C10 and c30 break experts, I need help, advice

[CarlsLQ9SS]

On my build, I went from a c10 master cylinder to a c30 master cylinder. What I’m confused about is the size of the lines/fittings on the c30 master cylinder. When I changed it, it was like the lines wad flip flopped (the front line was smaller on the c10 master cylinder and was larger on the c30 master, and the rear on the c10 was larger than the c30). So when I installed the c30 master cylinder, I moved the lines, one for the other but had to cut and reflare my brake lines to change the fittings for the proportioning valve. My question is does the front of a c10 master cylinder control the front brakes and the rear control the rear? And is it the same for the c30, or should I have kept the lines where they was on the proportioning valve and run the front on the master to the rear on the proportioning valve and the rear to the front?

[PepperTreeGarage]

I'm not an expert, but here are a couple pics. First one is the setup on our '72 C20. Second one is from the Service Manuel. You can see that the lines are crossed. Front line from Master Cylinder to the rear port of the Combination Valve. Hope this helps.

[CarlsLQ9SS]

Both pictures confirms my thoughts. Looks like I have to redo my lines coming from the master cylinder. Thank you for the input and the pictures, much appreciated!
Carl

[truckster]

I'm not certain about the C30, but on the C10 front is front and rear is rear; on the C20 front is rear and rear is front.

[geezer#99]

Originally Posted by truckster

I'm not certain about the C30, but on the C10 front is front and rear is rear; on the C20 front is rear and rear is front.

It shows that in the service manual pic.
I know it’s not the question but I’m curious.
Why the need for the c30 master?

[Sheepdip]

You don't say what year your truck is but what you got is a 71-72 master for front disc brakes. The bigger reservoir goes to the front disc.

[CarlsLQ9SS]

My truck is a 71 GMC. I wanted the c20/c30 for the bigger bore in the master. May not make a difference but that’s what I went with, plus it was cheaper. Thanks for all the input and now I have to redo my brake lines. Not a big deal but hard to move around right now after a 4 level disc fusion in my lower back.

[geezer#99]

I’d just leave the lines alone.
And bigger bore isn’t always better.

[CarlsLQ9SS]

Originally Posted by geezer#99

I’d just leave the lines alone.
And bigger bore isn’t always better.

I was worried it would put more pressure to the back than the front, do you think it will matter?

[Sheepdip]

You need to plumb it correctly. You also might want to read this:
https://techtalk.mpbrakes.com/combin...ination-valves

[CarlsLQ9SS]

Originally Posted by Sheepdip

You need to plumb it correctly. You also might want to read this:
https://techtalk.mpbrakes.com/combin...ination-valves

That was a good read! I will deffinately replumb the master cylinder. Thanks for posting a link for me. Carl