64 C20 Dual Master Cylinder Conversion – Drum/Drum Questions

[cshanek]

Happy holidays everyone,

I’m on the homestretch of a front-end rebuild on my 1964 Chevy C20 (long-bed) and could use some guidance as I work through the details of converting to a dual-reservoir master cylinder.

The truck is currently drum/drum, with new shoes, drums, and front brake components. I’ve watched a bunch of videos and read several threads, but my setup feels just unique enough that I figured I’d start my own thread.

Here are my questions:

1. Is a brake booster worth it? I’m leaning toward using one since a lot of the eBay kits include a booster for under $200. Curious if people think it’s money well spent on these trucks.
   
2. Thoughts on eBay kits vs Brothers/JEGS/Ecklers kits? The price difference is huge. Are the cheaper, $200 kits generally fine, or am I asking for trouble? Any kit recommendations?
   
3. Parking brake: My truck still has a newly rebuilt driveshaft-mounted parking brake. Am I right in assuming I can just ignore/remove the parts in these kits meant for rear cable-style parking brakes?
   
4. Master cylinder bore size for drum/drum C20? I see 1", 1-1/8", etc. What bore are you running and how’s the pedal feel?
   
5. “Disc/drum” vs “disc/disc” kits & valves: A lot of kits are labeled for disc setups. I thought proportioning valves could be configured for different combinations. What’s the right approach for a drum/drum truck?
   
6. Brake line routing:
   I had to remove all the front brake lines during the rebuild and bought a replacement set. The rear line from the front passenger side to the back is still intact. Can I just run a second front line set and keep my rear line on the passenger side, even though most pre-bent kits go down the driver side?
   
7. Brake line clips: Where can I find the metal clips that secure the brake lines to the front crossmember? Do they sell them for dual lines?

I appreciate any insight or lessons learned from folks who’ve done this swap Thanks in advance!

[AcampoDave]

I've run manual disc/drum and drum/drum with a dual MC on half tons. The pedal effort was no problem in both cases. It still felt like a regular stock C-10. I was unsure how manual disc brakes would feel, but it turned out to be no different really, just stopped better which was perfect for me. I just didn't want a booster. I find them unattractive, heavy, and just one more thing to fail. I bought my MC, rotors, drums, flexible lines etc from my local O'Reilly's. I kept my old rear metal line on the passenger's side and connected an extension to it up to the MC. I got my dual brake line clips from a junkyard squarebody and even used the front metal lines and prop valve from the same. Drum/drum set-ups don't actually use a prop valve, but some do have some type of residual pressure distriution block with a warning light sensor where the lines meet.

[AcampoDave]

Here is one of those distriution blocks with a sensor on my drum/drum. I think I took it off a '69 in the junkyard.

Edit: I was under the mistaken assumption that it was a prop valve when I put it on there and later found out it wasn't. I just left it alone at that point since it was already done and working well.

[cshanek]

Originally Posted by AcampoDave

Here is one of those distriution blocks with a sensor on my drum/drum. I think I took it off a '69 in the junkyard.

Edit: I was under the mistaken assumption that it was a prop valve when I put it on there and later found out it wasn't. I just left it alone at that point since it was already done and working well.

I like your setup. The booster isn't a single point of failure correct? I'd imagine if it fails the brakes still work, just not "boosted"?

[cshanek]

This is the proportioning valve I purchased ...
https://www.ecklers.com/1947-1987-ch...-132389-1.html

[DPowers]

The Brake Area in the 60-66 FAQ section will give you all the info you need.

[AcampoDave]

Originally Posted by cshanek

I like your setup. The booster isn't a single point of failure correct? I'd imagine if it fails the brakes still work, just not "boosted"?

Not sure really, but that's what i'd assume.

Edit: AI had this to say about that valve thing on my drum set up: For a true 4-wheel drum brake system, you generally don't need a proportioning valve to reduce pressure, as drums can handle similar pressure, but you do need a 10-psi residual valve to keep pressure and ensure quick brake response.

So i guess that's what it is.

[cshanek]

Originally Posted by AcampoDave

Not sure really, but that's what i'd assume.

Edit: AI had this to say about that valve thing on my drum set up: For a true 4-wheel drum brake system, you generally don't need a proportioning valve to reduce pressure, as drums can handle similar pressure, but you do need a 10-psi residual valve to keep pressure and ensure quick brake response.

So i guess that's what it is.

I did a little reading tonight and have found some conflicting info. Everyone seems to agree a proportioning valve isn't needed from drum/drum setups, but some sellers have specifically mentioned in Q&As that they can be used for drum/drum setups. My limited post-reading understanding is that a metering valve (which often one of the few things in a proportioning valve), is recommended to delay rear brake application. I have found 1-2 dual res master cylinders with an attached metering valve for about $100, but nothing that fits my truck. I also read some (most?) of the modern dual reservoir MC have the 10PSI residual pressure built in, but I can't seem to find any mention of this in their specs.

[Agv63]

When i got my 63 it was those dual brake and clutch units, single circuits, when i changed to v8 and auto box i went with a 9" booster that i got from a friend and dual circuit master that came from an impala that was for drum brakes.
No distribution block, i just split the lines for front and rear , it worked fine, only issue i had is keeping the 455 from creeping at the lights, had that sort of a choppy idle similar to a blower motor but not as rowdy, but it crept with every chop, got fed up and changed to disc brakes and made a new stepped link from pedal to booster rod for correct ratio and thats when i found it was the booster that made the vacuum leak causing that choppy idle when brakes were on.
I got a booster master combo from ebay, with prop valve, only one availible at that stage was a 1" master, but im happy with it, it brakes real good and to some it might be touchy on the pedal, but im happy with it .only issue i had was that the prop valve blew a o-ring seal internally giving me bleeding issues rear, replaced with viton o-ring and its been almost 3 years now no issues.

[lilpoindexter]

Originally Posted by AcampoDave

Here is one of those distriution blocks with a sensor on my drum/drum. I think I took it off a '69 in the junkyard.

Edit: I was under the mistaken assumption that it was a prop valve when I put it on there and later found out it wasn't. I just left it alone at that point since it was already done and working well.

Jesus Christ, that is beautiful under the hood.

[Keith Seymore]

Originally Posted by cshanek

I like your setup. The booster isn't a single point of failure correct? I'd imagine if it fails the brakes still work, just not "boosted"?

Yes - correct.

K

[Keith Seymore]

Originally Posted by cshanek

but some sellers have specifically mentioned in Q&As that they can be used for drum/drum setups.

They would be incorrect.

The "proportioning" in a drum/drum set up is designed in via the differing wheel cylinder sizes front vs rear.

Adding/leaving a proportioning valve is just carrying around another component that is not required, reducing functionality of the rear brakes.

K

[theastronaut]

Here's the math on what changes with a booster, or with different diameter master cylinders. When you add a booster you have to change the pedal ratio (you lose leverage) and the bore size goes up (you lose hydraulic pressure), so if the booster dies you're way down on manual braking power. The total line pressure of a typical booster setup and a correctly sized manual setup isn't all that different, its just that the booster adds more assist at lower pedal input so it's a little easier to stop in normal day to day traffic, but they're about the same in a panic stop. I prefer the linear input to output of manual brakes, and braking power doesn't vary with engine vacuum. Remember also that drum brakes are self energizing due to the shoe/pivot geometry so they need less input pressure to be as effective as discs. Drums work very well when they're done correctly.


Another thing I never see mentioned about older non-abs systems when upgrading them; you do not want an easy brake pedal if you do not have abs. If the pedal effort is low/easy then it'll be very easy to lock up the brakes and slide uncontrollably in a panic stop situation. Newer cars with abs can get away with overly boosted brakes to make it easy for a frail grandma to stop the car, but abs kicks in to keep it from locking up. It should take some pedal effort to lock up the brakes, enough that you have to work at it a little so you can modulate the pedal right at the point of maximum tire traction. "You" are the abs, and a linear pedal response with manual brakes makes braking as hard as possible without locking up easier and more intuitive.

Manual pedal ratio is generally around 6:1, power brakes are generally around 4:1. My stock '66 C10 has a 6.25 ratio. The higher the number, the more leverage the pedal has on the master cylinder.

When you convert to power brakes you do two things to take away manual braking power- reduce the pedal ratio by about 35%, and you generally go from a 1" bore MC to a 1.125" bore MC, reducing output PSI by 20%. The booster is supposed to more than make up for that, right?

With a stock 6.25 pedal ratio and 100 pounds of input force on the pedal you get 625 pounds of force into the MC, which is hydraulically multiplied via the MC's piston surface area of .785 square inch. 625 pounds in turns into 790 PSI at the caliper if you have the typical 1" bore MC.

With a power brake conversion, 100 pounds of force on the pedal gets multiplied by 4 via the pedal ratio for 400 pounds into the pushrod/booster. CPP says a 7" dual diaphragm booster adds 493 pounds of force for 893 pounds of force into the MC. The typical 1.125" MC will turn 893 pounds of input force into the same 893- the bore size has 1 square inch of area so it doesn't multiply the input force any so you have 893 psi at the caliper. This is about 13% more psi than the typical manual setup ** BUT ** this is assuming 23" of manifold vacuum. How many trucks actually make 23" of vacuum? That 13 % advantage quickly fades in the real world.

What happens if we stay with manual brakes but drop down to a smaller master cylinder bore to multiply input force more? With a 15/16" bore you get 905 psi at the caliper, and 7/8" would have 1041 psi. Brake pedal input to output is totally linear and not variable with manifold vacuum, and they work 100% the same with the engine on or off. It's cheaper and easier because pedal modifications aren't needed.

But, everyone says you need power brakes "because they stop better" and brake companies need you to keep believing that so you keep buying their aftermarket "upgrade" parts...

The one benefit to power brakes is that they can have less pedal travel than manual brakes, because of the shorter pedal throw due to the lower ratio, and because the bigger MC bore displaces more fluid for the same amount of travel. Is this an issue in the real world? Its not on my '66 with the stock 6.25 pedal ratio, 1" bore MC, the usual squarebody D52 12" discs in front, and stock rear drums. The pedal drops a normal amount and pedal effort is pretty normal too. If you have air in the lines then you'll have more pedal travel since air is compressible, but you shouldn't have air in the lines if you bleed them correctly. If you have bigger/multi-piston calipers then you'd need to design the whole system around the amount of volume/pressure they need. But for the typical D52 disc setup, a manual 1" bore MC works great and makes plenty of hydraulic pressure without the pedal travel being any more than normal.

[cshanek]

Originally Posted by theastronaut

Here's the math on what changes with a booster, or with different diameter master cylinders. When you add a booster you have to change the pedal ratio (you lose leverage) and the bore size goes up (you lose hydraulic pressure), so if the booster dies you're way down on manual braking power. The total line pressure of a typical booster setup and a correctly sized manual setup isn't all that different, its just that the booster adds more assist at lower pedal input so it's a little easier to stop in normal day to day traffic, but they're about the same in a panic stop. I prefer the linear input to output of manual brakes, and braking power doesn't vary with engine vacuum. Remember also that drum brakes are self energizing due to the shoe/pivot geometry so they need less input pressure to be as effective as discs. Drums work very well when they're done correctly.


Another thing I never see mentioned about older non-abs systems when upgrading them; you do not want an easy brake pedal if you do not have abs. If the pedal effort is low/easy then it'll be very easy to lock up the brakes and slide uncontrollably in a panic stop situation. Newer cars with abs can get away with overly boosted brakes to make it easy for a frail grandma to stop the car, but abs kicks in to keep it from locking up. It should take some pedal effort to lock up the brakes, enough that you have to work at it a little so you can modulate the pedal right at the point of maximum tire traction. "You" are the abs, and a linear pedal response with manual brakes makes braking as hard as possible without locking up easier and more intuitive.

Interesting writeup, and love the details. I actually just bought https://www.ebay.com/itm/236509754375 prior to reading your post. I was on the fence on the booster vs no booster debate honestly, but figured I would go with the booster just to reduce pedal play.

I am still trying to determine if I should throw in 10lb residual valves with that setup.

[cshanek]

Originally Posted by Agv63

When i got my 63 it was those dual brake and clutch units, single circuits, when i changed to v8 and auto box i went with a 9" booster that i got from a friend and dual circuit master that came from an impala that was for drum brakes.
No distribution block, i just split the lines for front and rear , it worked fine, only issue i had is keeping the 455 from creeping at the lights, had that sort of a choppy idle similar to a blower motor but not as rowdy, but it crept with every chop, got fed up and changed to disc brakes and made a new stepped link from pedal to booster rod for correct ratio and thats when i found it was the booster that made the vacuum leak causing that choppy idle when brakes were on.
I got a booster master combo from ebay, with prop valve, only one availible at that stage was a 1" master, but im happy with it, it brakes real good and to some it might be touchy on the pedal, but im happy with it .only issue i had was that the prop valve blew a o-ring seal internally giving me bleeding issues rear, replaced with viton o-ring and its been almost 3 years now no issues.

I ended up purchasing something similar. Fingers crossed.

[cshanek]

Originally Posted by Keith Seymore

They would be incorrect.

The "proportioning" in a drum/drum set up is designed in via the differing wheel cylinder sizes front vs rear.

Adding/leaving a proportioning valve is just carrying around another component that is not required, reducing functionality of the rear brakes.

K

Interesting. I just bought a drum/drum booster/MC/prop valve that was an otherwise identical listing to the same setup for disc/drum. I suppose I can just not mount the proportioning valve ("Bottom Mount Drum/Drum Proportioning Valve Kit")

[theastronaut]

After I've gotten deeper into brake parts and what all is needed to set up a correctly engineered system, it's baffling to see the amount of parts places and even "brake companies" selling a combination valve as a "proportioning valve", or they only have one "proportioning valve" that "fits" every application. And so many of them never say anything about the need for redoing the pedal ratio when adding a booster, so then the brakes are touchy and the pedal is soft with too much travel if it even builds enough pressure. I've seen disc or drum master cylinders... like, do they have residual valves built in or not, because it can't be both ways... I've even seen low drag calipers in kits than have to have a step bore/quick take up master cylinder to make them work and nothing in the kit mentioned this at all. It's the wild west so you have to really know brakes to get something pieced together that works correctly now, because the parts places and some of the brake parts places are just parts resellers, not really brake people.

[theastronaut]

This is a good explanation of the different types of valves. Check what you actually have vs what the seller listed it as. You can also put a pressure gauge on the outlet of the master cylinder after bench bleeding it to see if it has built in residual valves or not.

As said already, drum/drum front to rear stopping balance is set by staggering the wheel cylinder bore sizes so a proportioning valve isn't needed to keep the rear from locking up first.


https://www.youtube.com/watch?v=dD1dbNtH-6s

[jocko]

cshanek, good luck on your dual res mc conversion, sounds like you're on the right track. Agree there is no benefit in adding a prop valve, essentially irrelevant for your drum/drum setup. A distribution block would provide you notification that the front or rear circuit has failed (assuming you also wire up a brake fail light of some sort to your dash). Below is a link to how I did my dual res mc conversion - and note I did not add a distribution block - I kinda looked at this this way: I've already doubled the safety/reliability compared to what GM was ok with up until 1967 just by adding the dual res mc without a distribution block, so I'll just look under the truck now and then and make sure I'm not leaking brake fluid all over the place. I'd submit that, if properly adjusted, you'd also feel a braking performance degradation if one circuit failed. Anyway, good luck with your project, this was a fun one. my dual res mc conversion thread (on a 66, but same as yours)

Also concur on the parking brake note, will function same as always. Since I could never remember to release it, I added a little homemade "parking brake" notification ala newer trucks, not perfect, but it worked well. Parking Brake warning light

[cshanek]

Seems like there is an opportunity to attempt to distill all of this down into a reference graphic. Does the graphic below jive with everyone's understanding?
I probably should have mentioned a check valve is just a residual valve built into OEM-style Drum/Drum MCs.

I received my dual MC and booster kit over the weekend. I did not do my homework however. It appears there are two bolt heads right in the way that mount the dash/steering column brace. This also means I cannot make the pushrod/plunder hole large without cutting into that brace. Mulligan. Just planning on installing the MC and Pressure Differential switch for now.

[1964FarmFreshC10]

I just did this a few months ago on my 64. Super easy an no additional valves are needed. If you lose either the front or rear brakes you won't need a light to inform you it will be VERY obvious unless you're asleep at the wheel! LOL

[cshanek]

Finished the front end rebuild, had to find a ton of inverted flare/NPT fittings (various sizes) for everything. Held off on installing the booster since I think it would require cutting/trimming steering bracket in the cab. Thanks for all the help.

[1964FarmFreshC10]

Looks good.

You shouldn't need to cut the steering bracket just drill a couple of new holes through it and the firewall.

Originally Posted by cshanek

Finished the front end rebuild, had to find a ton of inverted flare/NPT fittings (various sizes) for everything. Held off on installing the booster since I think it would require cutting/trimming steering bracket in the cab. Thanks for all the help.