Difference between Brake Master Cylinders?

[custom10nut]

On another thread, it was mentioned that to do an upgrade to front disc brakes, one would have to replace the Master Cylinder.
Is there a difference in MCs or is the difference in the Proportioning Block.
I’m about to pull the trigger on an upgrade kit from Scarebird, but was under the impression that I can use my current MC and just change to the disc/drum proportioning block.
I’m gonna do what I have to do, but need to know before I start the work.
Thanks in advance

[57taskforce]

Originally Posted by custom10nut

On another thread, it was mentioned that to do an upgrade to front disc brakes, one would have to replace the Master Cylinder.
Is there a difference in MCs or is the difference in the Proportioning Block.
I’m about to pull the trigger on an upgrade kit from Scarebird, but was under the impression that I can use my current MC and just change to the disc/drum proportioning block.
I’m gonna do what I have to do, but need to know before I start the work.
Thanks in advance

I believe both the cylinder and valve are different but for sure the master cylinder reservoir sizes are different. I wouldn’t be surprised if the master cylinder bores were different too.

[Steeveedee]

Not all master cylinders have unequal reservoirs when disc brakes are used. I don't know what the engineering criteria are for that. But there are internal differences between disc/drum and drum/drum brake master cylinders. I would run the proper master cylinder for your application, personally.

[custom10nut]

Thank you both. I have a complete extra power disc brake setup that I took off a 71. The one thing I did notice was the difference in the outward appearance of the MC.
Also I now know I’m going to replace all my brake lines, when I do my conversion.
Again, thank you

[RichardJ]

>> (reservoir size) I don't know what the engineering criteria are for that. <<

The criteria is simply that you don't run out of fluid before the pads wear out.
Very little fluid transfers from the MC to the caliper when you apply the brakes. When the pads are brand new, there is very little fluid sitting behind the caliper piston. When the pads have become worn down, there is a large volume of fluid that takes up permanent residence behind the caliper piston.

[Bigdav160]

Drum brakes need a residual pressure valve to keep the lip seals in the wheel cylinders inflated. (usually in the master cylinder outlet)

Drum disc combination needs a metering valve and proportioning valve (combination valve)

[rpmerf]

When you get to the late 70s and 80s braking systems, there are 2. JB6 and JB7. JB7 uses wider rotors and a larger master cylinder - 1-1/4". Forget what JB6 uses, 1-1/8"? I would aim to match the master to the rest of the system as best you can.

[Bigdav160]

But wasn't JB7 standard 3/4 ton fare? Slightly larger pistons in the caliper and wheel cylinder? Makes sense to keep the leverage ratio the same GM would install a larger bore MC.

GMT400 1/2 trucks had JB5 with leading/trailing rear drums, low drag calipers and a quick take up MC. The wheel cylinders were 1 3/16" vs the 1" WC in the more powerful dual servo drum brakes.

[Steeveedee]

Originally Posted by RichardJ

>> (reservoir size) I don't know what the engineering criteria are for that. <<

The criteria is simply that you don't run out of fluid before the pads wear out.
Very little fluid transfers from the MC to the caliper when you apply the brakes. When the pads are brand new, there is very little fluid sitting behind the caliper piston. When the pads have become worn down, there is a large volume of fluid that takes up permanent residence behind the caliper piston.

I know about the volume of fluid transferred. But I've seen master cylinders for disc/drum systems with equal volume reservoirs and some with a larger reservoir on the disc brake side. That's what I was referring to. Also, I'd like to think that the people maintaining their vehicle check the brake fluid more often than the time interval for the brakes to wear out!

[mike16]

when I did a disc brake conversion on a 63C20 I went through all this.
MC's
drum/drum are 7/8th in. diameter
disc/drum are 1 inch dia
disc disc are 1 1/8th inch diameter

the disc portion of the reservoir is larger because dis's use more fluid.

the rear drum wheel cylinders are 7/8th in. dia on drum drum set ups but 15/16th on disc drum wheel cylinders

there is also a difference between the mc's if they are power boosted vs manual. the rear of the piston that is exposed when you look at the mc shows this difference.

I think the residual pressure on a disc set up is only 2 psibut for a drum set up its like 10 psi

this book, Brakes by fred Puhn and published by HP books is an excellent read on these very issues. A copy is currently for sale on Amazon for only about 5 bucks