Brake master cylinder help needed.......

[72K5er]

I am working on my '72 K5 and am replacing the master cylinder. It has disc front and drum rear with a proportioning valve. I have reworked the brakes already but I need help with the master cylinder. The mc it came with when I got it has the larger res in the front and the smaller res in the rear. It has a black bendix booster. I ordered a new mc for my application and the one I recieved has 2 reserviors of the same size. From what I have read, this is more common with the bendix booster. Which one should I use? Is there a reason the one has a larger front res? I have attatched pics of both. Thanks for the help.

[dwcsr]

It doesn't matter which you use as long as the bore is the same on the replacement

[72K5er]

Thanks. What is the purpose of the larger front res though?

[RichardJ]

Drum brakes have self adjusters or the owner is expected to periodically adjust manually adjusted brakes as the brake shoes wear down. Disc calipers have no adjustment for wear on the pads. The caliper pistons are fairly large and will hold a lot of fluid when the pads are worn. The large reservoir is needed so it doesn't run dry before the squealer signals the low pads.