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Discussion Starter · #1 ·
has anyone tried using a master cylinder for power brakes on a manual brake setup?
what exactly is the difference between the power and manual master?
I ask because late 78 gm use a master that might fit my MG space constriction.
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Discussion Starter · #3 ·
bore size and pedal geometry are what counts ..
Not sure how to figure out geometry. Wilwood told me the same thing and I froze like a deer 🦌 in the headlights. Lol. Any info would be appreciated.
 
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Here's my 2 cents Mario. My 69 Ford 1/2 ton has 4 wheel drum brakes and a dual master. I at one time considered going with a disc brake conversion for the front. I asked about the useability of the existing master and simply adding a vacuum booster and was told it would work. I did not do the conversion so I can't say if that info was correct. I also asked my buddy who owned the local parts store if there was a separate part number for the non-power brakes master and the power brake master and they were the same. Your bore size and pedal geometry should be ok because you're not changing the location or brake line size. As you know the booster simply bolts on where the master did and the master then bolts to the booster which should come with a rod of its own.
 

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Discussion Starter · #5 ·
I agree. Thanks guys.
 

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Discussion Starter · #6 ·
Let me elaborate on my issue. I have a lucas MG original Master hooked up to a 70’s mid size Chevy disc drum setup with a proportioning valve to match manual brakes. When I hit the pedal it goes down passed the halfway mark and brakes don’t feel like they’re really grabbing hard so I hit it again and they grab better but not enough to say I’m comfortable taking a blast down the road by telephone poles. Hey that sounds familiar. Lol. So I feel a Gm master would be a better match for the right fluid shot at the brakes. Any comments welcome.
 

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Mario, this latest information changes the dynamics. What Crazyhorse suggested about bore size might indeed be relevant now with your information about what is probably a smaller bore master trying to push fluid to a larger brake setup from a mid-sized American car. What you say about your current pedal travel my first thought was air (do you have access to a power bleeder) in the line but now I'm thinking more about what is probably a to small a bore size master that isn't pushing the volume of fluid you need to operate those larger sized American brake components; especially the front disc calipers. Do you know the setting of your proportioning valve? Is it set to allow the much more needed fluid to the front brakes than to the rear drums. Also, how is the adjustment on your rear brakes? To loose an adjustment is going to use up a whole lot of pedal travel to move the rear brake pistons. Remember the basics buddy.
 

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Yep bore size and pedal geometry smaller bore equals more psi being applied to the calipers ..so to make it simple... if you have a pedal setup that when you apply 50 lbs of foot pressure to a 1" piston in the clyd. you get 50lbs not taking into account the pedal and rod geo.....now if you make that 1" piston a 1/2" piston then that 50lbs. becomes a 100lbs psi. as the brake rod is moved up the pedal arm you increase the pressure being applied to the master..the pedal arm is a lever..the longer the length from the point of application the great the applied force, most masters can not be bleed right on the car with out a pressurer bleeder... and if tilled up like most will not manuel bleed with out removing and bench bleeding
 

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Mario, pedal geometry is a different leverage for power vs. manual brakes. Manual is usually around 6:1 where power is around 5:1 ratio. When I used the Subaru power brake booster and master on my Austin it had about 14"-15" vacuum, and worked. When I swapped in the bigger cam I lost brakes. I planned to add a vacuum pump, but didn't like the idea. So instead I removed the booster, and converted to manual brakes, using the same master. I cut the brake pedal arm, and extended it about 1.5" longer, and even with the power master my car stops great.
I plan to do the same modification to my '39 this winter. I used a disc-drum manual master on it, but have the same Subaru brake pedal assembly. It needs more leverage, so I'm going to cut the pedal off and weld on an extension to the arm to give better leverage.
 

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Discussion Starter · #10 ·
Mario, this latest information changes the dynamics. What Crazyhorse suggested about bore size might indeed be relevant now with your information about what is probably a smaller bore master trying to push fluid to a larger brake setup from a mid-sized American car. What you say about your current pedal travel my first thought was air (do you have access to a power bleeder) in the line but now I'm thinking more about what is probably a to small a bore size master that isn't pushing the volume of fluid you need to operate those larger sized American brake components; especially the front disc calipers. Do you know the setting of your proportioning valve? Is it set to allow the much more needed fluid to the front brakes than to the rear drums. Also, how is the adjustment on your rear brakes? To loose an adjustment is going to use up a whole lot of pedal travel to move the rear brake pistons. Remember the basics buddy.
Yes and that’s why I was looking at a Malibu master. The only mystery I need to figure out is the pedal travel. As far as the proportion valve it’s factory gm to match the brakes I’m using. Right side front brake line does have a rise above the valve and Then coming back down again but not higher than the master. So I believe I have all the air out being it hits both front evenly. Never pulled to left since I assembled it. I’m gonna look at this again today. i am going to see how much pep boys wants for a master And hopefully have one in stock. Otherwise it’s Amazon. Thanks guys.
 

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Discussion Starter · #11 · (Edited)
I called around and all the chain stores couldn’t help me with a master today. Had to be ordered For Monday. Then I remembered the slogan “if you can’t find it on earth you can find it on Mars”. Mars autoparts in Bayshore LI. Been around since the 60’s. I called them and asked for a 1978 Chevy Malibu master for manual or power brakes, whichever he had. Personally I never knew they still made manual brake cars in 78 but he had a manual one in his store. Believe it or not. So I went and picked it up. 60 bucks for remanufactured, somewhat pricey but I have to pay for same day convenience. Worth it to me. The shape is exactly what the MG needs to clear the fender and the hood. I’m going to try it in a bit.
got to love that auto parts. I used to buy stuff there in the mid seventies. Store hasn’t changed a bit. Look at the wood floors. Been there forever.
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That's good news Mario!! Just out of curiosity what's the bore size of both masters? I might add here that after I mentioned about checking the adjustment of your rear brakes because of added pedal travel as they wear I went out, put my truck in the air and checked my brakes. My pedal travel had increased a lot along with the travel of the e-brake push pedal. Sure enough, all 4 wheels needed adjustment and now the pedal travel is back where it should be.
 

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Discussion Starter · #13 ·
That's good news Mario!! Just out of curiosity what's the bore size of both masters? I might add here that after I mentioned about checking the adjustment of your rear brakes because of added pedal travel as they wear I went out, put my truck in the air and checked my brakes. My pedal travel had increased a lot along with the travel of the e-brake push pedal. Sure enough, all 4 wheels needed adjustment and now the pedal travel is back where it should be.
On the bore size I don’t know yet. I will this afternoon. I’m sure the Malibu is larger than the MG. also I didn’t make an emergency brake cable system yet. I wonder if that will affect my pedal travel too. I have the cables mounted to a bracket on each side but never came up with a pulley system to use the brake. I seem to remember dealer procedure required e brake adjusted properly and rear shoes adjusted properly before bleeding.
 

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GM G body master should be a 7/8" bore...hope you have a stiff mounting point for it...I am doing like Vall did with the pedal length on the 1924 db and making the pedal arm longer and I am off setting it to clear the gas pedal more
 

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Discussion Starter · #15 ·
The outside diameter at the pushrod cup is 1”. The MG says 3/4 right on the side of the casting. there is a sizable difference between the two.
 

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There's that difference in volume I was talking about Mario. So you're going to be pushing a larger volume through the same size line which will increase your line pressure. Did you get a chance to check your rear brake adjustment yet?
 

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Discussion Starter · #17 ·
There's that difference in volume I was talking about Mario. So you're going to be pushing a larger volume through the same size line which will increase your line pressure. Did you get a chance to check your rear brake adjustment yet?
No Steve not yet. Maybe in a couple of days. My son is ceramic coating his Kia in the garage and I can’t move around like I want to.
 

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Discussion Starter · #18 ·
This is where I’m heading with the install. I gotta pull the brake pedal assembly and slide over the brake pedal.
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Mario doing the math on both masters works out to be with a 4 to 1 mec. at pedal and a average pedal pressure of 150lbs being applied on the pedal works out to be ... MG master at 3/4 inch would be about 910 psi.. the 1 inch GM witch is really a 7/8 inch bore works out to about 670 psi so you will be losing brakeing power that is being applied to the calipers...as far as volume you will shorten the amount of stroke lost with the GM master compared to the MG master, but if you are not running a residual line presure valve on your rear drum brakes adding one will some what compensate for pedal travel loss you are having.. the volume increase will not change the psi at calipers enough to make a differenc since it is dead head pressure and will be lost due to having to rebalance front to rear fluid flow... I would look for a smaller MG master...or maybe have it sleeved down with another piston seal set up
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There are definitely a couple of different schools of thought being presented here. Take for example a 1" diameter garden hose. It's moving a large volume of water, more than a 5/8" dia. or 3/4" diameter hose and will fill up a container much quicker than the other 2. Now, you restrict that flow by decreasing the size of the line that it's flowing to (your master cylinder piston and bore reduced to your smaller brake lines. Your pressure is increased dramatically but your volume stays the same to be able to fill the cavities of the disc brake pistons along with the help of your proportioning valve. Lash is right about the "dead head" but it has to be or there would be no pressure at all for obvious reasons.
 
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