Master cylinder for Scarebird brakes

Moderator: Moderators

Post Reply
MilesnMiles
Posts: 7309
Joined: Fri Nov 04, 05 8:40 pm
Location: Cornwall

Master cylinder for Scarebird brakes

Post by MilesnMiles »

I’ve recently acquired an old stock Scarebird disc brake conversion kit for my Coronet. Looking at various US threads on the topic I’m concerned about what master cylinder to choose. The kit uses the large Dodge Ram calipers which are about the biggest available.
Just wondering if anyone has done a similar conversion or can provide some advice.
User avatar
Dave999
Posts: 9427
Joined: Thu Feb 24, 05 10:31 am
Location: Twickenham,London, England

Re: Master cylinder for Scarebird brakes

Post by Dave999 »

this might help see attached
table 2 is the relevant bit to identify a car that has the cylinder style you want before you hit rockauto

you need a disc drum one

original Master Cylinder for Dodge Ram- was a Large Bore 1 5/16 inch bore.

id suggest that was specified not only for the caliper but also for the length of the peddle

suggest one that is narrower gives longer peddle travel, easier effort more progressive brakeing but if you go too narrow, peddle will hit floor before the brakes are fully on

if you can get a B or E body one disk drum with the pipes in the right places in theory you should not be too far wrong

A brake bias for the rear might also be nice if they tend to lock up first.
in an ideal world you'd put 9 inch post 73 drums on the back. don't think its easy....to regain some balance, big 10 or 11 inch 1960s duo servos may well be ditch finders when they have to operate without their drum based partners up front, when the pressure needed to work the front disks is applied to them via their rear circuit .i.e you foot activates both..when the fronts just start to grip the rotor the rears may well be 98% full on
hnce some kind of bias may be necessary

whilst some funky looking current mopar master cylinders fit or fit on a 2 bolt to 4 adapter they do not have the machined place for the pushrod retainer rubber to fit into.... i'd be wary

Dave
Attachments
Moparts_Complete_Guide_to_Mopar_Master_Cylinders_1967-19746.pdf
(1.03 MiB) Downloaded 51 times
The Greater Knapweed near the Mugwort by the Buckthorn tree is dying
MilesnMiles
Posts: 7309
Joined: Fri Nov 04, 05 8:40 pm
Location: Cornwall

Re: Master cylinder for Scarebird brakes

Post by MilesnMiles »

Thanks Dave, good stuff as usual. Didn’t realise how big the Ram M/C was. I have read that a 1 1/8” will suffice, it as you say, it’s making it compatible and yes, I’ll be having a proportioning valve fitted.
I’d use Dr Diff, but he doesn’t list a 1 1/8”. Im looking for a decent pedal and it seems it’s not always straightforward.
User avatar
Dave999
Posts: 9427
Joined: Thu Feb 24, 05 10:31 am
Location: Twickenham,London, England

Re: Master cylinder for Scarebird brakes

Post by Dave999 »

depend as well on if you have a servo or not

for exmaple a servo and bigger mastercylinder that would be hard work on the leg to brake if it was unassisted would work well with the servo helping

if you are going full manual than a thinner bore is necessary becasue you have no assistance from the servo

its all just a pressure= Force/Area thing presure in the system being higher applies the brakes harder

if the master cylinder was a block of wood and the peddle was pressing a drawing pin into it... you could do it with 1 finger very little force, concetrated at the tiny end of the pin = massive pressure in that small area and it goes into the wood
if you repalced the drawing pin with a 10 mm bolt it aint going in to that block of wood without two feet pressing... force is spread over a circular area that is huge in comparison pressure at the wood is not high enough to presss it in

same thing going on
narrow cylinder you can make much higher pressure in the system for the force of 1 leg than you get with a fat master cylinder

takes much more force to get the same pressure in the fluid in the system with the fatter cylinder

and becasue the cylinder is round the area = PixR^2 pi r squared area of a circle, the ralationship has a squared bit in it so its not even linea
a small increase or decreas in the radius is multipled by itself in the r square part
so doubling master cylinder size does not double the effort necessary, the graph aint a line its an exponential curve its way way more effort

if advice says 1 1/8 works then thats a goodun BUT was the advice for servo assisted or Manual brakes

if the poop really hits the fan, you can allways raise the pivot hole in the brake paddle 1/2 an inch drill a new hole and get a lonegr peddle for more leverage i.e distance from your foot to the pivot is made longer


Dave
The Greater Knapweed near the Mugwort by the Buckthorn tree is dying
MilesnMiles
Posts: 7309
Joined: Fri Nov 04, 05 8:40 pm
Location: Cornwall

Re: Master cylinder for Scarebird brakes

Post by MilesnMiles »

Still got the teacher skills mate😬. Yes, full manual and I belive there is jiggery pokers sometimes necessary with the the push rod. I like the adjustable one my Roadrunner had: did it a bit tight and brakes stuck on after the first push🙈
User avatar
Pete
Posts: 21944
Joined: Sun Apr 18, 04 10:49 pm
Location: MMA Chairman

Re: Master cylinder for Scarebird brakes

Post by Pete »

In this scenario, Pedal Ratio is king, 7:1 or better....
Pete Wiseman; Cambridge.

Mopar by the grace of God
User avatar
Dave999
Posts: 9427
Joined: Thu Feb 24, 05 10:31 am
Location: Twickenham,London, England

Re: Master cylinder for Scarebird brakes

Post by Dave999 »

MilesnMiles wrote: Thu Apr 28, 22 2:28 pm Still got the teacher skills mate😬. Yes, full manual and I belive there is jiggery pokers sometimes necessary with the the push rod. I like the adjustable one my Roadrunner had: did it a bit tight and brakes stuck on after the first push🙈

got my GCSEs coming up...well my daughter does so it all front of mind :) apoligies if teaching to suck eggs it just all came out bit of a slow day :)

Dave
The Greater Knapweed near the Mugwort by the Buckthorn tree is dying
MilesnMiles
Posts: 7309
Joined: Fri Nov 04, 05 8:40 pm
Location: Cornwall

Re: Master cylinder for Scarebird brakes

Post by MilesnMiles »

All good stuff chaps 👏
Now I just need to calculate how to get that 7:1 ratio that Pete mentioned.
Post Reply