Some Brake questions if you guys don't mind,

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

DANO65 wrote:

---

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.

__________________________________________________________________________________________

 The master cylinder bore is the same for both the 1965 single master and the 1967 Chevelle dual circuit master, 1 bore. 

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!

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.

It provides the pivot point for the old school "through the floor" brake pedal.

You could probably build and fab your own pivot point bracket and remove the master, but it likely would never be a robust as this casting.

You can see the pivot point here, to the right.

Yep, good idea, we are looking at that as well Bruce. We need to build a mockup to find the length close to what we'll need first though.

Problem is, I have no master core to build off of yet! Soon.

Good advice

10lbs. at the pedal creates 60lbs Force at the master (6:1 ratio)

Force/Area = Pressure.       60 lbs / .7854 sq in = 76.39 psi

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).