My master cylinder is leaking on my 1997 half ton Ive replaced the rear brakes cylinder to the 1 ton dully. I was reading in the Haynes and it said the 3/4 ton master cylinders have a bigger bore then the half ton so i thought of upgrading to the 3/4 master cylinder but the book said not to because it will cause improper braking and poor brake performance is this true. I would think with the bigger bore of the master cylinder my braking would be better.

 

Moving more fluid doesn't necessarily translate into better braking. You will still be working with the same proportioning valve, calipers and wheel cylinders, all of which are designed for a specific fluid flow. You would likely have to upgrade all these components to see any improvement from a master cylinder swap.

 

The 94-96 half tons have a inch and one eighth bore and the 97 and up have a inch and a quarter bore. As far as it being a "upgrade", well, that depends on how bad your brakes feel now. I'm in the process of replacing my MC. I also have a 97 1/2 ton. I replaced it once already with a cardone reman. but the one they gave me was a defect. So I ordered a new Dorman from summit and I should receive it by Dec 2. It's on back order. If you wanna wait till then, I can give you a report on how it feels. I plan on doing a write up. BTW, Is your MC leaking at the reservoir grommet? After you press the brakes, does it feel like the brake pedal is "chirping" or "burping"? Just wondering if you have the same symptoms as me.

 

Yea mines leaking bad at the reservoir grommet.