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Albertoprop17
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PostPosted: Fri Aug 14, 2020 8:20 am    Post subject: Master cylinder question Reply with quote

Hi, I have front disc brakes with single 48mm calipers and stock post 68 rear drums. I will upgrade the rear to discs with 38mm calipers. Now I have a 19mm M/C and I feel the brake pedal on the soft side, it comes near to the floor (the car brakes nice)

Will the rear 38mm calipers increase the softee feel with the current M/C? Should I upgrade it to 20.6mm or 23mm (porsche?) M/C?
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Howard 111
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PostPosted: Fri Aug 14, 2020 9:22 am    Post subject: Re: Master cylinder question Reply with quote

The CB kit for 4 wheel discs come with a 20.5 M.C. If it feels soft with what you have now, I would definitely upgrade.
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raygreenwood
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PostPosted: Fri Aug 14, 2020 12:11 pm    Post subject: Re: Master cylinder question Reply with quote

Albertoprop17 wrote:
Hi, I have front disc brakes with single 48mm calipers and stock post 68 rear drums. I will upgrade the rear to discs with 38mm calipers. Now I have a 19mm M/C and I feel the brake pedal on the soft side, it comes near to the floor (the car brakes nice)

Will the rear 38mm calipers increase the softee feel with the current M/C? Should I upgrade it to 20.6mm or 23mm (porsche?) M/C?



Put the discs on first before you change MC.

Yes the front piston is pretty big...but is that a single piston caliper?

The 38mm for the rear is about right...especially if they are single piston.

I have not found that many front and rear disc brake set ups that could not do just fine with the correct 19mm dual circuit cylinder. Of course....part of what matters is how the stroke length is limited by the pistons inside.

Bear in mind....for example:

Porsche 914 with 42mm twin pistons each...calipers in front...and twin pistons each 33mm (which were a little small...could have used 36 or 38 to better effect)......and t did just fine with a 17mm master cylinder.

https://www.pelicanparts.com/914/technical_specs/914_brake_data.htm

Part of this as well....which you may or may not have control over....is the ratio at the foot pedal.

The other most important part....is fluid volume requirement. Right now you are judging this with drums.

While the wheel cylinders of drum brakes seem to be MUCH smaller diameter....they are also twin piston in reality. So a 22mm drum brake piston has one on each side of the bore which is equal to a single 44mm piston.....and....drum brake shoes retract a lot father so you are talking about a longer throw on the drum piston....more volume.

The brakes I used in the example above...914 Porsche....they have a "windage" setting adjustment for the piston. So the gap between piston/pad and rotor is very small...0.2mm which is .0078"...which means the pads only have to travel .008" to clamp the rotor.....so not very much fluid volume at all.

The front calipers only have 0.2 to 0.5mm windage or rotor clearance....so .00" to .019" clearance...again...not much fluid volume required to operate these.

One of the biggest falings I have seen with people installing rear discs with front discs and complaining about lack of pedal with a standard 19mm dual circuit MC....is not doing anything but installing the new calipers.

They typically have not addressed or adjusted "windage" or pad to rotor starting and at rest gap.

Bear in mind.....the way calipers work....they only have to move about .010" to work....and then retraction of the pad and piston when you let of f the pedal.... is not accomplished by springs. Its accomplished by the ring seal flexing back. It only flexes about .005" to .010" maximum. Ray
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FreeBug
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PostPosted: Fri Aug 14, 2020 12:24 pm    Post subject: Re: Master cylinder question Reply with quote

Don't go taking any risks!
If someone knows, Ray' the one.
Discs can be tricky to master..

I messed up on my bad disks
It just slipped out, L5, S1,
And I wish it would heal faster.

And doctors really can be dicks:
my doctor says my back is done
and I didn't even ask her...

Dem's the brakes!
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Albertoprop17
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PostPosted: Fri Aug 14, 2020 12:34 pm    Post subject: Re: Master cylinder question Reply with quote

My front calipers are single 48mm piston, they are from an mk2 vw golf, they are fully rebuilt (empi ones were crap and can't be rebuilt)

The rear ones are going to come from a mk3 golf gti, also fully rebuilt.

I will install the rear discs without touching the M/C.

Also I have a '73 superbeetle with the same "problem". I put dual 48mm piston calipers from a Talbot Horizon, and I keep rear drums. The car brakes much better, it can lock front wheels fairly easy, but the pedal travel is too much for my taste. Should I put rear disc brakes on it too or can I replace the brake cylinder or swap for type 3 rear brakes

Thanks guys for your answers
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raygreenwood
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PostPosted: Fri Aug 14, 2020 12:59 pm    Post subject: Re: Master cylinder question Reply with quote

Albertoprop17 wrote:
My front calipers are single 48mm piston, they are from an mk2 vw golf, they are fully rebuilt (empi ones were crap and can't be rebuilt)

The rear ones are going to come from a mk3 golf gti, also fully rebuilt.

I will install the rear discs without touching the M/C.

Also I have a '73 superbeetle with the same "problem". I put dual 48mm piston calipers from a Talbot Horizon, and I keep rear drums. The car brakes much better, it can lock front wheels fairly easy, but the pedal travel is too much for my taste. Should I put rear disc brakes on it too or can I replace the brake cylinder or swap for type 3 rear brakes

Thanks guys for your answers


Cool! Those are both actually good calipers. Wit ha single 48mm sliding caliper piston....thats not that much piston area.

So your front circuit is feeding a total of two 48mm pistons....compared to say....a VW type 3 or 4....with FOUR 42mm pistons. Its nt that much fluid volume.

Now....that being said...and this is where a little bit of the MC sizing issue comes from. With single sliding calipers...since the piston is doing the work of two....you can have almost twice the "windage" adjustment between piston and inner pad...because it has to allow enough retraction to produce a gap on each side...instead of having each piston provide its own gap in a twin piston set up.

And...I feel that sometimes they are set up for excessive windage gap...which is what makes the windage gap adjustment all that much more important.

So in other words...a lot of times what you see...is that as you push the pedal...the pistons have to move too far to contact the pad and contact it to the rotor. In many cases this is not because the pistons require more volume....its because the gap they have to traverse is too big.

Most European sliding rear calipers with E-brake...have some form of windage adjustment. This is required...because they have to keep the gap between pad and rotor correct for the E-brake cable travel.

Most of these have some function of wear adjusting as you use the hand brake. They have a screw type ratcheting pin inside that turns the piston outward slightly each time to take up an excess slack.....but you have to set the windage right from the start...and really should adjust it at 100 miles after the new pads have broken in and worn to fit rotors.

The fronts.....they have only the flex/retract range of the seal inside the cylinder.

Depending on how much braking percentage the fronts do....and how fast the pad you are using wears...every once in a while the retraction rate gets out of calibration from the pad wear rate...so windage gap increases and so does pedal travel...but usually only for a short period and then they catch up and it returns to normal. Disconcerting.

Its worth it on many...to buy a good set of pad shims for the front . It makes the pad to rotor windage gap...slightly excessively tight during break in. But its helps to keep the windage preloaded toward the thin side.

You may need a bigger MC....but my point is that you will not know until you put the brakes together and adjust and break in everything. Ray
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FreeBug
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PostPosted: Fri Aug 14, 2020 1:17 pm    Post subject: Re: Master cylinder question Reply with quote

In my thinking (but I'm dumb),
the pedal travel has to do
less with the discs than with the drums:
the springs pull back the shoe,
that's what the volume's from.

Like Ray says, for pedal feel:
discs really don't move much,
It's just the flexing of the seal,
but a drum is different stuff,
the shoes have far to go!
More volume of brake fluid...
When you switch from go to slow,
you need more fluid to do it.

Recently a question was posed
by some smart and curious individual,
asking about travel and the purpose of those
brake valves, residual.

I think it's an interesting thought,
at least worthy of exploring,
But more than that, i know not,
I'm far too drum-adoring.

It's my silly thinkin, I guess,
they come with in-built ABS!
They stop from eighty to fifty, heating-prone...
But from fifty down, you're on your own!
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PostPosted: Fri Aug 14, 2020 3:01 pm    Post subject: Re: Master cylinder question Reply with quote

FreeBug wrote:
In my thinking (but I'm dumb),
the pedal travel has to do
less with the discs than with the drums:
the springs pull back the shoe,
that's what the volume's from.

Like Ray says, for pedal feel:
discs really don't move much,
It's just the flexing of the seal,
but a drum is different stuff,
the shoes have far to go!
More volume of brake fluid...
When you switch from go to slow,
you need more fluid to do it.

Recently a question was posed
by some smart and curious individual,
asking about travel and the purpose of those
brake valves, residual.

I think it's an interesting thought,
at least worthy of exploring,
But more than that, i know not,
I'm far too drum-adoring.

It's my silly thinkin, I guess,
they come with in-built ABS!
They stop from eighty to fifty, heating-prone...
But from fifty down, you're on your own!


Yes....^^^^^.....its usually a longer throw at the drum shoes......but just the same.....on fsirly normal sized calipers you are thinking about.....its usually the "throw" or travel distance of the pistons....and not the piston volume itself. Ray
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modok
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PostPosted: Fri Aug 14, 2020 10:59 pm    Post subject: Re: Master cylinder question Reply with quote

raygreenwood wrote:


Cool! Those are both actually good calipers. Wit ha single 48mm sliding caliper piston....thats not that much piston area.

So your front circuit is feeding a total of two 48mm pistons....compared to say....a VW type 3 or 4....with FOUR 42mm pistons. Its nt that much fluid volume.

Ray


I fail to see how having one piston or two makes any difference, if the two pistons are opposite each other, then the same fluid volume will produce the same clamping movement

you have made TWO errors in saying a drum brake 22 is like a 44
unless you meant that four 22mm pistons, in parallel, is the same as one 44?
Series or parallel, rather important difference Shocked

in any case, I agree with you, basically.

BUT, I prefer larger master cylinders, as long as the driver has a strong leg.
Wink
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PostPosted: Sat Aug 15, 2020 9:19 am    Post subject: Re: Master cylinder question Reply with quote

modok wrote:
raygreenwood wrote:


Cool! Those are both actually good calipers. Wit ha single 48mm sliding caliper piston....thats not that much piston area.

So your front circuit is feeding a total of two 48mm pistons....compared to say....a VW type 3 or 4....with FOUR 42mm pistons. Its nt that much fluid volume.

Ray


I fail to see how having one piston or two makes any difference, if the two pistons are opposite each other, then the same fluid volume will produce the same clamping movement

you have made TWO errors in saying a drum brake 22 is like a 44
unless you meant that four 22mm pistons, in parallel, is the same as one 44?
Series or parallel, rather important difference Shocked

in any case, I agree with you, basically.

BUT, I prefer larger master cylinders, as long as the driver has a strong leg.
Wink


The drum brake cylinder...on just one side.....if its 22mm diameter.....has two 22mm pistons....one on each end. They both have to move. Unless you are using something exotic like dual diagonal or staggered lines at the MC.....the wheel cylinders from the rear drums are "parallel" .....both fed from equal sides of the same Tee fitting from the single MC feed line.
All the pistons move at once unless controlled/limited at the shoe by a spring or if you have something really exotic that uses different size pistons at each shoe.

Anyway......my point was.....that most people miss.....is that the piston AREA of "wheel cylinders" in drum brakes.......is not that SMALL....when converted to volume USAGE with stroke.

Most people make this automatic assumption when trying to install discs in place of drums.....that the wheel cylinders are so small that the master cylinder feeding them....must be undersized for the calipers they just bought.

In reality.....with the drums having two small pistons per side....four small pistons total.....but roughly twice the piston travel/retraction because of the spring assist retraction on most drums and the fact that they generally pivot at either top or bottom.......the actual fluid volume required to operate the drum......is not that much different than the fluid requirement for a disc brake caliper.....with much bigger piston diameter.

And there are always exceptions...some drums have cylinders top and bottom.

And no.....not necessarily does it ,require four 22mm to equal one 44mm. Wink

That would depend if the 44mm was a single piston sliding caliper.....or a twin piston fixed caliper......and.....the wild card.....is the piston travel or stroke required to operate it.

The fixed twin piston caliper....most of them.....require less windage ...meaning less piston stroke.....than the sliding caliper....about half or a little less. And it typically has two smaller pistons than the sliding calipers one bigger one.....and the sliding caliper also has a longer stroke.

Volume required to operate....is not just piston diameter/area. The piston stroke is in that equation as well. This is where the volume on wheel cylinders really gets larger than people assume it is.

Its the total stroke required to operate a caliper or a wheel cylinder that most fail to take into account.

Back to your original point......about my point of the single 48mm piston sliding caliper versus the 42mm twin piston fixed caliper.....you noted you. "Fail to see why having one piston or two makes any difference if both pistons are opposite each other the same fluid volume will produce the same clamping movement".

Yes....it might....but also it may not. Again.....it depends on the piston STROKE required to operate each type of caliper.

The point I was making....or trying to make.....is that the 48mm single piston sliding caliper......is not that far off in total fluid volume usage than a fixed caliper with twin 42mm pistons......because the 48mm single piston sliding caliper requires about TWICE as much piston STROKE to operate.....than the twin piston fixed caliper.

We are technically saying the same thing. How so?

Whether you use fixed twin piston or sliding single piston.....you have two brake pads with two pad to rotor gaps. Lets say they are the same gap on either set up of 0.5mm which is 0.0197". Thats just the gaps from pad to rotor after the piston retracts when you let off the brake. Thats the "windage".

BUT.....on the fixed single piston caliper.....you have ONE piston that must retract far enough to produce a 0.5mm windage gap between TWO pads.....BOTH pads.

On the twin piston fixed caliper you have two pistons that retract the same amount each.....to produce 0.5mm of windage gap between each of their own single respective pads.

The fixed single piston caliper...has TWICE the retraction distance. It has to retract 1.0mm to produce two 0.5mm windage gaps. Therefore it has twice the stroke distance to fill back up with fluid to being the pads back into contact with the rotor.

The 42mm twin piston unit.....both pistons only stroke and retract 0.5mm.

My point was that because of this......the 48mm single piston volume requirement is only slightly more than a twin piston 42mm unit....and the stock MC already operates the twin piston unit just fine.

For reference, the 48mm single piston caliper with a 1.0mm stroke requires 0.11 cubic inches of fluid volume.

The 42mm twin piston unit with 0.5mm of stroke for each piston.....requires 0.042 cubic inches of fluid volume....X2 = 0.084 cubic inches. The difference between the two is 0.026 cubic inches more of fluid volume to operate the larger single piston sliding caliper.

Not insignificant but not huge. So two 48mm sliding calipers will use 0.22 cubic inches. Two twin piston 42mm fixed calipers will use 0.168 cubic inches.

A typical 19mm master cylinder has a 30mm maximum stroke ....and can produce 0.49 cubic inches per total stroke....max.

Yes.....that means that the two 48mm calipers with their bigger windage retraction gap needs right at 12.5mm of master cylinder/pedal stroke.....to move the pistons and close the windage gap.

The two twin piston 42mm caliper units will require a little less than 9.5mm of master cylinder/pedal travel to close the pad windage gap.

When we start getting into looking at the rear wheel cylinders and comparing what fluid volume they use versus calipers......you have to take into account that the wheel cylinder/drum/shoe set up.....has MUCH more piston travel to produce contact between shoes and drum. Ray
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PostPosted: Sat Aug 15, 2020 10:21 am    Post subject: Re: Master cylinder question Reply with quote

NOTE.....bear in mind that the volume numbers above are just talking ahout the fluid volume required to move the pistons....which are already in contact with the pads.... into contact with the rotor surface.
That's just about pedal TRAVEL. I did not get into the more or less pedal PRESSURE required for stopping.
Ray
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PostPosted: Sat Aug 15, 2020 12:37 pm    Post subject: Re: Master cylinder question Reply with quote

Agreed.
Tho I would say that drums require far more stroke because they have far more flex. Force is exerted over the circumference of the shoes and drum, a distance of several feet any way you look at it Shocked

If the drum brake has one, two, or four pistons(and some designs do) it would be the same scenario, the pistons are all in "series" so the volume/travel is only related to the bore size, no matter the number of pistons.

And in case you were wondering the four piston design can allow a double leading configuration that also works in reverse also.

30% larger master goes with the 30% larger pistons, makes sense IMO.
I'd go for the 20.5, OR, even bigger


Last edited by modok on Sat Aug 15, 2020 12:51 pm; edited 1 time in total
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PostPosted: Sat Aug 15, 2020 12:51 pm    Post subject: Re: Master cylinder question Reply with quote

modok wrote:
Agreed.
Tho I would say that drums require far more stroke because they have far more flex. Force is exerted over the circumference of the shoes and drum, a distance of several feet any way you look at it Shocked

If the drum brake has one, two, or four pistons(and some designs do) it would be the same scenario, the pistons are all in "series" so the volume/travel is only related to the bore size, no matter the number of pistons.

And in case you were wondering the four piston design can allow a double leading configuration that also works in reverse also.



Yep! Understand on the series point. On a drum brake like that.....in series...they all move as one with the same volume change.

Hmmm.....more flex? So....do you mean like....in a singe cylinder drum brake like the average VW.....because the shoes are pivoted/fixed at lower end.....and actuated at the top by the single cylinder.....that along with having to close the large windage gap between the shoe and drum......we also get some flex to the metal.....of shoe......or once the shoe contacts the drum....flex of the drum? .....or both?

That makes sense and I had not thought about that factor!
Ray
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PostPosted: Sat Aug 15, 2020 12:52 pm    Post subject: Re: Master cylinder question Reply with quote

EVERYTHING flexes. it's downright spooky
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PostPosted: Sat Aug 15, 2020 2:47 pm    Post subject: Re: Master cylinder question Reply with quote

modok wrote:
EVERYTHING flexes. it's downright spooky


Come to think of it.....I should have known that.

On the type 4 cars and 914's......one of the differences in the early to late rotors.....is that the late ones have a flange in the center that makes them "hub-centric". Most people think they did that to make the wheel easier to install.

In fact....it was done to make the rotor hat stiffer. The earlier non-hub-centric rotors....required the cal8per to have guide pins in the piston bores that went into l9cking chucks in the back of the pistons.

When you were cornering.....and braking.....the rotor hat flexed enough that it leveraged the pads into an angle......which misaligned the pistons in the bore enough to cause seal wear and leakage.

Yes.....flex. Ray
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PostPosted: Sat Aug 15, 2020 4:31 pm    Post subject: Re: Master cylinder question Reply with quote

modok wrote:
EVERYTHING flexes. it's downright spooky


I remember the first day
I put a dial indicator
to measure the endplay,
and then a little later,

I discovered (OMG!)
that me just me,
could bend a flywheel easily.

YEAH!

I wish that day could've lasted longer,
from that day on, I've never felt, alas, any stronger.
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PostPosted: Sat Aug 15, 2020 5:06 pm    Post subject: Re: Master cylinder question Reply with quote

You need a bigger master. I had Wilwood fronts and t3 rear drums, 20.6 master. Car stopped good pedal felt normal. I switched to strange two piston rears and the car stopped fine, but the pedal was spongy. It felt like air in the lines. After many attempts at bleeding and pressure bleeding, and residual pressure valves and all of that I upgraded to a 1-1/16 bore master and itís been great since.

Smaller masters move less volume and generate pressure with less effort. Discs need more volume and less pressure.
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