Dual master cylinder bore size

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jtschug
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Dual master cylinder bore size

Post by jtschug »

I know this has been discussed before, and it has been on my To-do list for a long time, but it is bubbling up to the top of my list.

I am refurbing the 9 3/8" rear axle out of my parts car. I'm going to remove the center section and replace with a nodular 9", 3.5:1 gears, and a TruTrac limited slip differential. It will get new seals and new wheel bearings, but I'm keeping the 31 spline axles.

While I am at it I am replacing the read drums with this 4-piston Wilwood kit:
https://wilwood.com/brakekits/BrakeKits ... =140-11402

So the system with have the stock Kelsey-Hayes 4-piston calipers in the front and now these new 4-piston calipers in the back.

I'm debating between a 1" bore master cylinder and a 1-1/8" bore.

Looking around at other cars, it seems I have the smaller Bendix type booster. My MC sits almost 2" further back than other cars I saw at the recent All T-bird show. I wonder if I have space to fit a GM Universal style like this which comes in either 1" or 1-1/8":
Image

It is more expensive, but I would be willing to pay more to get this one if it would fit without cutting the shock tower brace also available in 1" or 1-1/8":
Image

However there are many more options if I only need a 1" bore Master Cylinder. Does anyone have any thoughts on the bore size I need?

BBlendo
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Re: Dual master cylinder bore size

Post by BBlendo »

I am in the middle of a resto on a 65 convertible i went with Baer products for Mc and rear disc the MC is a REMASTER ALONG WITH A HYDROBOOST FROM HYDRATECH WITH THE HYDRO THEY RECOMENDED A 1 1/8 bore to soften the pedal a 1" was hought to be too sensitive for. A hydro boost app you may want to check with wilwood to see what they recommend for there mc i also used a saginaw type ps pump to increase pressure max @1500psi compared to original @ 850psi.
Regarding rear brakes before you install new bearings you may want to dry fit rotors I didn't and had take it all apart reove the axle flange and turn down the hub to accoomadate the parking brake top hat but wilwoods may be different if you are going to use a parking brake good luck and it was definetly worth it these are the best brakes i have ever had!!!
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stubbie
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Re: Dual master cylinder bore size

Post by stubbie »

I'm no brake expert but if you have more pistons then you have more surface area involved with stopping and will affect the pedal in some way. I might be tempted to go bigger but as I said I'm no expert on brakes. I was reading an article about brake piston number(1,2,4) and area affect on pedal ratio last week. I'll see if I can find it. There are calculators on the net that may help. Or ask Willwood when you buy the kit.

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RedBird64
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Re: Dual master cylinder bore size

Post by RedBird64 »

I spent countless hours researching this very question for my own application of Disc/drums.
The bigger the bore, the more power you have at the calipers but it also increases pedal effort significantly. I have a 1" bore with my 65~67 setup and it's well balanced but that's with rear drums. However, with the hydroboost you will have plenty of power assist and since you will have discs in back (which take more power than drums) I would definitely go with a 1 1/8" bore if you have room for that fat of a MC.
FWIW, I am positive you will not be as happy with a 1" bore.

A couple of weeks ago someone started to make an illegal u-turn by turning in front of me. It was a great feeling to stab the brakes and have it stop hard and straight without burying the pedal.

I'm totally envious of the system you're building and may have to shamelessly copy it!

Scott
Last edited by RedBird64 on Sat Nov 02, 2019 4:56 pm, edited 2 times in total.
1964 Coupe Wimbledon white/Rangoon Red w/black int. Owned for 42 years. It was my folks car before that (second owners). VTCI # 12013.

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redstangbob
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Re: Dual master cylinder bore size

Post by redstangbob »

I would consider your original single M/C is 15/16 inch bore and supplied fluid for both front calipers as well as the rear cylinders, also 15/16 bore. You'll now have a M/C piston supplying the front only, and another M/C piston supplying the rear only. Going larger on the bore will affect pedal feel, you'll have lots of capacity IMO. good luck, Bob C
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RedBird64
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Re: Dual master cylinder bore size

Post by RedBird64 »

It takes some time but you can also research Late 60's and 70's vintage American cars that had disc/drum or disc/disc. They kept getting bigger MC's as brakes evolved. You will find a lot of them had 1 1/8 bores. Some were even bigger!
Your hydroboost is sourced from the HD truck market. They have huge MC's so assist isn't going to be an issue.

S
1964 Coupe Wimbledon white/Rangoon Red w/black int. Owned for 42 years. It was my folks car before that (second owners). VTCI # 12013.

stubbie
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Re: Dual master cylinder bore size

Post by stubbie »

Quote - The bigger the master cylinder is, the more fluid you get but less pressure at the caliper. The smaller the master cylinder is, the less fluid you get but you’ll have more pressure at the caliper

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RedBird64
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Re: Dual master cylinder bore size

Post by RedBird64 »

stubbie wrote:
Sat Nov 02, 2019 10:26 pm
Quote - The bigger the master cylinder is, the more fluid you get but less pressure at the caliper. The smaller the master cylinder is, the less fluid you get but you’ll have more pressure at the caliper
I've seen that on the Google search. Problem is, that doesn't take everything into acount like pedal effort, power assist and caliper volume. All these things not only change the effort, they change the overall travel of the caliper. Too small a MC could result in having to get the pedal down to the floor in order to move enough fluid to apply the brake for a light braking action. A hard, panick stop may be impossible. The quote leaves out that while the pressure is lower with a bigger MC and no other changes, it's actually the same or higher with the appropriate assist for a set amount of foot pressure and travel. Keep in mind that the OP will have way, way more assist than the old system cold ever have mustered. The wrong sizing could cause him to end up with dangerously grabby brakes.
So I would not take that quote and make any choices from it alone.

Frankly, I think the stock disc/drum setup would be better off with a 1 1/8 MC but there's no way the booster could support that. Pedal effort would be like having manual brakes!
Cars with a similar setup had 1 1/8" MC a few years later but they had much better boosters - 10~12" or better.

Scott
1964 Coupe Wimbledon white/Rangoon Red w/black int. Owned for 42 years. It was my folks car before that (second owners). VTCI # 12013.

jtschug
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Re: Dual master cylinder bore size

Post by jtschug »

So, I've got the car out a couple times with this Wilwood setup, and I definitely do not have enough boost.

The first half of the pedal travel is great, and it slows down much faster than it did with drum brakes, but then it seems to just hit a wall and the next part of pedal travel is like manual brakes. In a dry parking lot at 40 MPH, I can't lock the brakes even with both feet pushing on the pedal. It isn't unsafe, it stops maybe slightly better than before, but it isn't the big improvement I was hoping to get. It was a real pain getting the MC in, and I had to do it twice because the push rod was too long and the brakes dragged, but now I'm thinking I need either a bigger booster or hydroboost.

stubbie
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Re: Dual master cylinder bore size

Post by stubbie »

There is also that if you increase or decrease the master cylinder size then you also need to make that adjustment to wheel cylinder size.

jtschug
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Re: Dual master cylinder bore size

Post by jtschug »

By adding disc brakes in the back I did change the wheel cylinder size, which is why I changed the master cylinder to 1 1/8”

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