high to place your foot on the pedal. It felt like a safety issue where rotating your foot from the gas to the brake may hit the side and miss. Is there a precise measurement for setting the pushrod length thus the pedal height?


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Poqman, this is a super basic question - but did you ever switch out the brake pedal for a power brakes version?

Manual brakes have a longer throw for more leverage, vs. the power brake pedal. Maybe this travel problem is because you're using the manual version?
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I'm struggling how to properly adjust the pushrod. I did set the gap between the MC and the booster rod to zero using a Baer tool so I don't think there is slack. The pushrod is currently adjusted so the brake pedal is about 2" higher than the gas pedal. That position leaves about 1/2" gap at the top of the pedal where it bottoms out. I tried adjusting the pushrod longer/pedal higher, but it didn't feel right having to lift your foot so high to place your foot on the pedal. It felt like a safety issue where rotating your foot from the gas to the brake may hit the side and miss. Is there a precise measurement for setting the pushrod length thus the pedal height?

I'm reacting to this earlier statement of yours (post 1):
Quote:
If I reach down and pull the pedal up with the engine running, it returns with a small pop to a full pedal and the brakes function normally again. So someone help me explain what is happening. It's as if the booster diaphragm bottoms out once the vacuum is gone and doesn't return to normal until you pull the pedal up. I'm assuming the problem is just in the booster and not the MC.
It seems to me that the rod needs to be pushed into the booster further. Is it a friction fit that you are overcoming when you pull up on the pedal?

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Poqman, this is a super basic question - but did you ever switch out the brake pedal for a power brakes version?

Manual brakes have a longer throw for more leverage, vs. the power brake pedal. Maybe this travel problem is because you're using the manual version?
Good question, but also consider that the conversion kit should have addressed this. I have not done the conversion on this particular model but I have with a '65 Galaxie with automatic transmission, and that used the manual brake pedal. I could be wrong, but I thought the only difference was that the manual pedal was wider (to allow two foot braking).

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I can't speak to Galaxies, but on Mustangs and Cougars of this vintage, the MB version has a different mounting point for the brake rod. It's higher, closer to the pivot, allowing greater leverage, at the expense of more travel. The power brake version mounts the brake rod lower, and requires a pedal change for factory brakes. I suppose the reason for the change is to allow better modulation of the brakes, since they're more sensitive.

In fact, with factory brakes, if you use a manual pedal with a PB booster, you will put the rod at such a steep angle that it will bind or break things.
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The SSBC power brake kit I installed in my '66 came with an extension for the brake pedal (shown in the figure below) that changed the pivot point. As mentioned in an earlier post in this thread, I never got this kit to work properly and removed it. I did not reinstall the extension for the Master Power kit I now have, because the push rod is straight w/o it; i.e., not at an angle. I suspect the mounting for the MP MC is different than that of the stock MC.
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post #23 of 27 (permalink) Old 01-12-2018 Thread Starter
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Originally Posted by Grimbrand View Post
Poqman, this is a super basic question - but did you ever switch out the brake pedal for a power brakes version?

Manual brakes have a longer throw for more leverage, vs. the power brake pedal. Maybe this travel problem is because you're using the manual version?
The conversion was done during a restoration before I got the car, but I suspect that it was replaced. The pedal had been freshly painted as if it were new and the pivot point is in line with the booster so there are no angles when pushing.

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I'm reacting to this earlier statement of yours (post 1):

It seems to me that the rod needs to be pushed into the booster further. Is it a friction fit that you are overcoming when you pull up on the pedal?
It was, but the sticking was resolved when I installed a new booster and MC from SSBC. It seems I traded one problem for another. The old booster and MC had a better feel when first applying the brakes, but stuck at the end of its travel and had to be helped up to return. The new booster and MC has about 1" travel at top before feeling the brakes take hold, then you feel the booster effect help and the the pedal travels about 2" from the floor where it is solid, but hard to push to complete the stop. It's like the booster has gone beyond its assisted travel length and then you are left pushing the MC manually. Nowhere in it's travel does it feel spongy. SSBC technical swears it can only be air still in the system. I've bleed 8 times and used 4 quarts of brake fluid. I've done the gravity bleed, the pressure bleed, the assistant pump the pedal bleed and a vacuum bleed. If there is air in the system, I'm at a loss why it hasn't been removed. But this is why I'm looking for a place where a small bubble could hide. I was looking at the distribution block (DB) as a place where air could be in a chamber, but from responses, I understand the DB on a '66 has no moving piston like the '67 model did with a brake warning light. I did find air at the SSBC proportioning valve (PV) after several bleeds that never came out until I cracked open the fittings and bleed the PV at the fittings. Only then did the rear wheel cylinders start flowing again doing a gravity bleed.

I already bought the Baer brake tool to set the gap between the booster rod and MC so I know the 1" travel is not due to a gap there. Has anyone used a brake pressure gauge to help isolate where the problem could be? It's expensive and a pita test, but I'm running out of things to try.

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I think SBC is all wet here. If the pedal is not spongy, there isn't air in the system. What you're describing can only be an issue with the booster.

A vacuum booster is one of the simplest pieces of equipment in the car. All it does is magnify your pedal effort with a large vacuum diaphragm, and to do so consistently over the travel length of the pedal rod. It sounds to me like the booster doesn't have enough internal travel. Is it a "slim line" type? Maybe that doesn't provide enough travel length.

The only other things I can think of are:
  • Make sure you have at least a 3/8" diameter fitting on the manifold and a 3/8" hose connected directly to the booster.
  • Make sure the MC didn't leak into the booster.

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Don't worry about what the tool tells you, there's slack in the booster that needs to be taken up. Lengthen the pushrod. If it feels better, lengthen it more. The pedal height should be defined by a bumper that hits the pedal support just behind where it mounts to the dashboard. If that's not touching, the pushrod is way too short. For a while, I dealt with a pedal that wouldn't firm up until an inch above the floor that no amount of bleeding and vacuum system inspection would fix. Ultimately, it was the pushrod that was the issue. Even though it was touching the booster and in the cup, it wasn't engaging the master cylinder.

Just try it. If it doesn't work, it's easy to revert.

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Don't worry about what the tool tells you, there's slack in the booster that needs to be taken up. Lengthen the pushrod. If it feels better, lengthen it more. The pedal height should be defined by a bumper that hits the pedal support just behind where it mounts to the dashboard. If that's not touching, the pushrod is way too short. For a while, I dealt with a pedal that wouldn't firm up until an inch above the floor that no amount of bleeding and vacuum system inspection would fix. Ultimately, it was the pushrod that was the issue. Even though it was touching the booster and in the cup, it wasn't engaging the master cylinder.

Just try it. If it doesn't work, it's easy to revert.
Sounds like a plan even though I don't understand where the slack is coming from. There is about 1/2" currently between the pedal support and bumper so there is plenty of room to lengthen the rod.

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