Wilwood master cylinder sizing recommendations

[keith4909]

Switching rear drum to discs soon. Have a 7/8 master now. Pedal has a lot of travel. Pressures check out really good. Is it best to stick with the 7/8 or switch to the 15/16th master. I don't like how far the pedal travels. Will I sacrifice safety and performance with a slightly bigger mc.

1967 Gto
Front
12.88/6 piston front
12.88/4 piston rear
Thank you

[hotrodalex]

Bigger MC bore will reduce pedal travel and increase effort, so that sounds like good thing for your situation. I just switched to rear disks and the pedal travel increased, so I think I need a larger MC as well.

[andrewb70]

Sean,

Please post your tech questions in the appropriate tech area and not in the open discussion. Thanks.

Andrew

[keith4909]

What size is your master now. I know Wilwood has 4 sizes. Will the 15/16 be adequate? Any thoughts?

[hotrodalex]

I'm running power brakes, so I have a 1". But the 15/16" is popular with manual brakes.

[Peter Mc Mahon]

Your pedal has a lot of travel until it engages or has a lot of travel through the actual braking?

[Kcorrigan2]

You never did answer if you have power brakes (vacuum assisted) or purely manual. This makes a difference. With power brakes you can run 1.125" bore. The tipping point is 1"....depending on the above question. I personally like to know that I can always get plenty of pressure at the calipers and would sacrifice a bit of pedal travel for that...now if the pedal travel is excessive i would go up one bore size. You will not sacrifice safety at 15/16". You will have to use a bit more leg if your brakes are not assisted but it should not be excessive.

[keith4909]

Through braking. Works right away but can go all the way to the floor if pressed very hard. 7/8ths now. Manual brakes. Sorry

[andrewb70]

Does your pedal bottom out on the floor? Does the MC bottom out? I run a 7/8" MC and I have a long pedal but the brakes work very well.

Andrew

[Apogee]

You have the 7/8" bore MC now, so why not try it out with the new rear disc brakes before you make your decision? The rear discs will require less volume for actuation than the drums do, so your pedal should only improve. Whether it improves as much as you'd like remains to be seen. Stepping up front the 7/8" bore MC to a 15/16" unit will decrease both your pedal travel and the output pressure for any given effort by about 12% compared to the 7/8" bore unit.

Tobin
KORE3

[keith4909]

So I know the Wilwoods master cylinders have a 1.1 stroke. Are there master cylinders with the same 7/8 bore and possibly a longer stroke? I did the math and with a 5.5/1 pedal ratio and 100 pounds of force each master cylinder will produce

917psi 7/8
797psi 15/16

I would need to apply 100-130 ft lbs to obtain the correct pressure with the 15/16 m/c

I'm a big dude and leg strength is no issue.

[Apogee]

The '67 GTO should have a 6 to 6.25:1 manual pedal ratio depending on where you measure to on the pedal itself, so that should only improve things versus your numbers above. I don't think that longer stroke MC's are the answer unless your floor boards are further away from your brake pedal than they are in most A-body applications. I personally would run the 7/8" bore unit and increase the brake pad CoF if you wanted to increase the brake torque for any given pedal input. IIRC, 115# of pedal force is the maximum allowed force to achieve lockup per the DOT FMVSS (Federal Motor Vehicle Safety Standards).

If you still feel the need or desire to reduce your pedal travel by 12% and increase your pedal effort by the same amount, then you could change over to the 15/16" bore MC.

Tobin
KORE3

[CTSV]

you might need to experiment because everyone's opinion on brakes is very different.

i now have a 15/16 baer remaster with the wilwood aerolite 6 all around and it feels great. im finally happy with it and i am not touching it. i tried the 1 inch remaster and the car just didnt stop at all. you must consider piston area and pads and exactly what you want out of it. i am running BP40 pads all around and they are awesome after i tried a dozen or so other pads over the years these are for keeps.

[keith4909]

Like I said. The pressures are great. The travel just makes me uneasy. I can get the pedal to almost hit the floor Will the 4 piston caliper in the rear instead of the stock drum make the pedal harder assuming I keep the 7/8 mc. Which uses more fluid? The wheel cylinder or the caliper? A second note. I could very well be off on the pedal ratio. If Tobin is right my pressures will be great with the 15/16. It sucks,that size wasn't available from Wilwood a year ago when I bought the 7/8 master cylinder.

[Apogee]

Like I said. The pressures are great. The travel just makes me uneasy. I can get the pedal to almost hit the floor Will the 4 piston caliper in the rear instead of the stock drum make the pedal harder assuming I keep the 7/8 mc. Which uses more fluid? The wheel cylinder or the caliper? A second note. I could very well be off on the pedal ratio. If Tobin is right my pressures will be great with the 15/16. It sucks,that size wasn't available from Wilwood a year ago when I bought the 7/8 master cylinder.

Per my post (#10) above, the wheel cylinders require more volume for actuation than the Wilwood FNSL4R calipers with either 1.98 or 2.46 square inches of effective piston area, so your pedal should improve slightly with the conversion to disc. Any air in the system will extend your pedal travel as well, so it should go without saying that you should not judge your pedal travel until the system is fully bleed.