going back to manual disc brakes from power

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1973dust

1973dust

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I'm trying to determine if I have everything I need to go back to a manual master cylinder and the more I search, the more confused I'm becoming. My 73 Duster was originally 4 wheel manual drums and I had converted it to power disc/drum from a 74 Valiant. I still have the original manual brake pushrod but it appears that I may also need a plastic "spacer" plate that the pushrod bellows snaps onto? I also am trying to find a new rubber bellows and, so far, all I have located is one for $60!!! from Year One. I am hoping that someone has another source for that item. I'm also wondering how important the bellows is and what purpose it really serves. Does it help stop a fluid leak into the interior if the rear seal on the master blows? I'm converting to an aluminum 2 bolt master from Dr. Diff as I need a replacement anyway and I've never liked the feel of the power brakes. Thanks.
 
1973dust said:
I'm trying to determine if I have everything I need to go back to a manual master cylinder and the more I search, the more confused I'm becoming. My 73 Duster was originally 4 wheel manual drums and I had converted it to power disc/drum from a 74 Valiant. I still have the original manual brake pushrod but it appears that I may also need a plastic "spacer" plate that the pushrod bellows snaps onto? I also am trying to find a new rubber bellows and, so far, all I have located is one for $60!!! from Year One. I am hoping that someone has another source for that item. I'm also wondering how important the bellows is and what purpose it really serves. Does it help stop a fluid leak into the interior if the rear seal on the master blows? I'm converting to an aluminum 2 bolt master from Dr. Diff as I need a replacement anyway and I've never liked the feel of the power brakes. Thanks.
Click to expand...
If you're going to the 2 bolt master, you'll need the adapter plate for the swap.
Mopar Master Cylinder Adapter
That adds depth to the master mounting point, which means your stock manual pushrod is no longer useable- you'll need the adjustable pushrod to gain the extra length (or modify yours if you're feeling creative).
Mopar '62 -'70 Adjustable Master Cylinder Push Rod
I don't believe the bellows ever came on a 2 bolt master, and the adapter would probably not allow use of it anyhow.
The adjustable pushrod comes with the rubber bushing that locks the pushrod into the master.
 
Thanks, I also have the adapter plate coming. Dr Diff's description claimed the original rod would work but I won't be surprised if that's not the case. While searching, some said it's fine and some said you need an adjustable. It seems nothing is ever easy in Mopar-land.
 
I can't speak for the 2 bolt master but the 4 bolt uses the bellows to keep dust out of the cyl bore of the master. As for keeping leaking master from the inside of the car... The missing paint on the drivers floorboard of my 67 dart says no! (Big sticky mess in and under the carpet)

As for the gasket that holds the bellows...

It is not reproduced. It serves 2 purposes,

1 is to secure the bellows, before 70 something there was a metal part that bolted to the master that kept the piston in the bore and attached the bellows. After 70 something the master had a snap ring to retain the piston so the gasket was introduced

2 is to act as a seal between the master and the body.
 
1973dust said:
Thanks, I also have the adapter plate coming. Dr Diff's description claimed the original rod would work but I won't be surprised if that's not the case. While searching, some said it's fine and some said you need an adjustable. It seems nothing is ever easy in Mopar-land.
Click to expand...
The original drum brake pushrod is fine. If the master you ordered does not come with the rubber lock ring you’ll need it. The rubber bellows doesn’t really fit well with the aluminum 2 post master anyway. It’s just a dust seal of sorts. These cars of ours aren’t daily drivers and seldom see inclement weather. Not a concern for me.
 
1973dust said:
I've never liked the feel of the power brakes.
Click to expand...
The booster/brake "feel" is tunable.
I agree that the factory system was ridiculously sensitive.
As to M/Cs, there are three or 4 sizes to choose from, each smaller one offers more pedal travel and requires less leg- power.
As to the booster, you can adjust when and how much assist you get by adjusting the pushrod length within a narrow window, between too short and/or too long..
My happy place was the 15/16ths M/C .
 
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AJ/FormS said:
The booster/brake "feel" is tunable.
I agree that the factory system was ridiculously sensitive.
As to M/Cs, there are three or 4 sizes to choose from, each smaller one offers more pedal travel and requires less leg- power.
As to the booster, you can adjust when and how much assist you get by adjusting the pushrod length within a narrow window, between too short and/or too long..
My happy place was the 15/16ths M/C .
Click to expand...
15/16" bore is what I've been using and I still felt that it was too sensitive. I think I tried every pushrod length possible. If I ever go back to the booster I think I'll try a 1 1/32" bore as that may give a little more travel and less sensitivity? I'm hoping I like full manual. It will definitely be easier access to bulkhead connectors and such with the booster out of the way. As a side note, the FSM I have on CD lists 15/16" bore for manual disc/drum and 1 1/32" for power disc/drum which seems to be the opposite of what I've found anywhere else. Are all the parts stores wrong?
 
the 1&1/32, will displace MORE fluid with the same pedal travel, NOT LESS;
and because of that, will require MORE leg power to get it done.
That's where the booster comes in, assisting your leg.

Because the 15/16th requires more piston stroke, at an easier leg-power, I find the Booster sees less resistance at the beginning of the stroke, and the booster hit is softer.
As the piston nears hydraulic lock-up , it resists the pushrod, which with continued increasing pedal pressure, increases the boost assistance.
Finally, nearing lock-up , pushrod resistance is highest and you are pushing at near max leg power, and the atmospheric valve inside the booster is wide open, which offers maximum assistance.
This is how I have mine set up.
But I have one other variable, engine vacuum. At idle my vacuum is about 10/11 inches. My booster requires ~13 to offer max assistance. If I sit at a light for long enough, idling at 11 with my foot on the brake, on a hill (I have a manual trans) I can feel the assistance start to fade, and it requires more leg power to hold her on the hill.
As soon as the light turns green, and I step on the gas the vacuum goes up, and so, I am ready for the next application.
To mitigate this phenomenon, I re-wired my Line-loc to be a hill-holder. So when I come to that situation again, I just set the lock with the line pressure high enough to hold me there, and take my foot off the brake-pedal. To take off, I give her clutch and simultaneously release the lock as I hear/feel/see the engine bog down. Badaboom!
It took me several tries to get the pushrod length adjusted to where I got the feedback just right. Now it's a dream.

My booster is a big Single diaphragm off a mid seventies F-body.
My M/C is a generic 4-bolter I picked up from the Jobber; I have no idea of the original application. I asked for a 15/16ths and that is what the retired mechanic gave me. It looks like a mid-seventies Mopar.
Good luck.
 
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1973dust said:
15/16" bore is what I've been using and I still felt that it was too sensitive. I think I tried every pushrod length possible. If I ever go back to the booster I think I'll try a 1 1/32" bore as that may give a little more travel and less sensitivity? I'm hoping I like full manual. It will definitely be easier access to bulkhead connectors and such with the booster out of the way. As a side note, the FSM I have on CD lists 15/16" bore for manual disc/drum and 1 1/32" for power disc/drum which seems to be the opposite of what I've found anywhere else. Are all the parts stores wrong?
Click to expand...


You were using a 15/16 MC on power brakes??

The FSM is correct.

I prefer manual disc over power anything but with that 15/16 MC on there that would have been uncomfortable for me to drive.
 
Newbomb Turk said:
You were using a 15/16 MC on power brakes??

The FSM is correct.

I prefer manual disc over power anything but with that 15/16 MC on there that would have been uncomfortable for me to drive.
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74 dart’s and dart sport (likely all 74 mopar A bodies) with factory power disc brakes came with 15/16 bore. It’s fine.
 
Yes, 15/16" is what rock auto and pretty much everyone else specs for that application for power disc. I'm just now questioning that being correct and, yes, the transition between normal stopping and full panic was a fine line.
 
4mulas said:
74 dart’s and dart sport (likely all 74 mopar A bodies) with factory power disc brakes came with 15/16 bore. It’s fine.
Click to expand...


I’ll have to look in my 1973 FSM and see what it says.

My memory is at least up to 1972 they were 1 1/32.
 
Regarding master cylinder bore size, check the factory part manuals and FSMs- I don't trust ANY of the aftermarket listings anymore. They've homogenized their listings to the point of accuracy being a secondary consideration.
According to the FSM, the factory used the 1 1/32 master on manual systems, and the 15/16 master on power systems from '73-'75 with the 2.6" diameter caliper piston.
In '76 they went to a 2.75" caliper piston and used a 1 1/32 master on both power and manual, IIRC- but I don't have the '76 FSM to confirm my memory on the master size.
 
Newbomb Turk said:
I’ll have to look in my 1973 FSM and see what it says.

My memory is at least up to 1972 they were 1 1/32.
Click to expand...
Yes. On the 74 it was. And on my manual disc 70’ duster.
15/16 bore equates to a longer softer travel/pedal but better clamping force. The larger bore masters equate to firmer higher pedal but less clamping force.
 
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Post #13.
I suspect the m/c size would be the other way round: 15/16" for manual, 1 1/32" for power brakes.
 
Even though the caliper increased in size to 2.75 inches in 1976, the master cylinder was 1 1/32 on manual disc and 15/16 on power disc which were the same sizes as the 73-75 Abodies. Info from the 1976 Chrysler service manual.
According to the Chrysler factory service manuals, post 13 is correct on the 73 thru 75 years.
Of course you can use any size master cylinder you like but the above numbers are from the factory service manuals.
 
Professor Fate said:
If you're going to the 2 bolt master, you'll need the adapter plate for the swap.
Mopar Master Cylinder Adapter
That adds depth to the master mounting point, which means your stock manual pushrod is no longer useable- you'll need the adjustable pushrod to gain the extra length (or modify yours if you're feeling creative).
Mopar '62 -'70 Adjustable Master Cylinder Push Rod
I don't believe the bellows ever came on a 2 bolt master, and the adapter would probably not allow use of it anyhow.
The adjustable pushrod comes with the rubber bushing that locks the pushrod into the master.
Click to expand...

The original brake pedal push rod works just fine with the 2 bolt master cylinder and adaptor.
1973dust said:
Thanks, I also have the adapter plate coming. Dr Diff's description claimed the original rod would work but I won't be surprised if that's not the case. While searching, some said it's fine and some said you need an adjustable. It seems nothing is ever easy in Mopar-land.
Click to expand...
The original push rod works just fine. I have an original manual brake pushrod in my Duster with the DoctorDiff 2-bolt aluminum master cylinder and adaptor with a 15/16" MC for manual disk brakes. The pedal remains more or less in the same place as with the factory MC.

Bewy said:
Post #13.
I suspect the m/c size would be the other way round: 15/16" for manual, 1 1/32" for power brakes.
Click to expand...
Nope.

Stock A-body master cylinders were 15/16" for 9" drums, 1-1/32" for 10" drums, 1-1/32" for manual single piston (73+) disks, 15/16" for power 73+ disks, and 1" for the KH disks. From the '73 service manual, "V-L" is the A-body designation. The 2.75" calipers were B/E/R/F/M/J body fare, pin style in '73 hence the "floating" designation, the 2.6's were A-body single piston slider calipers. Since this chart is from a '73 manual it doesn't include the '76 A-body 2.75" piston sliders, but they used the same MC as the 2.6" calipers on A-bodies anyway.
From the '73 FSM
screen-shot-2016-08-18-at-4-01-07-pm-png.png


From the '70 FSM for the KH brakes
screen-shot-2016-08-18-at-4-12-22-pm-png.png


As you can see, A-bodies are actually backward when it comes to manual and power disk master cylinder bore sizing. That's because the power booster linkage changes the pedal ratio (A-bodies use an angled power booster bracket). So the power disk cars actually got a smaller bore master cylinder than the manual disk cars.
 
Hey guys. I finally got a chance to install the Dr. Diff 15/16" 2 bolt master today and I'm wondering what the normal brake pedal height is for manual disc/drum brakes. With the power brakes, I could pivot on my heel back and forth from brake to gas and now I have to slide my foot back to not catch the side of the brake pedal. I'm guessing the brake pedal is about a 1/2" higher now. I used the original non-adjustable manual pushrod from the factory 4-wheel drum master. I, of course, had to back off the brake light switch and there is still room for it to move back more so it may just be that manual brakes have a slightly higher pedal than power and I just need to get used to it.
 
1973dust said:
Hey guys. I finally got a chance to install the Dr. Diff 15/16" 2 bolt master today and I'm wondering what the normal brake pedal height is for manual disc/drum brakes. With the power brakes, I could pivot on my heel back and forth from brake to gas and now I have to slide my foot back to not catch the side of the brake pedal. I'm guessing the brake pedal is about a 1/2" higher now. I used the original non-adjustable manual pushrod from the factory 4-wheel drum master. I, of course, had to back off the brake light switch and there is still room for it to move back more so it may just be that manual brakes have a slightly higher pedal than power and I just need to get used to it.
Click to expand...

That sounds about right, the manual brake pedal does sit relatively high. I went from power disks to manual disks and converted to a 4 speed at the same time and the brake pedal was about even with the clutch pedal height. I also had to readjust the brake switch but it sat with room to adjust still as well.

I’ve got a hydraulic clutch set up now so I may switch to an adjustable push rod and lower the brake pedal a bit since I can adjust both the clutch and brake to match.
 
72bluNblu said:
That sounds about right, the manual brake pedal does sit relatively high. I went from power disks to manual disks and converted to a 4 speed at the same time and the brake pedal was about even with the clutch pedal height. I also had to readjust the brake switch but it sat with room to adjust still as well.

I’ve got a hydraulic clutch set up now so I may switch to an adjustable push rod and lower the brake pedal a bit since I can adjust both the clutch and brake to match.
Click to expand...
Ok, I assumed it was normal.
Dana67Dart said:
Factory 67 Dart manual disk / drumView attachment 1716297793

Hard press! 4.5"View attachment 1716297794
Click to expand...
Thanks for the measurements. I'll check it out for comparison tomorrow. If I can't get used to it, I think I'll probably make a shim to space the master away from the firewall to lower the pedal a little.
 
1973dust said:
Thanks for the measurements. I'll check it out for comparison tomorrow. If I can't get used to it, I think I'll probably make a shim to space the master away from the firewall to lower the pedal a little
Click to expand...
Something to keep in mind....

If one of the brake systems goes out you need as much pedal travel as you can get to be able to apply pressure to the other system.
 
That's a very good point. I probably should try to just get used to it as it sounds like it's how they were when new.
 
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