Bore Size for Master Cylinders

[johnm]

Can you help, I am using a Dual Pedal, with two master cylinders for the brakes and one for the clutch. The Car is using C4 brakes all the way around, will 1 inch bore master cylinders for front and rear be adequate, or should I use something bigger.



John

[andrewb70]

If this will be a manual brake setup, then I think a 1 inch bore might be a little on the large side.

Do you know what your pedal ratio is?

Andrew

[johnm]

6.25:1, Keep in mind, it is manual, and I need a MC for the Front and one for the back.

THANKS!
John

[andrewb70]

I would probably start with something like 7/8" front and rear. The C4 calipers do not have huge pistons so the volume requirement is not super high. Also remember that with a dual setup you do not plumb a prop valve into the rear line. The bias is adjusted with the balance bar.

Andrew

[johnm]

Thanks for the help Andrew.

[chicane67]

7/8" front...... 1" rear.

[johnm]

Why a bigger one in the rear? THANKS!

[andrewb70]

Why a bigger one in the rear? THANKS!

The larger the bore the less pressure that will be applied to the caliper piston. With less pressure the rear will be less likely to lock up.

Thanks for the input Tom. I was hoping one of the ATS guys would speak up!

Andrew

[fuzzyonion]

Isn't that what the balance bar is for? Especially when the rear calipers are already about half the size of the front(?)
I thought the different size masters were mostly used to compensate for when all 4 wheels had the same calipers.

The 7/8" is used by some with a regular tandem master, C4s, and the 6.2 pedal found in first gen Fbodies.
Early C4s used 7/8 with power brakes.
Judging from posts, it seems to work OK, though most end up tweaking it to get the pedal more responsive. With dual masters you might be able to run smaller for a more responsive manual pedal.

Go to stoptech.com and read thru their technical whitepapers. There is a new one on setting up dual masters. Looks like they would recommend closer to 3/4" bore.

[chicane67]

Well.... there are two schools of thought here.

1. Run a 7/8 front with a 1.0 rear

or

2. 3/4 front and a 7/8 rear.

The problem we have is that there are disimmilar line sizes in a factory F body (and others as well). The front is 3/16 and the rear is 1/4. The biggest reason for the larger rear size is to move the volume required for the drum cylinder size.

I just recently ordered a 3/16, one piece line from InLine Tube..... for near dirt in cost..... just so that I may replicate the master and line sizes that have been used with the AP Racing six-pot front, four-pot rear brake set-up. I even inquired Tilton for master sizing.... and they stated 3/4 F and a 7/8 R. But....... some other cats in the business that run the same exact set up found that they liked the 7/8 - 1.0 set up much better with the 3/16 line front and rear. Anything smaller cylinder size..... and it locks up too easy as well as loosing the good modulation characteristics.

I kinda lean their way because we share alot of the same driving styles and have had the same chassis and brake set-ups in the past. Considering that, I also know that if it were in question...... purchasing another cylinder (3/4) for the pedal set-up is a cheap price to pay to get it right.

As for the balance bar.... that is a no brainer. Its the only way to fly and can be eaisly adapted to any pedal set up. Its nice to have that kind of adjustment at you fingertips.......

[fuzzyonion]

Well, now we have a discussion :D
I have to point out that the kits you mention might hold more volume in the calipers than the C4 and so would need larger masters. And also might have near the same volumes for front and rear calipers, possibly requiring different sized masters in addition to the bar. You dont mention their volumes.
But it is a matter of taste on how hard you like to push. I dont like to push, but I also dont like a soft pedal, so I guess I'm never really satisfied... but I accept that ;)

I know the C4's have much smaller rears than front calipers. I'm just going by the math though. It's the best place to start IMO. It tells me they should have the same size bore masters on a dual setup.

BTW, not all vehicles use 1/4" line to the rears. Many have 3/16" all the way.
I also have to differ on the 1/4" line needing more volume from the master... I cant wrap around that... the line is already full. It cant get any more full, so unless the metal line is swelling or flexing somehow, a larger bore master only means less pressure. The only fluid volume is being used by movement at the calipers, which is the same no matter the line size. You could check this against Pascal's law, and hydraulic principles. I could be wrong.

[SHANE 73Z]

I am running C4 sized (bore) calipers on my car as well. I am currently running dual 3/4" masters. With the poor pedal 5.2:1 ratio, I think my system leaves aliitle to be desired. I will be installing my in car bias adjuster this weekend, so I can do some fine tuning.

I am curious over the debate as far as the line size issue. I am still running stock line sizes, so this may be another aspect for me to look at.

Shane

[andrewb70]

I am running C4 sized (bore) calipers on my car as well. I am currently running dual 3/4" masters. With the poor pedal 5.2:1 ratio, I think my system leaves aliitle to be desired. I will be installing my in car bias adjuster this weekend, so I can do some fine tuning.

I am curious over the debate as far as the line size issue. I am still running stock line sizes, so this may be another aspect for me to look at.

Shane

Shane,

Can you please elaborate as to what you feel the system is lacking. The difference between 5.2:1 and 6.2:1 is significant.

Andrew

[SHANE 73Z]

Andrew,

The current setup lacks what I would call good "initial bite". It takes quite a bit of pressure before the brakes start to get any feel to them. Also, I still cannot get a front wheel to lock for any reason.

Some may say that you dont need the wheel to lock, but I would say that a locking point is a good place to judge your systems performance and capacity down from that point. I like a system that you know where you are at in its travel/effectiveness.

Shane

[SHANE 73Z]

Dam net nanny/double posts!!!!!!!!!!!

Shane