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Old 05-14-2020, 11:12 PM   #1
nikwho
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What master cylinder and proportioning valve for disc/disc

Hello all,
Quick question about my brake system setup, specifically regarding my master cylinder and proportioning valve. My '59 Apache has a '79 Camaro sub-frame and 10 bolt rear axle. Has stock diameter front cross drilled/slotted rotors with Wilwood dual piston calipers. Just wrapped up an LS1 rear disc brake swap, with matching cross drilled/slotted rotors and single piston calipers.

It currently has the booster/master cylinder/proportioning valve from the same '79 Camaro donor car. I now want to upgrade/replace the booster, master cylinder and proportion valve. I'm pretty happy with the firewall mounted setup, with the Camaro pedal, etc. I'm fairly open to keeping with firewall mounted, or frame mounted setup, though I'm not so sure that the frame mounted setup would fit, given my Camaro sub-frame swap.

So, assuming that I stay with the firewall mounted setup, what should I be looking for regarding master cylinder bore diameter? Booster diameter? Do I need to buy a disc/disc proportioning valve? I've currently got a Wilwood adjustable proportioning valve installed in line with the rear brake line. FWIW, I also have a line lock on the front brakes. My understanding was that the Wilwood adjustable valve would make my setup work, after some tuning, but upon further reading, I have read that the disc/drum proportioning valves (like mine) delay fluid reaching the front brakes, due to drums requiring more time (and fluid?) to actuate.

So, do I need to swap proportioning valves? What master cylinder bore diameter should I seek out? What effect will different bore diameters have on brake/pedal feel? What diameter booster diameter should I look for?

My 383 has about 10:1 compression, with a fairly mild cam that pulls decent, but not a ton of vacuum. I may run more cam in the future, that will decrease vacuum, and may end up needing an electric vacuum pump for the brakes.

Any help would be greatly appreciated!

Nik
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Old 05-14-2020, 11:16 PM   #2
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Re: What master cylinder and proportioning valve for disc/disc

Pic of the old truck as she sits.

ETA: truck currently has single piston calipers up front, but I have these dual piston Wilwood calipers enroute currently.
https://www.summitracing.com/parts/w...t/model/camaro

Thanks!!
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Old 05-15-2020, 01:55 AM   #3
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Re: What master cylinder and proportioning valve for disc/disc

you shouldnt need a proportioning valve for a disc/disc, usually the proportioning valve is needed on a disc/drum setup because the disc needs far and away more pressure to clamp than drums (why power boosted brakes became a thing around the time discs did). if a prop valve wasnt installed it would lock the rear brakes before the fronts even started clamping.

so a disc/disc setup properly balanced (in rotor size etc, not pressures) can use a regular master cylinder. if you really upsized the rears ( a waste of money, in my cruddy opinion, front brakes do the work in 99% of driving) or are experiencing lockup, then you may need a prop valve.

98-03 chevy blazers were around the same weight as a half ton fullsize truck, and had 11" front rotors with dual piston calipers and small rear discs with single piston calipers, so that master cylinder would probably work. but get the fittings from a salvage yard and install them on your lines, they are weird sizes. by the way, the blazer has no proportioning valve from the factory.

thats just my suggestions, others will have more.
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Old 05-15-2020, 04:18 AM   #4
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Re: What master cylinder and proportioning valve for disc/disc

nikwho

good lookin truck-------nice stance
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Old 05-15-2020, 11:01 AM   #5
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Re: What master cylinder and proportioning valve for disc/disc

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nikwho

good lookin truck-------nice stance
Thank you, sir! Been playing with this truck for the past 12 years, off and on. I got it where I like it, though, it sits REALLY low. Next up is my rear four link and coil-overs on all four corners. This little truck is fun! Feels like driving a slot car. I'm hopeful to get paint and body work done this year, but wanting to find a big window cab, or do a big window swap, as well as find a nice fleetside bed first. Getting closer!
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Old 05-15-2020, 11:05 AM   #6
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Re: What master cylinder and proportioning valve for disc/disc

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Originally Posted by joedoh View Post
you shouldnt need a proportioning valve for a disc/disc, usually the proportioning valve is needed on a disc/drum setup because the disc needs far and away more pressure to clamp than drums (why power boosted brakes became a thing around the time discs did). if a prop valve wasnt installed it would lock the rear brakes before the fronts even started clamping.

so a disc/disc setup properly balanced (in rotor size etc, not pressures) can use a regular master cylinder. if you really upsized the rears ( a waste of money, in my cruddy opinion, front brakes do the work in 99% of driving) or are experiencing lockup, then you may need a prop valve.

98-03 chevy blazers were around the same weight as a half ton fullsize truck, and had 11" front rotors with dual piston calipers and small rear discs with single piston calipers, so that master cylinder would probably work. but get the fittings from a salvage yard and install them on your lines, they are weird sizes. by the way, the blazer has no proportioning valve from the factory.

thats just my suggestions, others will have more.
Thank you! That all makes sense. I'm curious about master cylinder bore diameter. Seems as though I have read that smaller MC bore diameters give more pressure. So, I would imagine that a larger MC bore diameter would give more feel?

I guess that I will omit the disc/drum proportioning valve, but leave the Wilwood adjustable valve in place to fine tune it. Makes me wonder why they sell disc/disc proportioning valves, though! There must be a purpose!?

I just want to get it figured out, so I buy the correct parts.

Nik C.
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Old 05-15-2020, 12:43 PM   #7
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Re: What master cylinder and proportioning valve for disc/disc

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Makes me wonder why they sell disc/disc proportioning valves, though! There must be a purpose!?

big rear brakes, disc or drum, will lock up early because of weight transfer to the front/unloading of the rear wheels in panic stops. a factory disc/disc setup had literally hundreds of engineering hours in design and testing, where a disc rear upgrade is usually a matter of what fits. so you may need a disc/disc prop valve, especially in a racing or high performance/track application. I wasnt saying you wont need it, in fact I said you shouldnt need one. testing, thats where you will find out. and if you are leaving the wilwood valve in you will be well equipped if you need it.
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Old 05-15-2020, 02:24 PM   #8
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Re: What master cylinder and proportioning valve for disc/disc

Absolutely. Under heavy braking, the rear of the truck is unloaded to varying degrees (depending on many factors) and the front is loaded. So, equal braking force between them would cause the rear to lock prior to the front. My understanding is that with the adjustable Wilwood valve that I have, I will be able to adjust that bias into my brake system by slowly decreasing the pressure to the rear brakes until the front and rear brakes are locking at essentially the same time.

So, seems as though that is mostly worked out. Are there any brake guru's out there that can help me pick my master cylinder bore diameter? Am I correct in my assertion that smaller bores give more pressure to the brakes, while larger bore diameters would give more feel? I was tentatively planning on a 1" MC bore, but have seen 7/8", 15/16", 1", 1 1/16" and 11/8", I think. I figured that I should be in the 15/16" or 1" range, but only because those are quite common, not because of any data or actual information! LOL.

Thanks much!
Nik
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Last edited by nikwho; 05-15-2020 at 02:37 PM.
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Old 05-16-2020, 12:00 PM   #9
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Re: What master cylinder and proportioning valve for disc/disc

Nice Looking Truck!!!

I agree with joedoh
I am running disk/disk and am using a stock S10 master cylinder and stock AC delco booster along with the original factory OEM proportioning valve that I have never adjusted. My truck's brakes are perfect!

Also side note- there is a line size difference with OEM steel lines. If my memory serves me correctly the drums need the larger size steel lines that travel down the frame. Disk typically use the 3/16" diameter line. Drums 1/4"

That being said... I just used the stock 1/4" OEM rear S10 line (S10 originally used drums on rear) that travels my frame back to rear axle. with this set up-

My truck will stop on a postage stamp.

The ports on the master cylinder front and rear are sized 3/16" and 1/4" as well.
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Old 05-18-2020, 11:42 AM   #10
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Re: What master cylinder and proportioning valve for disc/disc

As far as bore diameter, it might be worth calling Wilwood, or one of the other aftermarket companies, and explain to them exactly what your combo is. They can then suggest the best size for your application. Even if you don't buy their product, it would still be worth your time to call the people who deal with these things on a daily basis.
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Old 05-18-2020, 02:38 PM   #11
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Re: What master cylinder and proportioning valve for disc/disc

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Originally Posted by Rickysnickers View Post
As far as bore diameter, it might be worth calling Wilwood, or one of the other aftermarket companies, and explain to them exactly what your combo is. They can then suggest the best size for your application. Even if you don't buy their product, it would still be worth your time to call the people who deal with these things on a daily basis.
Good idea! I would actually like to buy a Wilwood MC. I've always liked their products. Ill contact them.

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Old 05-18-2020, 09:13 PM   #12
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Re: What master cylinder and proportioning valve for disc/disc

good idea to contact wildwood, since you bought wildwood, they should be able to help you out with a bore size.
the S10 idea for booster is a great idea. like said, trucks are similar in size. one thing to note is that some of the S10 boosters are dual diaphragm and some are single. try rock auto for some part numbers or option numbers. even the stock S10 master would likely be fine, you could try a used one for cheap and if it works, you're on your way. grab the brake lines from the master as well so you have the correct fittings to fit, then you can easily swap them onto a new brake line and just do a new flare after.
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Old 05-18-2020, 09:15 PM   #13
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Re: What master cylinder and proportioning valve for disc/disc

oh yeah, nice truck too.
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Old 05-18-2020, 09:20 PM   #14
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Re: What master cylinder and proportioning valve for disc/disc

booster for a zr2 S10, dual diaphragm

https://www.rockauto.com/en/moreinfo...oQ0VFpaZTFM%3D


master cylinder for ZR@ S10

https://www.rockauto.com/en/moreinfo...oQ0VFpaZTFM%3D
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Old 05-19-2020, 03:59 PM   #15
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Re: What master cylinder and proportioning valve for disc/disc

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oh yeah, nice truck too.
Thanks much!

And, thank you for the advice! I'll likely try that route.

Nik
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Old 05-20-2020, 04:29 PM   #16
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Re: What master cylinder and proportioning valve for disc/disc

For safest braking you should provide more stopping power at front wheels than rear. Brake master cylinder size should be selected based on braking torque generated at front and rear axles. Braking torque is a function of system pressure, rotor diameter, pad coefficient of friction, surface area of caliper piston(s), tire contact patch, weight distribution, weight transfer, and surface area of master cylinder piston. Homebrew guys rarely have the means to determine weight transfer or coefficient of friction but we can determine rotor diameter and caliper surface area as well as weight bias and tire contact patch. Without doing some napkin calculations at a minimum all you are doing is guessing imo.

I would say calling the pros is a good idea.
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