Bore size for Manual brakes

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Muswagon

Muswagon

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The power brake system cleared everything but I am making FA for vacuum. I may get a pump down the road but in the meantime I can try manual brakes.

The centric master I ordered has a 1.031 bore

The master on the booster setup has a 0.937 bore

Would the difference be significant?

They are identical physically so the reason I ask is first out of curiosity, and second the power one is already bench bled and ready to go. Not that it is a big deal to do the new one...

Oldmanmopar had suggested a 1" bore in my other thead so wondering if .937 is significant in the pressure/volume world
 
It depends more on what effort you want to use to stop the car. The smaller the diameter of the piston, the less effort it takes to brake. You may also get a slightly lower pedal when braking with a smaller diameter over the larger diameter.
 
What brakes are we talking about?

.937" is a 15/16" master cylinder, which is actually what I prefer for most manual disks on these cars. A larger bore will give you a harder pedal effort, but will take less travel to engage the brakes. The smaller bore means a lower pedal effort, but it also means that you need more pedal travel. The smaller bore also produces higher line pressure, so, you actually get more clamp force at the calipers. But again, it will take a bit more pedal travel to create that.

My personal opinion is that the factory 1-1/32" master cylinders are a bit too large for manual brakes on an A-body, they give a high/hard pedal that makes it tougher to modulate the brakes- they're much more on/off with that size. 15/16" gives a little more pedal travel, so you get a bit finer control over your braking force. But, you need more travel to fully engage the brakes so it can make it a little more challenging to just slam- lock up the brakes. Of course, you don't WANT the brakes locked for maximum braking, but you still need to be able to do it.

The difference between 1" and 15/16" is noticeable, but whether one is better than the other is more about preference than performance. And of course it does depend on the caliper bore size too, so it matters which brakes you have.
 
cudamark said:
Mine was the Reader's Digest version! :lol:
Click to expand...

And true!

I just find it's easier to make a decision with more of the details. There's a lot of factors that go into sizing the master cylinder for a particular car's set up, and a decent amount of driver's preference too.
 
Some of the engines I build don't have a lotta vacuum, so I toss (all) the boosters and go for the 15/16 M/C .
Raybestos #36406.

RAYBESTOS MC36406 Master Cylinder | RockAuto

RockAuto ships auto parts and body parts from over 300 manufacturers to customers' doors worldwide, all at warehouse prices. Easy to use parts catalog.
www.rockauto.com
Gives the most "mechanical" advantage without power assist, straight bolt on .
Good luck
 
Do you really want to be bored with all the specifics? Been there done that.

Get a 15/16th or 1in bore. Slap it on and drive the car. Are you really going to notice the difference? Probably not.

Get a 4 bolt to 2 bolt adapter plate to run the newer brake master cylinders.

I’ll give you some real good advice, do not get the one with a slanted brake fluid reservoir… the pedal feel is great but if you do any fun spirited driving it sloshes air out of the front lid. I have this problem now but am to lazy to redo the brakes right now so I just bleed them every so often.

IMG_4067.jpeg
 
So here is the math...

1690143683231.png


15/16" = 14% easier pedal but 14% farther travel

1-1/32" = 6% harder pedal but 6% less pedal travel

(As compared to factory 1" bore for either manual or power)

 
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DentalDart said:
Do you really want to be bored with all the specifics? Been there done that.

Get a 15/16th or 1in bore. Slap it on and drive the car. Are you really going to notice the difference? Probably not.

Get a 4 bolt to 2 bolt adapter plate to run the newer brake master cylinders.

I’ll give you some real good advice, do not get the one with a slanted brake fluid reservoir… the pedal feel is great but if you do any fun spirited driving it sloshes air out of the front lid. I have this problem now but am to lazy to redo the brakes right now so I just bleed them every so often.

View attachment 1716118619
Click to expand...

I run one like that. Just change the gasket on the cap, no more issues.
 
DentalDart said:
Really? I feel like I changed the gasket before, I’ll try changing it again.

I have a brand new MC sitting on the shelf now.
Click to expand...

Really. The caps and gaskets aren't all that great, I found with mine after opening and closing them a bunch of times they will start to seep a little. Replacing one or both solves it, and if you just put them on and leave them alone they won't leak.

IMG_4936.jpg


Also, on the cap that raised section of plastic on the cap is a vent, and it works best if that's at the highest point- ie, don't put that facing the front of the car, because when you stand on the brakes the fluid shift will put fluid out the vent.

IMG_4937.jpg
 
You want around 1200lbs at the caliper with manual brakes; 750psi line pressure with a 15/16"bore MC is'nt enough. Been there, done that.

TO get line pressures up, ensure your pedal ratio is around 6:1 plus and use a 7/8" bore MC . With 7/8"bore I had just on 1000 psi measured at the caliper with a hard push on the pedal. Car is 3500 lbs with me in it.

I ended up having to use a 3/4" bore MC on my combo that put out just over 1200 psi. Pedal cleared the floor at full push but not by much.
 
ratbastad said:
You want around 1200lbs at the caliper with manual brakes; 750psi line pressure with a 15/16"bore MC is'nt enough. Been there, done that.

TO get line pressures up, ensure your pedal ratio is around 6:1 plus and use a 7/8" bore MC . With 7/8"bore I had just on 1000 psi measured at the caliper with a hard push on the pedal. Car is 3500 lbs with me in it.

I ended up having to use a 3/4" bore MC on my combo that put out just over 1200 psi. Pedal cleared the floor at full push but not by much.
Click to expand...

That why I started using Toyota M/Cs when needed, - vast selection of bore sizes/mounting styles, reasonably priced, readily avail on the parts store shelf, divorced reservoir.
Cheers .
 
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