Good Afternoon, All,

We all know (or at least SHOULD know) that increasing the bore size of the brake master cylinder results in WORSE brake performance. (Lower line pressure per pound of pedal pressure....See TheOgre's cave for LOTS of clear information on the subject!)

It seems to me that the reverse would also be true: DECREASING the bore size will result in increased line pressure (and therefore clamping force) per pound of pedal pressure. The only cost I can see to this is increased pedal TRAVEL until the pads contact the rotors. So, smaller bore equals increased clamping force and, I assume, better modulation prior to locking up the tires which is a very good thing.

So, am I missing something?

And has anyone actually TRIED it? How did it work for you? Do you recommend it?

TIA,

Larry

Outside of the S-10 brake booster that increases pedal pressure I gave up on trying to re-engineer the Fiero brake system. With good wheel cylinders a new Master cylinder and the S-10 booster, I can lock up the wheels. That's good enough for me. Brakes are your essential safety mechanism. Figured my life is worth more than a brake project that may deliver questionable performance. I would stay with the stock system.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

quote Originally posted by imacflier: The only cost I can see to this is increased pedal TRAVEL until the pads contact the rotors.

Correct, the pedal will have to move further.

Actually, that's the greatest risk of this modification. If the pedal hits the floor before you're able to lock up the brakes, well then you'll never be able to do full emergency braking.

Normally there's supposed to be some "buffer zone"; extra travel beyond what is needed for lockup in case you get air in the system, or more squishiness for whatever reason.

I would not necessarily say automatically no to the modification (for a one-owner car), but you'll have to make sure that you have enough buffer zone. And it will be a smaller buffer zone than stock.

quote Originally posted by imacflier: better modulation prior to locking up the tires which is a very good thing.

I would say a more squishy brake pedal.

Whether that facilitates modulation is then up to the driver and his body.

For some, that makes modulation easier. The mountain bike crowd seems to like a soft brake lever.

Personally, I find brakes easier to use when they are firm.

quote Originally posted by imacflier: And has anyone actually TRIED it?

Not me...

Voluntarily reducing the buffer zone isn't my cup of tea, neither is making the pedal more squishy.

quote Originally posted by imacflier: DECREASING the bore size will result in increased line pressure (and therefore clamping force) per pound of pedal pressure.

That's correct, unless the pedal hits the floor.

[This message has been edited by pmbrunelle (edited 10-10-2020).]

"has anyone actually TRIED it?"

Not Likely they did because running small MC w/ stock system have same problems running Stock MC w/ GA and other calipers w/ big pistons and running out of fluid for front brakes at minimum.

OE Secondary MC piston and front brakes have very little Volume as is w/o "squeezing dry" by hitting travel limits.
Want "real fun?" Have the tiny leak in front brakes and not stop next red light... Rear brakes can live w/ same tiny leak but you may notice pedal slowly drop at a red light after stopping.

Most people only know front brake failure After hitting someone.
Hitting someone w/ bogus "upgrades" to the brakes can cause criminal and civil court problems that can bankrupt many people and their families.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

A small bore master cylinder would have similar results as larger bore calipers. And that is something I have experience with. For awhile, I had Grand Am brakes on my Fiero (front and back).

The larger caliper bores gave the hydraulic system more mechanical advantage. So it was easier to lock up the brakes. Actually, it was too easy. It made the brakes feel "grabby." I accidentally locked up the brakes on a couple occasions. The increased travel also made the brake pedal feel mushy. So as I pushed the brake pedal, it would initially do nothing as I took up the slack, then suddenly the brakes would grab hard. So modulating the brakes was tricky.

I also considered that to be a safety concern. So I ended up replacing the brake system with something completely different.

But in your case, I think the S10 booster upgrade would probably be a better choice. That's assuming you don't want to completely re-engineer the braking system. Also, get some high quality brake pads. It'll make a difference.