Non-power to power brake conversion...problems continue
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Senior User
Joined: Jun 1999
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Non-power to power brake conversion...problems continue
Hi all, Was doing well, I think, on my conversion. '78 F100 400/C6, disks on front, drums on rear. Ok, I swapped out my non-power MC and put in a booster and power MC. Solved the fitting problem and the brakes work. Although, they seem to be "spongy". I still have to push the pedal waaaayyyy down to get the truck to stop. I've checked the vacuum, 18". I adjusted the rear brakes, almost one turn of the wheel by hand and they stop. I can pump the pedal without the truck on and the pedal comes up and holds. When I start it, it goes down like it should. The rod from the booster to the MC is adjusted correctly I believe, albeit a bit long on the adjustment. It's at 0.975" vs the 0.931"-0.945" in the FoMoCo shop manual. So, what am I missing or doing wrong? The only thing I haven't switched is the rear wheel cylinders, I believe the power ones are of a larger bore than the non-power ones, but, would that make a difference?
Anyway, any help, comments, suggestions, prayers? Going to drink beer and ponder a little, maybe y'all with more wisdom and experience can help me! Thanks, Rick
Anyway, any help, comments, suggestions, prayers? Going to drink beer and ponder a little, maybe y'all with more wisdom and experience can help me! Thanks, Rick
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Joined: Nov 2000
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Non-power to power brake conversion...problems continue
Hi, I have had this problem when changing master cyl. before some of them seem to have a different piston in them and need more rod try adjusting the rod longer as long as the brakes don't hang up it should be fine John
Fleet Owner
Joined: Jun 1999
Posts: 24,552
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From: Creedmoor, North Carolina
Non-power to power brake conversion...problems continue
Rick , Did you change the prop. valve too?
John
jowilker
Club FTE since 01 01
NC Chapter
66 F100s
In the still cool hours of the night,
you can hear chevys rusting away.
John
jowilker
Club FTE since 01 01
NC Chapter
66 F100s
In the still cool hours of the night,
you can hear chevys rusting away.
Thread Starter
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Senior User
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Non-power to power brake conversion...problems continue
John,
No, I didn't change the proportioning valve. Since all I could find was one prop valve for 75-79 I assumed that mine would be correct. Also, it was working fine (I believe) before I did the swap. Meaning, that the brakes worked fine and the truck would stop well (now don't ask me why I swapped to power). I have tried to reset it via the instructions in FoMoCo manual. Is there a way to "test" it to see if it's bad? Is there, or should I, bleed it somehow? Off to bed and pondering! Rick
No, I didn't change the proportioning valve. Since all I could find was one prop valve for 75-79 I assumed that mine would be correct. Also, it was working fine (I believe) before I did the swap. Meaning, that the brakes worked fine and the truck would stop well (now don't ask me why I swapped to power). I have tried to reset it via the instructions in FoMoCo manual. Is there a way to "test" it to see if it's bad? Is there, or should I, bleed it somehow? Off to bed and pondering! Rick
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From: Virginia
Non-power to power brake conversion...problems continue
Some vehicles have different holes in the brake pedal where the
pushrod goes. The lower hole would be for power brakes. Does
your truck have two holes or require a different pedal arm?
This would vary the ratio of the pedal and give you more or less
travel. This wheel cylinder bore needs more attention too.
Did you verify that there are different bore sizes? It would
definitely affect pressure and pedal travel. Think about hydraulic brakes just as you would think of a hydraulic jack.
A small bore piston driving a large bore piston gives you a
lot of pressure to lift heavy loads, but you have to pump the
small piston many times. Same with brakes, if you have a small
bore master cylinder pushing a larger bore wheel cylinder you
will get lots of pressure but will also have lots of pedal travel. Also, I always adjust the shoes on a drum system all
the way out till I can't hardly turn the wheel. Then back the
adjustment off till you have a little drag.
pushrod goes. The lower hole would be for power brakes. Does
your truck have two holes or require a different pedal arm?
This would vary the ratio of the pedal and give you more or less
travel. This wheel cylinder bore needs more attention too.
Did you verify that there are different bore sizes? It would
definitely affect pressure and pedal travel. Think about hydraulic brakes just as you would think of a hydraulic jack.
A small bore piston driving a large bore piston gives you a
lot of pressure to lift heavy loads, but you have to pump the
small piston many times. Same with brakes, if you have a small
bore master cylinder pushing a larger bore wheel cylinder you
will get lots of pressure but will also have lots of pedal travel. Also, I always adjust the shoes on a drum system all
the way out till I can't hardly turn the wheel. Then back the
adjustment off till you have a little drag.
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
Non-power to power brake conversion...problems continue
Rick I am not a brake mechanic, but have followed all of the brake threads for the past two or so years. Most everyone recommends exchanging in a complete setup for less little problems, because you are bringing in a complete setup, with the exception of the rear drums.
You can try bleeding the valve, there may be enough difference in the setup to require a different setting.
John
jowilker
Club FTE since 01 01
NC Chapter
66 F100s
In the still cool hours of the night,
you can hear chevys rusting away.
You can try bleeding the valve, there may be enough difference in the setup to require a different setting.
John
jowilker
Club FTE since 01 01
NC Chapter
66 F100s
In the still cool hours of the night,
you can hear chevys rusting away.
Thread Starter
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Senior User
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Non-power to power brake conversion...problems continue
Hi all, Well solved all my problems, I think, I hope! Ok, the weak brake pedal was solved simply by adjusting the rod from the booster to the MC. Now my brakes are good, really good. On the wheel cylinder subject...I checked my donor vehicle, where the rear came from, and it had PB originally so they were correct. My brake light switch was solved by finding a donor vehicle with a "different" pedal and I swapped the pedals out. If you look at the pedals you'll notice that there are two different types of brackets for the switch mounting location. One is closer, and with a different angle, to the main frame of the pedal. The other is extended farther. So, needless to say I swapped and now my brakes and brake lights work too! So, keeping my fingers crossed! Thanks for all the help...Rick
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