Brake MS sizing vs pressure question

[TurboRA28]

Hi all, I recently 'upgraded' my master cylinder from a 14/16th to a 1" bore.

I say 'upgraded' as this seems to have caused a negative effect.

The peddal now is as hard as a rock, it basically feels unboosted/assisted in anyway.

There is very little peddal feel also. You really need to really stand on it before the brakes start to clamp down. There is very limited travel also, because the peddal is so hard you can not push it in much.

So i've been doing some reading and it seems that a larger master cylinder, while it can move a larger volume of fluid, the pressure coming out of it will be less?

So really a larger master cylinder isn't such a good thing as you lose pressure?

I'm guessing unless you have some peddal travel problem, and you are hitting the floor with the peddal, you don't need larger master cylinders?

Any info on this topic would be great.
Thanks
Joel

[Norbie]

A larger master cylinder has the same effect as making the pedal shorter: less travel, more effort.

You're correct, unless your pedal had too much travel to begin with and/or was too soft, there's nothing to be gained by upgrading to a larger MC.

[YLD-16L]

while it can move a larger volume of fluid, the pressure coming out of it will be less?

That's right, what size master cylinder (and booster diameter) does the vehicle use from which you have taken the brake calipers from? How does it compare to what you have?

Was your 7/8 bore giving too much pedal travel/spongy feel? It sounds like you might have actually needed a 15/16" bore if the 1" isn't providing enough pressure/too little pedal travel and modulation.

NB. Have you checked that your booster is definitely working? (with engine switched off, pump pedal several times then depress pedal as if to apply the brakes, turn engine on and the pedal should sink if the booster is operating properly).

[bobob]

here is a copy of something I found once while trying to work out if I change my calipers, do I need to change master as well:

What about the 19mm master cylinder upgrade?

This is pretty much a "feel" issue. The ratio of master cylinder piston area to total caliper piston area determines the mechanical advantage you get in applying pressure to the brake pads. The smaller the master cylinder piston (or the larger the caliper pistons), the more "advantage" you have (more force on the pads for less force on the pedal). However, while you apply less force, you have to push the pedal farther to get the same amount of movement at the pad. With a larger master cylinder piston (or smaller caliper pistons), you have LESS advantage, so you have to push the pedal harder, but you don't have to push it as far. The pedal feels firmer, but you're actually getting less braking. Some people like the feel this gives, so the upgrade isn't entirely a bad thing. However, many people make the upgrade because they think they're actually improving their brakes, when they're actually making them worse, just improving the feel.


Stolen from:

http://www.914club.com/bbs2/index.ph...0&#entry750550

[oldcorollas]

caliper vs master cylinder is a leverage thing.

the bigger the master cylinder, the smaller the caliper/master ratio is which means you have less multiplication of effort....


thinking of the master cylinder alone...

your foot preduces a force.
this force is resisted by all the fluid in the master at an equal pressure.
pressure works as force per unit area.
to resist the same force, a larger (MC) area has less pressure
a smaller area has more pressure (force per unit area)
higher pressure = more pressure in calipers = greater braking for given foot input.
bu tlarger MC = lower fluid pressure = less braking

or something.....

[bobob]

so it was say 20:1 ratio, now the master has been increased it is 30:1. But his leads to a drop in pressure or braking force.

The plus side is need to change to bigger calipers!!!

If you spent the time you could mathmatially work it out to give you the same as before.

Work out your old ratio, and change your calipers to get back to near that ratio, with the bonus of increased braking contact area!

[YLD-16L]

I think something that people with older cars often don't think about too is that a lot of modern cars have significantly larger diameter vacuum assist (brake boosters) than the old brake booster they retain in there car.

So whilst a heavily vacuum assisted pedal may feel good in the donor car the same brake setup in the modded car with the original smaller booster retained may require a lot more effort.

A lot of older cars don't have room for the larger boosters. When taking the brake parts from the donor car and taking master cylinders from the likes of Pajeros etc which have enormous boosters it is no wonder that the pedal may feel hard in the new modded setup even though the same master to slave piston ratio has been retained.

Just something to keep in mind .

[30psi 4agte]

I think something that people with older cars often don't think about too is that a lot of modern cars have significantly larger diameter vacuum assist (brake boosters) than the old brake booster they retain in there car.

So whilst a heavily vacuum assisted pedal may feel good in the donor car the same brake setup in the modded car with the original smaller booster retained may require a lot more effort.

A lot of older cars don't have room for the larger boosters. When taking the brake parts from the donor car and taking master cylinders from the likes of Pajeros etc which have enormous boosters it is no wonder that the pedal may feel hard in the new modded setup even though the same master to slave piston ratio has been retained.

Just something to keep in mind .

VERY good point about the older cars having smaller boosters and when people upgrade their master to a bigger diameter the booster is often overlooked and cause the issue of a firmer pedal !

[TA-022]

what brakes are you running joel?

going on this as a ratio game Master:Calipers

ive gone from RA28Master:RA28Calipers

to

MA70Master:HILUXCalipers

and i know eraly landcruiser / hilux used 1" master and considering im using the hilux/landy calipers I shouldnt have these issues mentioned ...

i think?

[oldcorollas]

compare the piston area to the master cylinder area.
remember that if you have changed from sliding calipers to a fixed caliper, you neeed to multiply the sliding caliper piston area by 2.

[TurboRA28]

I have hilux 4 spot calipers up front, and st141 single piston sliding calipers on the rear.

The hilux/landcruisers i've seen that the calipers came from are mostly 14/16ths or 15/16th.
But must be some 1" ones out there also?

Anyhow... I'm going back to the 14/16th it felt much better

[takai]

Which type of Hilux caliper? The early model or the late model. Early had differentially sized pistons and took an RX7 pad, the late had same sized pistons and take the 200SX/R32/Z32 pad.

For reference i have just put a 7/8th unboosted cylinder in mine, will see what its like tomorrow. I have just gone up to the late model caliper too.

[TurboRA28]

Early model, with the rx7 pads.

[takai]

Yeah, i woudl go to the 15/16ths, pedal feel was much better with that size piston and those calipers for me. Im hoping that the 7/8th unboosted will be enough for the late model calipers.
The 1" GTR master i had on there before the 15/16ths Starion was quite heavy and really didnt give a very convincing feel.

[TA-022]

i wonder if there is a fluid capacity difference between the calipers and old slave cylinders on the rear? and if that is enuf to affect this ratio?

also what proportioning valves are you using now joel??? maybe their a restriction ... increasing pressure and giving the tough pedal perhaps???