The Stovebolt Site Forums The Shop Area Engine Shop Messin' with brake sizes? MC

My big truck has dual master, one for clutch. I found one just like it, cept the Brake bore is 1-1/8" and is 1-1/4" on orig. Orig was $180, this other one is $37 rebuilt. NOW, my brake man says it won't work, that 1/8" is a BIG diff in hydraulics. He says I won't have enough push left in pedal to stop the truck as it was meant to be. Any other opinions? I'm gonna try it, I'm cheap! Might be buying correct one if don't work, and spend MORE than just using correct. Anyone else ever mess with bore sizes not being orig? Results?

GL you could always have the bigger one sleved to make it a 1" bore. Just an idea.

Wasn't there a write up about this in one of the Vintage Truck magazine. I will have to go home and look for it. I think it was 1 or 2 issuses ago.

If I understand your post correctly, you're reducing the diameter of the master cylinder by 1/8"? Yes, your mechanic is right, raise your insurance coverage limit, and try to hit something cheap! You won't be able to displace enough fluid to get the brake shoes firmly into contact with the drums, without pumping the pedal, or adjusting the shoes way too close to the drums. I'd definitely advise against that fix! The first accident is going to cost a LOT more than the difference in the parts cost.
Jerry

opps my bad I thought you were increasing your bore not decreasing. Size does matter! Sorry for my error.

I hate to disagree with hot- rod. He is very
sharp at this stuff. Reducing the size of the
master cyl. will give you more brakeing power.
I don,t think the additional pedal travel will
be noticable. I,d try it.

"The bigger the master cylinder is, the more fluid you get but you'll have less pressure at the caliper" (from Speedway Motors)

Different application but FWIW I ran in to this because I am putting 4 wheel discs on my truck and using a dual master cylinders. I called Wilwood and they recommended a 7/8" bore for the front and a 1" bore for the rear. More pressure in the front because that is where the braking is.

Vintage Truck in the April issue Bob Adler states that going to a one inch bore will increse the pressure, I changed mine I would say he's correct. old navy 51..................................

If there's enough fluid displaced to get the shoes firmly applied to the drums, the smaller cylinder will require less pedal effort. There's more hydraulic advantage, which equates to less pedal effort for the same braking effect. HOWEVER, and this one's very important, even a slight amount of brake lining wear will allow the pedal travel to increase to a dangerous distance. You'd need to be very careful to keep the brakes adjusted often. I've had problems with a Dodge 1-ton motorhome running out of pedal stroke when a very slightly too-small MC was installed. It almost wrecked us on more than one occasion. The problem turned out to be caused by a MC with a reservoir that was too small for the size of the rear brake cylinders. A disc/drum MC had been substituted for the correct drum-only cylinder.
Jerry

Again, Jerry's right. 23% more pressure with the smaller cylinder, but it also takes that much more pedal movement. So if you pedal gets pushed down 4" for brake application now, it will be 5" with the new m/c. If I were doing the work for hire, I wouldn't due to liability issues.