Does Master cylinder bore size really make a difference.

[Inertia]

I posted this at the end of a previous thread, - but felt it fills an often asked question .
I mess with bore sizes for custom brakes .

I recently had the owner of a 67 Barracuda bring the car back, even tho he had had the brakes done previously, he just wasn't happy with the brakes, adding he wasn't really happy with the brakes since purchased, new brakes didn't seem any better.
Drive it and Confirmed the brakes weren't "great", but 9" drums ?
I took the 9" drums off, the wear on the shoes and drums was near total contact.
My immediate thought was the Master had been changed thru the decades, and probably had a larger bore M/C installed for whatever reason.
Removed it, - Nope, was the 1" bore it was supposed to be. hmmmmm
I stock the 15/16 bore M/C, so thought what the hell.
It's a disk m/c with a large reservoir, - makes no difference, yeah, I know, yada yada. .
Well. Hot damn.
The difference of 1/16 smaller bore made all the difference, locks up, nice feeling, hard pedal.
Some will recognize this car, -
So I explained that at shows and such, someone will notice the disc m/c and ask about the disc brakes, which would lead to why the WRONG m/c, and - YOU CAN'T DO THAT ! !
I suggested he just nod his head and smile, knowing he has the best brakes ever .
Customer is super happy !
Pics of removed m/c measured .

 

[jos51700]

Pressure is force over area. Hell yeah it matters.

If it didn't matter, there wouldn't be different sizes. The factory wasn't changing those bores to make friends and influence people....

 

[mvh]

Your description isn't entirely clear — are you saying you installed a disc/drum master cylinder on a car with drum brakes?

That would not be as bad as using a drum/drum MC with disc brakes. But it still leaves open the possibility of a few odd side effects.

Also, the smaller bore MC should make the pedal seem SOFTER, not harder. So I suspect there was a problem with the original MC, and the improvement is due to fitting a properly working MC rather than the size difference.

 

[Inertia]

Your description isn't entirely clear — are you saying you installed a disc/drum master cylinder on a car with drum brakes?

That would not be as bad as using a drum/drum MC with disc brakes. But it still leaves open the possibility of a few odd side effects.

Also, the smaller bore MC should make the pedal seem SOFTER, not harder. So I suspect there was a problem with the original MC, and the improvement is due to fitting a properly working MC rather than the size difference.

There's the yada yada .

You can't see, - but I'm just smiling and nodding my head, lol

 

[1WildRT]

He's Gonna Die!!!!!! Yeah, we all are... Sooner or later...

 

[jos51700]

He's Gonna Die!!!!!! Yeah, we all are... Sooner or later...

I always figured my death with be more like... An obliteration, like in a bomb or something. Not some lame death like my wife finding out I died while getting a bj from my girlfriend.

But I could see my tombstone saying something like, "he forgot to put the little rubber thingy on the master cylinder pushrod. Nobody else got hurt. A-body 8 3/4 with 3.55 SG, unharmed, $850"

 

[4spdragtop]

Hyperbole vs experience!

 

[yangiant51]

i just got my 69 Swinger 340 back on the road after sitting over 20 years, I had a guy go over the 10" brakes and he replaced everything including the master cylinder (he also used Dot 5 silicone brake fluid). The brakes always worked great before I parked it and now the pedal is high but the pedal feel is hard and the car doesn't stop well at all. I then took it to another brake specialist and he readjusted the brakes and checked everything out and it stops a little better but the pedal is still hard and if I stand on the pedal as hard as I can the brakes wont lock up. Do you think a smaller master cylinder bore would help? Thanks Joe

 

[340sFastback]

i just got my 69 Swinger 340 back on the road after sitting over 20 years, I had a guy go over the 10" brakes and he replaced everything including the master cylinder (he also used Dot 5 silicone brake fluid). The brakes always worked great before I parked it and now the pedal is high but the pedal feel is hard and the car doesn't stop well at all. I then took it to another brake specialist and he readjusted the brakes and checked everything out and it stops a little better but the pedal is still hard and if I stand on the pedal as hard as I can the brakes wont lock up. Do you think a smaller master cylinder bore would help? Thanks Joe

If you have a bigger bore master cylinder that would explain it. A smaller bore will give you a lot more stopping power with less leg effort. Smaller bore will increase brake pedal travel.

 

[matthon]

I have original manual KH disc, recently I replaced the mc as it failed, and due to availability or stupidity or whatever, I replaced my 15/16 ("less leg effort") mc with a 1 1/32 ("more effort") mc.

The only difference I noticed with the new master was less pedal travel and the brakes came on sooner, which I like, prefer with the manual brakes.

Bottom line, the pedal is not difficult by any means and the car stops incredibly well.

 

[67Dart273]

Someone mentioned cylinder area. That is very easy to figure. But there MAY be another factor. You take one off, you slap on another. BUT YOU MAY possibly have JUST SLIGHTLY changed the leverage. WHAT?? How. Because "maybe" the new cylinder has a slightly different depth at the rear. This changes the pedal in it's arc, and the pedal leverage CHANGES as it goes through that arc. Pedal leverage is going to be maximum when the pedal is effectively at 90 degrees to the pushrod. If it's either up or down from that, the arc will cause a change in that advantage.

Mine had 9", manual brakes, and the factory cylinder. You could squal all for tires at 70. But you could not do that very many times!!! Brake fade. I have no idea what type of lining were on them.

When I had the first "white whale", the 1/2T 2WD Dodge, 97? I forget. It stopped "fair." I read online about fooling with cylinder size and calipers. I believe 3/4T calipers and on a 1/2T pickup. Turned that thing from stopping fair into something that would put you right through the windshield.

 

[Inertia]

i just got my 69 Swinger 340 back on the road after sitting over 20 years, I had a guy go over the 10" brakes and he replaced everything including the master cylinder (he also used Dot 5 silicone brake fluid). The brakes always worked great before I parked it and now the pedal is high but the pedal feel is hard and the car doesn't stop well at all. I then took it to another brake specialist and he readjusted the brakes and checked everything out and it stops a little better but the pedal is still hard and if I stand on the pedal as hard as I can the brakes wont lock up. Do you think a smaller master cylinder bore would help? Thanks Joe

The smaller bore does create more line pressure, and many spirited drivers prefer it for better brake control/modulation.
These days the replacement shoes come about 0.030 oversize, and most shops these days don't have the drum turning, shoe matching "arc" grinder, to match the shoe to the drum diameter.
It seems acceptable, and not uncommon to send a vehicle out with less than perfect brakes, but " wearing-in " to better braking is expected.
You just may be in that " wearing-in" period with better brakes yet to come.
Were it me, I'd put some miles on it, expecting it to get better, and if it doesn't, take it back.
Having said that, - I'd put the smaller m/c regardless.
I put the small m/c on everything, Chargers, Challengers, Station Wagons, .

Good Luck

 

[Inertia]

Pedal travel between different bores is often mentioned.
I've never tried to measure the difference, between many sizes, but it is negligible .
Properly adjusted brakes should have firm/hard pedal in about an inchish of travel regardless.

 

[matthon]

Let's not forget shoes these days suck and can easily glaze the drums and the result is crappy brakes that give out heart failure.

My wagon with all new power drum brakes wouldn't stop for dookie.

Found an old timer with good 'ol parts store shoes, the toxic kind, thing will throw you and all 8 other passengers through the windshield.
Well, 2 would go through the rear window once stopped.

 

[66Dartdriver]

How would you know that a master cylinder is a disc or a drum one?

 

[1WildRT]

How would you know that a master cylinder is a disc or a drum one?

Drum brake master has two fluid reservoirs both are about the same size..
Disc brake master has two fluid reservoirs both are different sizes.. The one for discs is bigger since as disc pads wear the piston moves out to compensate for wear... That has no real effect on the master, it doesn't care what it is supplying fluid to...

The drum brake unit also may or may not have residual pressure valves hidden in the ports... And the disc brake master may have a residual pressure valve in the port to the rear brakes.... Early design wheel cylinders needed residual pressure to keep the cups sealed... They were redesigned in the seventies & really don't need to residual pressure anymore...

 

[RustyRatRod]

Yes.

 

[harrisonm]

Let's not forget shoes these days suck and can easily glaze the drums and the result is crappy brakes that give out heart failure.

My wagon with all new power drum brakes wouldn't stop for dookie.

Found an old timer with good 'ol parts store shoes, the toxic kind, thing will throw you and all 8 other passengers through the windshield.
Well, 2 would go through the rear window once stopped.

Dookie?? LMAO. I haven't heard that one in years. Thanks. I got a good laugh out of that.

 

[Bewy]

Joe [ yangiant ].
Find out what was the original m/c size for 10" brake cars. Remove your m/c & check that it is the correct size.