Distribution block to proportioning valve

[Belsh]

Hey guys, my 1970 C10 has a distribution block for the drum drum set up.

Im converting my front end to discs, having read a load of posts on here am I right in saying the blocks are only for drum drum and I need to convert to a proportioning valve for the disk drum set up?

Thankyou as always

[Rust_never_sleeps]

You have that right. You'll need a disk/drum type proportioning valve(technically a combination valve)

[custom10nut]

I’m in the process of doing the same on my truck.
Yes the drum/drum setup has the smaller “Distribution Block”, and the Disc/Drum system requires what I call a “Proportioning Valve”.
Be advised that the Disc/Drum System also has a different Master Cylinder, and the fittings on the Brake lines are different sizes/shapes from the factory Drum/Drum System.
I’m waiting for my son to come home on leave next week to put my Conversion Kit on, so I took the time to mock up and test fit everything, and that’s when I found all the differences.
Good luck with your project

[Belsh]

Originally Posted by custom10nut

I’m in the process of doing the same on my truck.
Yes the drum/drum setup has the smaller “Distribution Block”, and the Disc/Drum system requires what I call a “Proportioning Valve”.
Be advised that the Disc/Drum System also has a different Master Cylinder, and the fittings on the Brake lines are different sizes/shapes from the factory Drum/Drum System.
I’m waiting for my son to come home on leave next week to put my Conversion Kit on, so I took the time to mock up and test fit everything, and that’s when I found all the differences.
Good luck with your project

Thanks for that Custom, I didnt realise I needed a different master cylinder!! Very disappointing as getting stuff in the UK is pretty difficult but hey ho I'll have an ask about and someone may have one lying around. I was going to change all the brake lines so also good to know they'll be different sizes
Good luck with the conversion, be nice to get working together with your son

[custom10nut]

If/when you purchase the Master cylinder, buy one for a 71/72.
They are a bit larger.
If I think about it today, I will make a picture to show them side by side.
And the difference between the two distribution blocks.
Do you already have Power Brakes?
I ask because you’ll need the Booster, the two mounting brackets, two support rods, and the longer actuator rod.
Good luck with your adventure, because getting parts in the UK must be a real challenge.

[Belsh]

Originally Posted by custom10nut

If/when you purchase the Master cylinder, buy one for a 71/72.
They are a bit larger.
If I think about it today, I will make a picture to show them side by side.
And the difference between the two distribution blocks.
Do you already have Power Brakes?
I ask because you’ll need the Booster, the two mounting brackets, two support rods, and the longer actuator rod.
Good luck with your adventure, because getting parts in the UK must be a real challenge.

Yeah I've already got a booster, the master cylinder looks a decent size, I wonder if at some stage someone has bought a later booster and cylinder.
If you get a chance could you have a quick measure of the cylinder please, that would be a huge help. Stuck a quick picture below of what i currently have

[70STOVEBOLT]

The physical size of the master is not what is meant by "bigger" I believe the difference is in the bore size for the master cylinder piston. The master in the pic is for drum/drum brakes and will not work on a disc/drum setup. You need a master cylinder for a 71/72 C10 (half ton). The booster should work fine.

[70STOVEBOLT]

So after further research, the main difference between a drum/drum master and a disc/drum master is the reservoir size. The reservoir for disc brakes has to be larger because the caliper requires more fluid to push the piston out to make contact with the brake pads. Also, it uses a 2 lb residual valve for the disc brake vs a 10 lb residual valve for the drum brake. Bore size may or may not be different between the two masters.

[RichardJ]

>>The reservoir for disc brakes has to be larger because the caliper requires more fluid to push the piston out to make contact with the brake pads.<<

Not true. Drum brakes have adjusters so as the shoe lining wears down, the wheel cylinders stay in the same relative position while operating.
Disc pistons have no adjustment to compensate for wear on the pads. Disc pistons only move about .003" - .004" when the brakes are applied and return to that same position when the brakes are released. When the pads are NEW, a small amount of fluid resides behind the piston. 40,000 miles later, a much larger amount of fluid resides behind the piston.
The amount of fluid transfer when the brakes are applied is relatively small.
If the reservoir is too small, the MC will start pumping air when the pads are worn down.
Modern MC's are now equipped with a float to turn ON a brake warning light when the pads have worn down. Many new cars have this timed just right. Most new MC's, whether disc/drum or disc/disc have a single reservoir and with a float don't worry about extra reserve capacity.
Light comes ON, the first thing to check is pad wear.

>>Also, it uses a 2 lb residual valve for the disc brake vs a 10 lb residual valve for the drum brake.<<

A 2 lb valve is only needed in situations where the MC is mounted below the frame and lower than the disc brakes. Pre 40's cars and HotRods looking for a completely clean firewall.
10 lb valves for drum wheel cylinder are not needed because of newer designed cylinder cup expanders or if the MC is mounted down low under the frame.


>>Bore size may or may not be different between the two masters.<<

That I can agree with. If the change to disc required too much foot pressure and you are not adding a booster, you may need a smaller MC bore.

[70STOVEBOLT]

Ok, disc brakes need a bigger reservoir because as the brake pads wear, the fluid level drops in the reservoir. Is that better?

[70STOVEBOLT]

Originally Posted by RichardJ

>>
10 lb valves for drum wheel cylinder are not needed because of newer designed cylinder cup expanders or if the MC is mounted down low under the frame..

Please define newer. We are talking about 50-year-old stuff here. This is from a post I saw on this forum, that was taken from an SAE test prep guide:

can:

"Many master cylinders designed for drum brakes incorporate a residual check valve to keep a slight amount of pressure on the system after the brake pedal has been released. These residual check valves are used on drum brake systems to keep around 5psi. up to 12psi. of pressure on the wheel cylinder's cup seals and resist any momentum of brake fluid returning to the master cylinder. Since the valve is keeping constant pressure on the system, any small leaks will result in fluid leaking out instead of air leaking in. Residual check valves keeps a constant pressure on the wheel cylinder's cup seals. These valves are located inside the master cylinder port leading to the rear drum brakes. They consist of a check valve and spring that allow brake fluid to enter the master cylinder only until a designated pressure is reached. Once this occurs, spring pressure closes the valve maintaining low pressure on the drum brakes. Many vehicle's today use cup expanders in the wheel cylinders that keep an air tight seal. This prevents air contamination eliminating the need for residual check valves."

I’m not an expert by any means, but I would think the society of automotive engineers would be.

[Belsh]

Ah man, now I’m even more confused….

[custom10nut]

Originally Posted by Belsh

Ah man, now I’m even more confused….

I’m not going to get into the technical parts of this conversation, but I will take some “Side by Side” photos of the Drum/Drum equipment, and then the Disc /Drum setup.
I just know that if you order a MC for a 70 (drums) it’s different than a 71(disc/drum)
I’ll also measure both of the MCs to compare them.

[RichardJ]

On older single system MC's, the residual valve was internal. In this example it is described as a "Fluid Check Valve".
In later dual systems a check valve was installed in both MC outlets.
The aftermarket sells inline check valves because some custom and really old vehicles have the MC mounted low, under the frame.

[RichardJ]

70stovebolt, did you read the quote in your last post?

Here are three vehicle pictures posted on this forum, showing applications that require the addition of aftermarket residual check valves.

If this isn't what you have, you don't need to add check valves.

[jocko]

Belsh, if you're converting your 70 to front disks and want a truly factory setup, keep in mind that disk brake lines are 3/16" rather than 1/4" for the front circuit. Many folks will tell you that your existing 1/4" front lines will work just fine, and that may be the case, but it's not a true duplication of the GM 71-72 disk brake setup. If it didn't matter, I doubt GM would have changed to 3/16" lines in 71 - in other words, given disk brakes were a "new" thing on GM trucks in 71, it would have been engineered as a holistic system - new mc, combo valve, and front brake line diameter (and I trust their judgment ).