Single to dual tandem on a stock manual drum system with a 6:1 pedal ratio. Do we keep the master bore the same as in the stock single reservoir? It's something I've wondered about.
Lets say, the car has a 1 1/8" single master. Its a old 50 Plymouth, so that old single master drives 4 single piston wheel cylinders up front, and 2 dual piston on the back, so in effect 8 pistons of area in total (7.8 in2). All the wheel bores are 1 1/8" (.975 in2).
So now, we add a dual tandem master, same 1 1/8" bore but times two. Essentially we will now drive 4 wheel pistons (now down to 3.9 in2) with one master piston, the other 4 (3.9 in2) with the other. So the total area for that (Each) 1 1/8" piston to react against is 1/2 of the 7.8 it was previous.
Logic tells me each master piston will have to move a significantly shorter distance for the same reaction (distance) the single master had on the wheel end before.
Will we end up with a hard pedal? Less travel? Less pressure for the same push? Or will it be the same feel as before? Is it all the same regardless?
Should (do we need to) drop down in bore size and trade some travel for pressure? Or have I lost my marbles?
Thoughts?
Cheers, Mark
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65 Laurentian post, 67 Grande Parisienne 4 door HT.
Mark... Can you try doing a comparison on another vehicle? Maybe a 66 Pontiac, for example and ease? You could buy a stripped 6cyl with no PB, then a loaded up Parisienne with PB. Then do a comparison like you have done above to see the pedal ratio and the movement of the pedal....
What if you just swap the master cylinder and then used an adjustable (length wise) push rod? (maybe more questions that confuse the issue)
One of the aftermarket master cylinder makers..... You can't be the ONLY guy who has had this problem...
Hey Mark all I can say is what I did without all the thought being put into it. When I redid the chassis for my 65 I swapped out the original single system master for a dual master from a 67 Chevelle as my Dad had a nearly new one he took off when he went to disks. It bolted right to my booster no issues and works just fine. Pedal feel is real nice. I do believe there is a little more travel and the brakes are not as supper sensitive as they were stock. I always found the stock power brakes to be a little too touchy.
By most accounts what I did was wrong, not using an Impala master but it seems to work just fine.
Hey Mark all I can say is what I did without all the thought being put into it. When I redid the chassis for my 65 I swapped out the original single system master for a dual master from a 67 Chevelle as my Dad had a nearly new one he took off when he went to disks. It bolted right to my booster no issues and works just fine. Pedal feel is real nice. I do believe there is a little more travel and the brakes are not as supper sensitive as they were stock. I always found the stock power brakes to be a little too touchy. By most accounts what I did was wrong, not using an Impala master but it seems to work just fine.
Probably just over thinking it. i just stick to somthing similar in size. as stated, 65-67 chevelles there is no more than a 1/16 difference in any wheel cyls between the 2 years, same hold true for fullsize. and all those swaps and conversions work just fine, i have a 67 dual on my 64/65 chevelles, and i have done the same with a 67 impala on a 65. all works great!
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Beaumontguru
MY BEAUMONT HAS 4 STUDDED TIRES AND 2 BLOCKHEATERS......AND LOTS OF OIL UNDERNEATH. The other one has a longer roof.
Thanks Guys. The fact you've stayed with basically the same bore when converted with no issues makes me feel better and answers the questions.
Overthinking things, made up a little visualizer diagram comparing Stock to the new dual. All pistons are 1".
Move A 1" and B should move .125". 1" of motion at A divided by 8.
Move C 1" and D should move .250" . 1" of motion at C divided by 4.
Just need some real math on it now. Anyone care to calculate force at each B/C? Lets say 10 lbs of pressure on the pedal to make it easier.
This Plymouth conversion is basically a total MacGyver. You take the old master, gut it, mount the new dual behind it on a DIY bracket, and run an adjustable new rod right through the old master to the new.
you were the guy to lead me down this path.... Why not using fabbed rod with heim joints? Has to be as strong/adjustable too! Might be the answer to your 'ratio" problem...
Drawing board...So you have nothing to mock up. Then "hotrod" it. Wilwood has all kinds of Mc's ? (try Hortons Rods in Milton)
Use this line of thought.. I'm building the car from scratch like a hotrod, building your own frame, suspension etc. So you have everything installed except the MC. Your looking at it from the MC to the brakes, maybe reverse the line of thinking from the brakes to the MC....
Just bouncing it around. It doesn't make sense, having to use the old MC to make this work, redundant as I see it.
We have ALL seen your other work... you have WAY more marbles than most of us!!!!
Hortons catalog is on line, Wilwood too, any hot rod shop, Heidts
Pressure is equal throughout the closed system, each cylinder will see the same 76.39psi (pressure will only build after ALL take up travel and resistance is met).
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72 Nova SS - Minitubbed 70 Nova SS - #'s L-78 Bench Stick 68 Acadian SS clone - factory air 67 Chevelle rag - SS 427 clone
