Master Cylinder Size

Local old timer who I've given a bunch of stuff to I'm not re-using - is redoing his own C20. He has a NIB original GM master cylinder he bought for another truck he had in the past and never used it. His original he believes is bad but is the light weight one with plastic cap reservoir. His new one is the metal reservoir and the PN refers to one online that notes "for 13" rear brakes". His are 11". He asked me if he can still use it - I have no idea, so hoping someone here knows.

Thx.

He probably can use it. But, the pedal pressure required to apply the brakes will be higher and he might have to reverse the two lines that screw into the ports.

Are those 2 lines the same threads?

Doppleganger said:

Are those 2 lines the same threads?

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More than likely the brake line size will be different front vs rear, so the threaded connections will be different also. They do make adapters so you can reverse them.

77 K20 said:

More than likely the brake line size will be different front vs rear, so the threaded connections will be different also. They do make adapters so you can reverse them.

Click to expand...

Usually the way GM did it, the lines are meant to go where they fit. For whatever reason, the front and rear circuits often swapped places in the master cylinder. It varied by year and application.

Are the boosters different between the 11" and 13" brakes? It looks like I'm going to have similar issues with my own.

Thanks fellas!

@bucket,

So are the line for the rear larger? Since the fluid has to go further? If that's the case and you are suggesting the line for the rear should go for the front brakes of the master cylinder, what would that do for the pressure going to the disc calipers? And what about the proportioning valve? It's starting to sound like it could become a "plumbers nightmare" adding adapters all over the place. The guy might be better off just getting the master cylinder for his year and system truck instead of getting something that doesn't go to his truck, and trying to make it work.

The primary and secondary position is swapped on many plastic reservoir MC. Rear port is primary, front port secondary. On the old style all metal MC the primary port is front and the secondary port is rear. Front = mounted pointing to front of vehicle.

I'm sure there are exclusions for different models and year and applications.

Brake lines for the rear from MC to proportioning valve to the axle have a bigger diameter.

He should get the right MC for that truck they are cheap and you don't have to hassle with adapters and sizes and port location, etc.

Your truck is a little newer than the trucks I did 3/4 ton axle swaps on. I had a '77 K5 and a '79 K15. I used my existing 1/2 ton power brake booster on both trucks. For the M/C I used the 3/4 ton version for the corresponding year. The 1/2 and 3/4 M/C were both metal and had the ports reversed. I had to reverse the lines.

I can't recall if you are using stock size tires or not. I'll give you my 2 cents on the rear brakes. When I did the K5 in the early 90's I didn't know a lot about GM trucks. The full floater I put in the rear had 11" drums (I believe 73 or 74 model year). That truck had had 36" tall tires and didn't stop worth a sh*t. I was going to swap to 13" drums, but sold the truck. My K15 had an '82 14 bolt SF with 13" brakes and 33" tires. It would lock the rear way to easy. I was going to add a manual proportioning valve to dial it back a bit, but I sold that to get my K30.