Master Cylinder Adventures

[jtminton]

So on Monday morning I took the day off to replace the MC in my 72 C-20. It was in pedal-to-the-floor-pump-it-5-times-to-stop-brake-idiot-light-on bad shape. I believe it was the stock one, or at least it has been on there since before my dad (the PO) got the truck in 85. I used my dad's shop and he had a new in the box (yet rusty on the outside from 5 years in Houston humidity) MC for me to use. Here is my exciting account of the event, followed by some questions I have for anybody that might know.

Here is what I started with (note where the lines run from the MC to the prop valve. it will come up later):


I unhooked the brake lines from the old MC to drain out some of that delicious fishy smelling brake fluid:


Here's some with the MC off:

(Note the lines that screw into the prop valve on this pic):


Now, after taking the MC off and pretty much eliminating any transportation I had, I decided to see if the new MC would screw into the lines okay. That's the kind of quick thinking you can expect from me. Anyway, what'dya know, they don't look the same. In fact, the lines to the prop valve seemed to be backwards. Here is a picture of the new MC after I got it all set up to bench bleed:


So now what? I called my dad for advice, and he said that I could see what steal off of his 55 Chevy panel that he had put a 72 front disc setup on some years back. Here is a picture of it. The MC looks EXACTLY like the new one I have, and the brake lines to the prop valve are set up the right way, instead of backwards like was on my truck:


So I decided to take the two little lines off the panel truck and put them onto my truck. They fit just right, or at least right enough for me to do a little finger bending to get them all hooked up. Here is one with the swapped lines:


Here is a shot of the new MC hooked up before I filled up the fluid:


Filled up the fluid, and it stops on a dime now, and there are no leaks. Here is my question:

Was the MC I had on there stock? What about the new one? Is there a difference between C-10 and C-20 MCs? Could that be the difference? Should I expect immediate brake failure and a firey death for using the wrong one?

Thanks!

[chevyfarmer]

I do not own a 3/4 ton but I am almost positive there is a difference in the master cylender.

[JimKshortstep4x4]

The C-10 master cylinder is different from the C-20 master cylinder. As you have discovered the C-20 front reservoir serves the rear brakes and the C-10 is just the opposite.

Since you have the master cylinder on the C-20 so that the rear reservoir serves the rear end, it should be ok as I am not aware of any issues in doing what you did.

I have always wonder if the differences were done to distinguish the C-10 from the C-20 since the C-20's came with a double diaphragm power brake booster.

Jim

[Ray_71Chev_in_PA]

JT,

I read your post because I am going to replace my Master Cylinder as well. My replacement has been tucked away in a box as well... Hope it is ok. Meanwhile, I only have a C10. Here's a shot:



I don't know if this is a typical as-built set-up. Like you, I'll find out when the time comes.

Best regards,

Ray

[rambler]

I'm picking up a 70 GMC C30 in a couple of weeks, it is missing the brake booster,, I was hoping to use my power brake booster setup from my 72 C10 parts truck,, I assumed there wouldn't be much difference..??? I guess I'm wrong? do I need to do some trading to get the proper brake booster? It needs to be power as I'm going to be hauling some loads and trailer with it ( I HOPE)!

[jtminton]

Originally Posted by rambler

I'm picking up a 70 GMC C30 in a couple of weeks, it is missing the brake booster,, I was hoping to use my power brake booster setup from my 72 C10 parts truck,, I assumed there wouldn't be much difference..??? I guess I'm wrong? do I need to do some trading to get the proper brake booster? It needs to be power as I'm going to be hauling some loads and trailer with it ( I HOPE)!

No idea. But I do have a broken master cylinder I'll sell you cheap!