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  #1  
Old August 19th 2008, 06:06
mabbo mabbo is offline
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master cylinder for stock front disc + empi rear discs

hello people,

i was going to upgrade to 944 brakes all around, but the guy i was buying the parts off has let me down so ill be sticking to what i have for the moment.

i have stock vw front discs/calipers with the empi rear disc kit. its a single slider caliper.

i fitted a new duel circuit vw master cylinder but i couldnt get a good pedal. i think the master cylinder may have been faulty. the pedal was firm but went all the way to the bulkhead. (with rear flexis blocked off the pedal felt great)

i fitted my mates 944 master cylinder (which he will need back soon) with the 19mm to the rear calipers and the bigger bore to the front calipers.

now the pedal is really firm, but i have to press the pedal really hard to get the brakes 2 stop quickly. my old stock front caliper/rear drum setup with stock master cylinder worked better

should i get another vw master cylinder or try something else?

the others i have considered are: audi 80/audi 100/csp upgraded one (around 20mm bore i belive)/stick with 944

what do you suggest?

cheers guys
mabbo
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  #2  
Old August 19th 2008, 07:19
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evilC evilC is offline
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Firstly what size is the Empi rear piston? The front if a 944 sliding caliper should be 53.5mm piston. The standard 19/19 m/c should be well up to the job if the bias front and rear are within reasonable limits . There should be no need to play around with m/c's and I wouldn't even speculate as to what will work without first assessing the front/rear bias.
Ideally, because of the rear weight bias the starting point is equal braking front and rear. This can then be adjusted to suit preferences and specific car mods.

evilC
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  #3  
Old August 19th 2008, 12:46
mabbo mabbo is offline
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dude
36mm i think. the front bug calipers are 40mm if i remember right?
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  #4  
Old August 19th 2008, 18:49
flat flat is offline
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I had the same problem with my stock front disks and empi rears. Just couldn't get things right. So I did the best thing, got rid of them and put on the Boxsters...

But, this should solve most of your probs till you can spend the bread for the Porkers:
http://www.cip1.ca/ProductDetails.as...13%2D16%2D9554

The 944 MC on the stock disk/empi setup will be horrible. The pedal effort will be huge! Unless you really need it I keep the master as small as possible to improve line pressure. (I'm using a stock 19mm on my Boxster S brakes with amazing results).

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  #5  
Old August 19th 2008, 19:42
C4 Metal Werks C4 Metal Werks is offline
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Someone please correct me if I am wrong here. The later Bay Window Buses and Vanagons both use a large master cylinder that the SB's and I think you could bolt on up if you had http://www.wolfgangint.com/Parts/Mas...r+Adapter+Kit/

On some of the European parts websites there are pictures of Vanagon looking master cylinder.
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  #6  
Old August 19th 2008, 20:37
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Yeah, but at 24mm the bus MC is way too big..... You'd need quads like Schwarzenegger to stop the beast.
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  #7  
Old August 19th 2008, 20:55
C4 Metal Werks C4 Metal Werks is offline
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Even a 71 bus with front discs and a brake booster uses a 26mm master cylinder? I'm pretty sure that is what I used on my 65 bus when I converted it to 4 wheels discs.

Quote:
Originally Posted by flat View Post
Yeah, but at 24mm the bus MC is way too big..... You'd need quads like Schwarzenegger to stop the beast.
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  #8  
Old August 20th 2008, 03:06
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Hi

I'm running early Type 3 front on my daily driver bug, they have 42 mm pistons and on the rear I have some 944 rotors with 2 spot Brembos from some sort of 911, these also have 42 mm pistons, the stock 19 mm MC works fine, the back end is very slightly over braked, I'm going to fit a Tilton proportioning valve to the rear circuit to correct this.

Steve
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  #9  
Old August 20th 2008, 08:31
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If you've got 40mm dual pot calipers to the front and single 36mm to the rear then the front/rear bias will be 2.47:1. That is too much front bias. With standard 944 single sliding pot calipers the front bias is 2.2:1 that can be moderated by the 944 24/19 stepped caliper that gives a 1.38:1 bias with the 944 caliper and 1.55:1 with the VW caliper. The underlining problem is that the rear caliper is only a single sliding unit of limited piston diameter. Change that to something better (say a 2 pot caliper) and your problem will be on the way to solution. Don't get hooked up on fluid volumes as the 19mm piston size in the standard dual cylinder should be more than man enough to move sufficient fluid to take up the slack before pad goes solid - assuming of course that there is no air in the system and the discs/rotors are within tolerence?? Also make sure that the front calipers are plumbed up to the rear m/c piston that is closest to the pedal in all cases.
Also, don't worry about over braking the rear as it is easy to install a brake proportioning valve to limit rear line pressure but just make sure you don't drive it without the valve as the vehicle will swap ends in the first panic stop.

evilC
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  #10  
Old August 20th 2008, 09:54
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Quote:
Originally Posted by C4 Metal Werks View Post
Even a 71 bus with front discs and a brake booster uses a 26mm master cylinder? I'm pretty sure that is what I used on my 65 bus when I converted it to 4 wheels discs.
The late bays use a 24mm dual circuit MC. The pedal ratio of an early bus allows the use of a larger MC without drastically increasing the pedal effort. A bug has less pedal ratio and hence you need more pedal effort.

For the record, I've got a 4 corner C2 (1991) setup on my 65 bus with a early 944 stepped MC (23/19), and I'm heavily leaning towards and early 911 (20.6mm) MC to increase line pressure and decrease pedal effort. Although I do admit that my pedal is very reasonable and it's doesn't take much foot to make it stop, but I'm sure it can be better.

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  #11  
Old August 21st 2008, 03:12
mabbo mabbo is offline
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thanks for the replys guys

well, im not 100% sure that the bug caliper has 2x 40mm pistons, im sure i read that online somewhere.

im quite suprised how front biased the brakes are with the stock front vw disc/caliper setup and a empi rear kit. when i couldnt get a firm pedal with my stock duel circuit master cylinder i thought it must be the problem because empi wouldnt sell so many rear disc kits if they havent been designed to work on a near to stock bug... guess i was wrong.

so, to improve the bias, im going to have to improve my rear calipers... again?

i thought the early 944 rear calipers also had 36mm sliding calipers on the back, with big (54mm?) sliding calipers on the front? so the rear 944 caliper and rear empi caliper are v.similar? what if i upgraded my front calipers to something bigger? maybe a 4 piston wilwood or something then keep the 944 master cylinder?

sorry if that goes against what you have just said - im really not all that clued up on brakes or braking systems - hence my posts

mabbo
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  #12  
Old August 21st 2008, 08:34
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Yes, the Empi 36mm single pot rear is very similar to the 944 rear that is also 36mm single pot. Simply transposing the 944 set up into a bug is not the way to go as the 944 is a heavy front engined car that needs a strong front brake bias. Even though it is reasonably balanced front to rear ~50/50 with the rear transaxle, under braking there is a substantial weight transfer, needing all the effort to the front. The bug however, is tail heavy ~37/63 and requires a 50/50 braking bias. Originally the 944 had a 19/19 m/c that gave a front bias of 68.75/31.25 but this was later moderated back to 59/41 with the stepped 23/19 m/c.
To make life simple for you to select your brake package lets assume that the ideal brake bias for the bug is still 50/50 then:
Total Front Piston Area/master cylinder piston area = Total Rear Piston Area/master cylinder piston area.
As previously said, if the rear is larger than the front then that is easy to deal with by incorporating a pressure limiting valve in the rear line.

evilC
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  #13  
Old August 21st 2008, 16:35
mabbo mabbo is offline
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hey dude.

thanks for that. how does that calculation work with twin or 4 piston calipers compared to single sliders?

i have just worked this out but i dunno if its right as the front calipers are 2 piston, and the rears single piston:

944 master cylinder:
40/23 = 36/19
1.74 = 1.89

bug master cylinder:
40/19 = 36/19
2.11 = 1.89

my maths is rubbish, i dunno how to change that to a ratio :S

mabbo
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  #14  
Old August 21st 2008, 17:57
mabbo mabbo is offline
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i have just been playing around with the excel calculator Angelo Amato posted on another thread:

http://myweb.tiscali.co.uk/angeloamato/BIAS.xls

with 40mm front, 36mm rear and 19.06 master cylinder = ratio 1.235 to front
with 40mm front, 36mm rear and 23.81/19.06 master cylinder = ratio 0.791 to front
with 53.5mm front, 36mm rear and 23.81/19.06 master cylinder = ratio 1.415 to front
with 53.5mm front, 36mm rear and 19.06 master cylinder = ratio 2.209 to front.

what sort of ratio is acceptable? i assume from the list above 1.235 is the best, so maybe a stock 19.06/19.06 master cylinder would be best, or, the 20.64/20.64 would give the same ratio with a firmer pedal.

am i on the right track?
mabbo
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  #15  
Old August 22nd 2008, 07:48
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Quote:
Originally Posted by mabbo View Post
hey dude.

thanks for that. how does that calculation work with twin or 4 piston calipers compared to single sliders?

i have just worked this out but i dunno if its right as the front calipers are 2 piston, and the rears single piston:

944 master cylinder:
40/23 = 36/19
1.74 = 1.89

bug master cylinder:
40/19 = 36/19
2.11 = 1.89

my maths is rubbish, i dunno how to change that to a ratio :S

mabbo
The calculation requires the TOTAL piston area for the caliper(s) it will therefore take care of the single pot, 2, 4, 6 or even 8 pot calipers, even those with different sized pistons in the same caliper.

Your maths is rubbish You have factored in the diameter not the area . you can discount pi in the calculation as it is a comparison and features on both sides of the equation therefore your comparison should read:

944 master cylinder
4 (2 pistons each side) x 1600 (40 squared)/529 (23 squared) = 12.10
2 (1 piston each side) x 1296 (36 squared)/361 (19 squared) = 7.18
Therefore the front rear ratio is 12.1/7.18:1
i.e 1.69:1

bug m/c
4 x 1600/361 = 17.7
12 x 296/361 = 7.18
Therefore the ratio is 17.7/7.18:1
i.e. 2.47:1

Joe Amato's figures are wrong insofar as they don't take into account the total piston area of the caliper. The hydraulic fluid is acting on both pistons simultaneously of a 2 pot caliper

evilC
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