73 duster brake question

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Zam1787

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I have a 1973 Plymouth duster.. has manual steering, manual brakes.. front disc with rear drums.. everything works fine in the car. I just recently built a 360 motor, aluminum heads with a procharger. Before I even attempt to drive this car down the road. I am looking to put a little more stopping power into this car. Could someone please forward me to a kit or to the necessary parts to convert my front disc brakes to power brakes. Looking to keep my rear drums. Everyone I see is converting there brakes to manual and I have not found any parts lists or directions to convert mine to power brakes. Thank you for your time!
 
If you convert to power brakes then you need a minimum amount of vacuum from the engine to use them or they will feel hard...

If you have a big cam and low vacuum, it's best to use manual discs....
 
Ok thank you for your reply! The car was a 318 car with a heavy cam and 360 heads.. just didn't have enough go for me..I was just worried that with that much motor i am putting in it, I'd be standing up on the pedal to make it stop
 
Someone is going to throw out the answer to go to 11.75 disk brakes vs 10.95 brakes. That's only a 7.3% increase for all the extra work plus probably 15 inch rims.
 
You can sort of "fine tune" the feel of manual brakes depending on the size of the bore of the master cylinder and size of the rear wheel cylinder bore. One of the brake experts maybe will chime in
 
As mentioned above. But I think I would start with checking the available vacuum you have at the engine. I have power disc brakes on my 69 Dart and thou it doesn't have a real big cam it is noticeable. It has a Hughes Engine HEH2328AL 223/228 @.050 and .534/.555 . It's always had enough vacuum to operate the booster and the car stops really well. I have the stock BBP disc brakes out front and 10 2 1/2" drum in the rear.
 
First, let me clear up a misconception. Just adding power brakes does nothing to increase your stopping power. Nothing. Adding a power booster reduces the amount of pedal effort needed, it does not increase the stopping power. Changing the master cylinder bore size can change the line pressure which does change the clamping force, but that's true without the power booster.

Someone is going to throw out the answer to go to 11.75 disk brakes vs 10.95 brakes. That's only a 7.3% increase for all the extra work plus probably 15 inch rims.

Um, yes, 11.75 is a 7.3% increase in the diameter of the rotor from 10.95, but again that has nothing to do with the change in stopping power. You need to use areas and lever arm calculations, not the percentage increase of the diameter. That number doesn't mean anything.

There are several things going on when you swap to 11.75" rotors. First, the diameter of the rotor is larger, so, the lever arm acting between the caliper and the center of the hub is longer, and that directly increases braking force. On the stock 10.95" disks, the middle of the brake pad falls at ~4.59" from the spindle axis. On the 11.75" disks, it's 4.975". That might not sound like a huge difference, but that represents an 8.4% increase in braking force because you multiply the force applied at the pad by the length of the lever arm.

You're also increasing the braking surface area. The pad area is the same, but because the pads are further out they sweep out a larger area as the rotor turns. Applying the same amount of force over a larger swept area means the brakes don't heat up as fast, and there's more area to dissipate heat as well. If you figure the outside of the rotor is 11.75" and the width of the braking surface is 1.8" ( I measured it), then the total braking area is π (11.75/2)^2 - π(9.95/2)^2 = 30.67 square inches for the 11.75" rotors. For the 10.98 rotors the braking area is π(10.98/2)^2 - π(9.18/2)^2 = 28.5 square inches . That's an 7.6% increase in surface area on the rotor.

And both of those assume that you keep the A-body caliper. The 73+ A-body calipers use 2.6" pistons. The B/E/F/M/J/R body calipers had 2.75" diameter pistons. Don't be fooled and think this is a 5.7% increase, you must use the area, not the diamter. The 2.75" piston has a surface area of 5.94 square inches. The 2.6" piston has an area of 5.31 square inches. That's an 11.9% increase in area for the piston, and if you keep the line pressure the same that's an 11.9% increase in braking force because you're multiplying the line pressure PSI by the surface area of the piston to get that braking force.

All of those things combine to increase the stopping power. The direct increase is the 8.4% if you keep the same calipers and upgrade from the 10.95" rotors to the 11.75" rotors, which you can do. Under heavy braking conditions, the larger surface area will also reduce brake fade by slowing down heating and speeding up cooling. That doesn't change braking force, but it will improve braking performance under difficult conditions, and that's a big deal. And you can change the calipers to the later larger diameter piston too. If you assume the same brake line pressure, that's an 11.9% increase in force on the pads. If you increase both the diameter of the rotor and the diameter of the caliper pistons, your 11.9% at the caliper multiplied through by the 8.4% increase in lever arm turns into 21.3% increases in braking force at the spindle which is what's acting on the wheel. That's a big difference.

But remember that just because you increased the braking power by 21.3% doesn't mean you've decreased your stopping distance by that much. And really, that's what counts. Stopping distance is tied directly into the tires, the better the compound you have there the more friction you have and the more you can benefit from the larger brakes. I've done the 11.75" rotor upgrade a couple of times now. On my Challenger all I did upgrade the rotors and caliper brackets. The calipers for the E-body were already 2.75", so, no increase there. Even just that upgrade was noticeable out on the streets, the brakes felt much better, and can lock up the 275/40/17's I have on the front of my Challenger just fine. On my Duster I went to 11.75" rotors and changed the calipers to the later ones as well. Again, a very noticeable improvement even with just the 225/60/15's I had on the car at the time. Now, I doubt my stopping distance made a big jump because it would be tire limited with those small tires. But braking felt a lot better, more confident. And I'm sure it reduced the stopping distance a little even under normal conditions. Under repeated heavy braking it would have held up better too.

You do need 15" rims to use the 11.75" rotors.
 
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I guess the fact that most every car, truck, SUV, CUV and cross over comes built with Power Assist brakes do so for no reason? It's all about the "effort" whether power assist brakes or a truck coming with more torque to increase the GVWR.
 
I guess maybe I should specify my exact needs... my car as is.. stops just fine.. manual discs with rear drums.. I do have to apply a lot of pressure with my foot to slow it down, I'm a 230lb rugby player, leg strength is no issue at all. I have a bench seat in the car and when I need to push on the brakes the person next to me feels it to. I was thinking power brake booster would take the load off the amount of pressure needed to
Push the brakes, not necessarily make the car stop faster. If I'm wrong please tell me, I'm not offended by criticism. I have slotted rotors and I'm running 17' rims already so having the room isn't an issue either. As far as vacuum goes, I have a full electronic ignition, and I'm using fuel injection. Everything is controlled electronically so I have very little draw from vacuum as it is. My question is.. will using a vacuum brake booster/master combo take the amount of necessary pressure down, and what parts are needed to do so... thank you guys for your time
 
I guess maybe I should specify my exact needs... my car as is.. stops just fine.. manual discs with rear drums.. I do have to apply a lot of pressure with my foot to slow it down, I'm a 230lb rugby player, leg strength is no issue at all. I have a bench seat in the car and when I need to push on the brakes the person next to me feels it to. I was thinking power brake booster would take the load off the amount of pressure needed to
Push the brakes, not necessarily make the car stop faster. If I'm wrong please tell me, I'm not offended by criticism. I have slotted rotors and I'm running 17' rims already so having the room isn't an issue either. As far as vacuum goes, I have a full electronic ignition, and I'm using fuel injection. Everything is controlled electronically so I have very little draw from vacuum as it is. My question is.. will using a vacuum brake booster/master combo take the amount of necessary pressure down, and what parts are needed to do so... thank you guys for your time
If I understand what your goal is reading your post, yes I believe adding a brake booster will satisfy your goal. You should be able to buy a brake booster/ master cylinder combo rebuilt. Other than that you will need the brackets which you can find used and a combination valve.
 
I guess the fact that most every car, truck, SUV, CUV and cross over comes built with Power Assist brakes do so for no reason? It's all about the "effort" whether power assist brakes or a truck coming with more torque to increase the GVWR.

I didn't say they came with power brakes for no reason. I said just adding a power booster doesn't improve your stopping power. And it doesn't. It does make the pedal easier to push, and that might be more necessary on larger vehicles because your need more braking force. That can lead to higher pedal efforts, which people don't like. But just adding a booster does not increase stopping power.

Now, if you design a brake system that has more braking force and higher pedal effort based around using a booster to make the pedal easier to push, you can have more braking force than would be easy to use with manual brakes. But that's not what's happening here.

I guess maybe I should specify my exact needs... my car as is.. stops just fine.. manual discs with rear drums.. I do have to apply a lot of pressure with my foot to slow it down, I'm a 230lb rugby player, leg strength is no issue at all. I have a bench seat in the car and when I need to push on the brakes the person next to me feels it to. I was thinking power brake booster would take the load off the amount of pressure needed to
Push the brakes, not necessarily make the car stop faster. If I'm wrong please tell me, I'm not offended by criticism. I have slotted rotors and I'm running 17' rims already so having the room isn't an issue either. As far as vacuum goes, I have a full electronic ignition, and I'm using fuel injection. Everything is controlled electronically so I have very little draw from vacuum as it is. My question is.. will using a vacuum brake booster/master combo take the amount of necessary pressure down, and what parts are needed to do so... thank you guys for your time

Thanks for the clarification. You're not wrong, adding the power booster would reduce the effort it takes to push the pedal and it sounds like that's what you want to take care of. To install the power booster you'd need the power booster and the mount/linkage that's used on the A-body platform to raise the booster and m/c on the firewall. You'd also likely want a new m/c, as the manual brake cars and power brake cars ran different cylinder bores.

Here's a picture of the A-body power booster set up. Power booster, angle bracket, and a pivot linkage for the push rod
IMG_2741.jpg


But, that's not your only way to reduce braking effort. You can change the size of the m/c bore, which also has an effect on pedal pressure. Stock '73 A-bodies with manual disks have a 1-1/32" m/c, and it gives a pretty high and hard pedal. I run all of my cars with manual disks with a 15/16" m/c. I even converted my Challenger from power brakes to manual. The stock power brakes just don't give you a whole lot of brake feel. The manual 15/16" master cylinder works well for that, the brake effort isn't bad at all in my opinion, and the longer pedal stroke allows for better brake modulation.
 
I didn't say they came with power brakes for no reason. I said just adding a power booster doesn't improve your stopping power. And it doesn't. It does make the pedal easier to push, and that might be more necessary on larger vehicles because your need more braking force. That can lead to higher pedal efforts, which people don't like. But just adding a booster does not increase stopping power.

Now, if you design a brake system that has more braking force and higher pedal effort based around using a booster to make the pedal easier to push, you can have more braking force than would be easy to use with manual brakes. But that's not what's happening here.



Thanks for the clarification. You're not wrong, adding the power booster would reduce the effort it takes to push the pedal and it sounds like that's what you want to take care of. To install the power booster you'd need the power booster and the mount/linkage that's used on the A-body platform to raise the booster and m/c on the firewall. You'd also likely want a new m/c, as the manual brake cars and power brake cars ran different cylinder bores.

Here's a picture of the A-body power booster set up. Power booster, angle bracket, and a pivot linkage for the push rod
View attachment 1715077673

But, that's not your only way to reduce braking effort. You can change the size of the m/c bore, which also has an effect on pedal pressure. Stock '73 A-bodies with manual disks have a 1-1/32" m/c, and it gives a pretty high and hard pedal. I run all of my cars with manual disks with a 15/16" m/c. I even converted my Challenger from power brakes to manual. The stock power brakes just don't give you a whole lot of brake feel. The manual 15/16" master cylinder works well for that, the brake effort isn't bad at all in my opinion, and the longer pedal stroke allows for better brake modulation.
This agrees with the hydraulic training manuals: Force= pressure x area. So for a given required braking force, increasing area ( master cylinder diameter) would require less travel distance. But at the cost of harder pedal as pressure is ramping up quicker.
 
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I guess maybe I should specify my exact needs... my car as is.. stops just fine.. manual discs with rear drums.. I do have to apply a lot of pressure with my foot to slow it down, I'm a 230lb rugby player, leg strength is no issue at all. I have a bench seat in the car and when I need to push on the brakes the person next to me feels it to. I was thinking power brake booster would take the load off the amount of pressure needed to
Push the brakes, not necessarily make the car stop faster. If I'm wrong please tell me, I'm not offended by criticism. I have slotted rotors and I'm running 17' rims already so having the room isn't an issue either. As far as vacuum goes, I have a full electronic ignition, and I'm using fuel injection. Everything is controlled electronically so I have very little draw from vacuum as it is. My question is.. will using a vacuum brake booster/master combo take the amount of necessary pressure down, and what parts are needed to do so... thank you guys for your time
The issue with vacuum is more to do with how much you have at idle, and at cruise, not so much with accessories using some of the vacuum. Bigger cams affect the amount of vacuum the engine will make. Stock motors with stock cams typically make between 15" to 19" of vacuum, and brake boosters are designed to work in that range. A bigger cam with more overlap reduces the vacuum, sometimes down betwen 10 - 12", and that may not be enough to make the booster work. Some people get around this by plumbing a vacuum booster between the engine and the power booster - sort of a reserve vacuum canister.
If it were me I'd keep the manual brakes, go with 11.75" in the front and go with discs in the rear. You can always go bigger in the front too, if you're running 17"s - the aftermarket has lots of options, other than the stock 11.75 stuff. But those 11.75's were the upgrade that make it onto B body cop cars of the era.
 
Just an FYI, understand that vacuum is the "suction" that is found in the intake manifold while the engine is running. The power booster has a rubber diaphram in side that is actuated by the suction from the intake manifold.
It is not an electrical thing, so has nothing to do with ignition, fuel injection computers or anything else.
Big, long duration cams work with low engine vacuum, especially when the throttle is applied. If you don't also have a reserve vacuum cannister or electric vacuum pump, if the engine dies you will probably only have power assist for the first pump of the pedal. After that it will be a real problem to modulate your braking.
Disc pad type/ compound also make a big difference. Street pads have good low temperature friction function. High speed or racing type pads work best when the pads are hot. SSo the first stop with a hard pad will not stop well but as they heat up they work better.
 
I guess maybe I should specify my exact needs... my car as is.. stops just fine.. manual discs with rear drums.. I do have to apply a lot of pressure with my foot to slow it down, I'm a 230lb rugby player, leg strength is no issue at all. I have a bench seat in the car and when I need to push on the brakes the person next to me feels it to. I was thinking power brake booster would take the load off the amount of pressure needed to

Are the existing brakes in good condition? Pads and shoes wearing evenly? I inspect mine once a year. I change my fluid every two years. I let it go for 4 years once, and I could feel an improvement in the pedal feel afterwards. I'm wondering if you have a partly blocked hose somewhere.
 
I have a 1973 Plymouth duster.. has manual steering, manual brakes.. front disc with rear drums.. everything works fine in the car. I just recently built a 360 motor, aluminum heads with a procharger. Before I even attempt to drive this car down the road. I am looking to put a little more stopping power into this car. Could someone please forward me to a kit or to the necessary parts to convert my front disc brakes to power brakes. Looking to keep my rear drums. Everyone I see is converting there brakes to manual and I have not found any parts lists or directions to convert mine to power brakes. Thank you for your time!

You do not need power brakes. However, you can-easily-upgrade what you have: use caliper brackets from a late 70s B/R body (most common on 2-doors and 79-81 R-bodies) and the late B-body 11.75" rotors. For years, this exact setup stopped Rick Ehrenberg's 500+HP, 150+MPH road-race Valiant.

If that's not enough, you can use the 11.75" rotors, 15" wheels...and Viper 4-piston calipers.
 
The issue with vacuum is more to do with how much you have at idle, and at cruise, not so much with accessories using some of the vacuum. Bigger cams affect the amount of vacuum the engine will make. Stock motors with stock cams typically make between 15" to 19" of vacuum, and brake boosters are designed to work in that range. A bigger cam with more overlap reduces the vacuum, sometimes down betwen 10 - 12", and that may not be enough to make the booster work. Some people get around this by plumbing a vacuum booster between the engine and the power booster - sort of a reserve vacuum canister.
If it were me I'd keep the manual brakes, go with 11.75" in the front and go with discs in the rear. You can always go bigger in the front too, if you're running 17"s - the aftermarket has lots of options, other than the stock 11.75 stuff. But those 11.75's were the upgrade that make it onto B body cop cars of the era.

You realize you replied to a thread that’s like 5 years old right?
 
For those concerned about vacuum, there are vacuum assist motors that wont break the bank and will remedy low vacuum issues
 
I guess maybe I should specify my exact needs... my car as is.. stops just fine.. manual discs with rear drums.. I do have to apply a lot of pressure with my foot to slow it down, I'm a 230lb rugby player, leg strength is no issue at all. I have a bench seat in the car and when I need to push on the brakes the person next to me feels it to. I was thinking power brake booster would take the load off the amount of pressure needed to
Push the brakes, not necessarily make the car stop faster. If I'm wrong please tell me, I'm not offended by criticism. I have slotted rotors and I'm running 17' rims already so having the room isn't an issue either. As far as vacuum goes, I have a full electronic ignition, and I'm using fuel injection. Everything is controlled electronically so I have very little draw from vacuum as it is. My question is.. will using a vacuum brake booster/master combo take the amount of necessary pressure down, and what parts are needed to do so... thank you guys for your time

Yur right on target and
I wouldn't worry about low vacuum off boost, this mostly occurs at really really slow speeds. I have no idea how that will play out with the Procharger, but make sure you don't pressurize the booster lol.
Here's my experience;
I ran a booster on my 360 4-speed with a 292/292/108 Mopar cam in it, and it stopped great. The only time it didn't was first start of the day, and backing up. A couple of blips on the throttle (no choke), charged it up, and she was good to go.

I highly advise against trying to use the Dual-Diaphragm, they take more vacuum to do what a regular single diaphragm does. I spent a lot of time with two of those (I already had them) but was never satisfied. I next installed a used booster off a 75 Dart, and badaboom,never looked back. I mean it was already 25 years old, and just worked. I run a 15/16 bore M/C. It takes a little more pedal travel to do the work but I like the feel of it. I worked my way down from two sizes bigger. Just make sure the rear adjusters work, and keep on working. I run that silicon fluid. I installed it in 1999, and have NEVER had problems with it.
I run Big-n-Littles; I have 235/60-14s on the front with the KH 4-piston calipers, and on the back 295/50-15s on 10 x 2s drums.
I run zero proportioning; and it stops real nice. I could run even more brake on the back, but I am happy as it is. You will be amazed at the difference. Be prepared to replace the rear shoes more often! lol.
Happy HotRodding
 
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