Ok I am about to swap to a dual master on my car.1951 plymouth I already have a disk brake conversion on the front. the current master bore size is 1" and the brakes seem fine. I am wanting the dual for safety. I am assuming I should stick with the 1" bore? In my car club we have a total of three 49-52 mopars so i would like to get this correct so we can dual all three cars. any help would be great.
Sounds like you are happy w/ current feel. An option is to go to a 15/16" piston. You trade increased pedal travel for ratio boost of about 13%. Not power brakes but definitely noticable.
Think about what you are proposing. Going from a single piston to a dual piston also with a 1" bore means you are going to push twice the volume of fluid that you did previously as you have two pistons pushing twice the area/volume . This means in effect your mechanical advantage will be halved. This will make for a very hard pedal. You will have little travel and you will have to push like hell to get the brakes to work. Given this dilema you need to factor this into the choice of cylinder you make. This means the new dual circuit cylinder will need to have a combined area of both pistons that equals the current single piston. Mathematically, if you have a piston diameter of .7071" this would produce the same combined area as a single piston of 1". This seems rather small . I would inquire as to what calipers were originally fitted to and then find out what size cylinder was used. Go from there.
Does a '51 Mopar master cylinder have the 4 bolt pattern? If so this '75 Mopar 15/16" bore may work - It's what came in my '62 Dodge Dart manual disc/drum kit: Centric 130.63017
The pedal force in and pressure out of a 1" single or tandem dual master is the same. If what you stated was true, manual pedal ratios would be in the 12-14:1 range vs the normal 6-7:1.
That's what I was thinking too Bob. The mechanical adavantage may be halved, but so would the work required, since each half of the master would only be doing 2 wheels instead of all 4. Does that make sense?
To properly understand what is going on here the relationship between the master cylinder and the wheel cylinders/calipers is akin to that of a fulcrum and lever. With a lever the longer the handle the greater the leverage. Now with the master cylinder and wheel cylinders the effective leverage, or gains in pressure at the wheel cylinder/caliper pistons is determined by the ratio of the area of all of the wheel cylinders and caliper pistons with the area of the piston in the master cylinder. This is an inverse relationship. This is fundamental to understanding what is going on here. For example lets say the combined area of all of the wheel cylinders pistons is say 14" square inches and the master cylinder piston has an area of 1 square inch . For every one pound of pressure on the master cylinder piston the effective pressure at the piston will be 14:1 . That is how you get increased leverage with any hydraulic system. In the present case the master cylinder piston area has doubled, all else being equal, therefore the effective pressure has halved. This means you will need to push twice as hard as before to get the same braking effect but with half the travel as you are pushing twice as much fluid volume.
Effects of Pedal Ratio and Bore Size on Hydraulic Pressure Output Pedal Ratio Bore Size lbs input PSI out 6:1 1 1/8 75 453 6:1 1 75 573 6:1 7/8 75 748 5:1 1 1/8 75 377 5:1 1 75 477 5:1 7/8 75 623 4:1 1 1/8 75 302 4:1 1 75 382 4:1 7/8 75 499
Have you thought this through? The same diameter of cylinder will move the same volume of fluid at the same pressure whether it has 1 or 2 pistons. In other words if you have to move 2 cubic inches of fluid, it doesn't matter if you have 1 piston moving 2 cubic inches or 2 pistons moving 1 cubic inch each. Since the wheel cylinders are the same, if the master cylinder is the same diameter it will have the same pressure and the same travel. It doesn't matter whether it is 1 piston feeding all the wheels or 2 pistons with 1 feeding the front brakes and the other feeding the back. I say if you are happy with the present brake feel and action, get the same diameter master cylinder. ........................................ If you want to make a change, a larger diameter will give a firmer pedal but you will have to push harder. May not matter if you have power brakes. A smaller diameter will give more braking for less pedal pressure but the pedal will go down farther. If you make a change in diameter be careful, a small difference in size can make a big difference at the pedal.
Don't fix it if it ain't broke. 1" dual should feel the same unless you have a booster and change to a different booster. For the "debate" A single well 1" master is pushing fluid to all 4 wheel cylinders with the same piston. A dual well master is also pushing fluid to all 4 wheel cylinders with the same piston. The only difference is the dual well has... DUAL wells!
Some of the info posted above is not correct............. When I did the dual conversion on my truck I found that using the SAME bore diameter as stock DID give the same pedal feel BUT when I cracked open the bleeder to test how it would work in an emergency with a failure I didn't have enough travel for it to actually have brakes....... so I went up a tad in bore size and it worked great. So, you may be OK with the same bore size, but be sure to test for failures in the front and rear... OR just go up in size a tad to be safe...there is no point in going to a dual reservoir IF you still loose all brakes in event of a failure!! A dual uses 2 pistons and a spring between them so when one side looses fluid that spring collapses all the way down taking a LOT of the pedal travel away before you actually start to push fluid with the other piston...and in some cases your pedal may be on the floor already!!! Please make sure you are safe!!!!
Very good advice. The "failures" can be easily checked and verified while bleeding, and making sure the open circuit master can be bottomed out before the pedal is critical to taking advantage of the dual master.
Hope there is something to that. I have found on my '62 Dodge Dart going from a stock drum/drum single 1.0" bore to a disc/drum dual 15/16" bore to be a bit spongy. The manufacturer that supplied everything right down the the frame clips says it should work. The whole system is new. None the less they are sending me a 1 1/32" bore master to try. If I set the parking brake it firms it up a bit, so some of it may just be adjustment of the rear drums. Bendix is coming out with a 1.125" bore dual MS for Mopar too I see.
The larger bore should fix it right up BUT as I mentioned double check your system with the new master they are sending by cracking open a bleeder in the front to test and one in the rear.
Isn't it ok since he has disc/drum...I thought the drums took more fluid travel, so to speak, than discs......so, to go to, say, dual discs, that might be an issue with a harder pedal? I know he's got both types, just sayin for reference sake.....and that I was considering dual discs on my 40 plymouth.
Using a smaller bore master will result in higher pressure out at the expense of longer travel for a given input. Just make sure the master can be fully bottomed out (open circuit) before the pedal hits the floor or firewall.
I'm having a problem also maby I can jump in I have a t bucket that had drum drum set up. Them swapped to disc in front. I have a rock hard pedal and does not stop for shit. I'm not sure of the bore. What bore size should I go to. I was told to go with a 1 in or smaller. Is that true?
You really need to know what brake parts you are working with before you start changing them, but generally a 1" master will work on most manual disc/drum systems, and is very common. Changing to front discs requires slightly higher pressure, and is why the dual 1" normally replaces the Lockheed 1 1/16" on old Fords when upgrading the brakes. Using anything smaller than 1" will almost guarantee the pedal will bottom before the master, something you want to avoid.
Ok the parts are ordered i am on call for work until Tuesday so it will probably be next weekend before I can get started. So i will keep you posted.
Disc brakes are harder than drum brakes because drum brakes are self energizing and discs are not. So it is not surprising they do not stop as well. 2 things you could do, change the leverage of the pedal or add a power booster. On a light car like a T bucket you should be able to get good braking without a booster if you have strength to give a good push on the pedal. As long as you have enough leverage. Or, a booster will make it stop like any car with power brakes.
Bloody brilliant test you did. I'll bet there are thousands of hot rods and customs out there that would not pass the same test you did. You didn't have enough stroke in the 1st MC. There wasn't enough volume (stroke) of brake fluid to fill the wheel cylinders and/or calipers when you hit the pedal hard with one circuit open. Most MC's with at least 1.1" or more of stroke will work with most any wheel cylinders (adjusted properly) or calipers out there. Grease gun analogy - If you took a regular size grease gun and one of those worthless minature ones and measured how much grease each gun put out with one complete pull of the handle, the smaller diameter gun would put out less. It's the same with master cylinders, the brake pedal arm is like the grease gun handle. Spongy pedal - If you're used to driving 50s and 60s barges with over boosted power brakes, a 1" or less diameter MC piston will naturally have more stroke in the brake pedal and is often described as spongy. Once you get used to the feel you'll find you can modulate the brakes on the edge of wheel lockup better with the longer travel pedal. If the car was a power brake car to begin with you may have to increase the available pedal travel to work with smaller diameter MC pistons. On my current Mopar there was a linkage to reduce pedal stroke because it was originally a boosted brake system. I had to remove the linkage, get a manual brake firewall bracket and drill the proper height hole in the brake pedal arm.
I know this is an older post but question on a smaller bore MS having a longer stroke. I understand this would happen on a single MS, to get the same volume, longer stroke. But on a dual MS, you are feeding only 2 wheel cylinders ( I know front and rear cylinder are not the same volume), so wouldn't the stroke be less to feed the 2 wheel cylinders? And the second system (cup) would fill the same volume as the first? So using the smaller single as an example, this would cut the stroke back by a half (1/2)? So the smaller dual would be less then the smaller single, but depending on the volume, smaller dual might be more stoke then the larger single? So am I correct? Or am I thinking too much? Please explain. Thanks.
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