Master cylinder thoughts please

[E55]

I’m looking to upgrade my 7 to dual circuit brakes and trying to decide which master cylinder to buy. I currently have a old Girling .625” that’s leaking. Caterham sells a standard .625” bore tandem MC for about $250+ but I can buy a .70” bore tandem MC from a Triumph Spitfire locally for about $65. 
 

Has anyone used the Spitfire unit who can provide some feedback or perhaps there’s another model I’m overlooking? It appears to be designed to originally mount at an angle hence the slanted reservoir. Does this matter?

https://apaindustries.com/parts/5930
 

Thanks!

 

[dallasdude]

I do know the casting number on both is the same.  I have been meaning to order one from Moss Motors and see if they do in fact interchange.  Most everything else is Spitfire in the braking system....might make sense they used the MC less the reservoir also.

[escondidoron]

Please note that the pedal effort required to attain any given braking force at the wheel is directly proportional to the master cylinder diameter.  So increasing master cylinder bore diameter increases the force required by the driver's foot to achieve the same braking force as you have presently with the smaller diameter MC.  If my calculation is correct going from a 5/8" MC (0.307 sq-in) to a 0.7" bore (0.385 sq-in) will yield a 25% increase in pedal effort required to achieve the same stopping force that you have presently.

[fastg]

escondidoron in correct, Hydraulics can are counter intuitive. It all about areas not diameter's. If you have a master cylinder that has a 1 Square Inch piston and you apply 100 pounds pressure to the pedal your hydraulic system is pressurized to 100psi. So if the area of the caliper pistons pressing on the pads is 2 Square Inch you have 200 pounds pressure at the pads. If you put in a larger master cylinder lets say 2 Square Inch but you still have the same 100 pounds pressure on the pedal you now only have 50psi in your system, with the same 2 square inch caliper pistons you only have 100psi at the pads. You have just halved your braking pressure.  But your pedal will feel a lot firmer as it will take 1/2 the pedal travel to move the pads the same amount.

 

Graham 

Edited March 26, 2021 by fastg
Spelling, not my strong suite

[E55]

Thanks for all the comments!  I have read some posts around the web of others using the .7" bore in place of the .625" so I figured I would give it a shot.  However, admittedly I did not run the actual calculations.  Also so far I have been unable to find a suitable replacement reservoir.  I will go ahead and buy the unit from Caterham with the matching bore size.

 

While I have your attention, what are your thoughts on proportioning valves etc?  What is the simplest way to account for the 9" drum brakes on the rear and Girling 14LF calipers up front?

[papak]

Check Wilwood. They may offer one.

[E55]

They were the first place I checked. Smallest bore is 1.00” for their tandem M/C. 

[TheDingo8MyBaby]

On 3/26/2021 at 12:13 PM, E55 said:

While I have your attention, what are your thoughts on proportioning valves etc?  What is the simplest way to account for the 9" drum brakes on the rear and Girling 14LF calipers up front?

 

I have 14LF's and 8" drums out rear (and a 0.625MC). The prior owner installed a proportioning valve plumbed to the drums and it is worthless as the car will always lock the fronts first. Generally I thought 14LF's and 9" drums is a pretty balanced setup. Are you saying that you have too much rear brake or too little? 

[E55]

I have only driven the car a handful of times so I am unsure.  It was parked for a long time and needed a major refresh.  I got it started and couldn't help but drive it a bit but then began tearing into it and decided to replumb the brakes because it was a real mess and stuff was leaking.  I am just trying to be proactive but based on what you say I may be better off not installing any specialized valves and see how the brakes perform after logging some miles.