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View Full Version : Brake M/C Plumbing?



Knestis
04-20-2005, 11:11 PM
The GTI is losing the ABS this week. We have all of the parts - I think - and are including a Tilton lever-style limiting valve in the rear branch of the system.

We'll run one line through the cockpit, T it at the rear axle, and run one leg to each of the rear wheels.

The question is where to connect the three lines to the new, non-ABS master cylinder. It's essentially the same as a MkII. In everyday life, it probably doesn't make any difference which port we plug but I've never thought about it very hard and it seems stupid to not at least ask around.

Are we neglecting any considerations?

K

924Guy
04-21-2005, 09:32 AM
Your considerations should be proportioning and plumbing. Sorry if this sounds obvious; let me explain myself. Also I have to speak in general terms, since I don't know exactly what that MC looks like.

You need to split the circuits in event of brake failure, so that you have even braking (X-split or TT-split) in case of losing one circuit. Most likely your MC has the 3 ports located such that 2 are on one circuit, and one is on another circuit? Keep in mind that one end of the MS is one circuit, the other end is the other circuit. So, for example, in my 924 with an ATE (Teves) MC, there's two ports on one circuit intended for the front wheels, and one for the rears on the other circuit - it's set up for a TT split.

Also the rear port is furthest from the firewall, and is a smaller diameter, resulting in the needed proportioning for my (stock) brake system. Not sure if yours is a smooth or stepped bore.

So you'll want to probably grab one port from each end (circuit) of the MC to establish two distinct circuits. If you have two ports at one end, you'd be best off using both of those for both front wheels to allow optimal flow rates, rather than just using one.

Apologize if this is a little repetitive or more than you needed...

------------------
Vaughan Scott
Detroit Region #280052
'79 924 #77 ITB/GTS1
www.vaughanscott.com

Eric Parham
04-21-2005, 11:13 AM
Kirk, I remember going through this and that my decision was to run the front brakes off of the rearmost section of the MC (closest to firewall) and the rear brakes with prop valve off of the frontmost section, but can't remember my exact reasonong (it was 5 yrs ago). I think it had something to do with which section of the MC seemed slightly more prone to failure (e.g., more complicated or less directly actuated).

vwmann1
04-21-2005, 11:17 AM
Kirk

The brake system(non-ABS) for the mkII and mkIII are the same. All the things you did to the mkII will transfer to the new car.
The MC is not stepped. Plug the port at the MC or tee them together to one then back to the prop valve.

The 924/944 brake system differs from the VW one in that it does not have the cross braking system. One part of the MC brakes the front and other brakes the rear. Proportioning is also different.

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Doug Hillmann

shwah
04-21-2005, 05:06 PM
I ran both fronts off the front ports, and ran a single line through the cockpit off one of the rear ports. Put a plug in the 4th port. Works fine, and gives a front/rear split system in case of failure - although I don't look forward to ever trying to get rid of very much speed using the rears only.

Chris

PS - Doug, I have two pairs of strut tubes now so you can pick the best of the litter http://ITForum.ImprovedTouring.com/smile.gif

Bill Miller
04-21-2005, 07:51 PM
Kirk,

I'd take Doug's route, and 'T' the two rear ports of the M/C, run them to the prop valve, and then split them to the L/R and R/R brake calipers / wheel cylinders.

------------------
MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608

Knestis
04-21-2005, 10:03 PM
Thanks for the ideas, all.

How does running two ports to one tube before the prop valve an improvement over just running one line from the MC and plugging the other port?

K

Bill Miller
04-22-2005, 08:22 AM
Originally posted by Knestis:
Thanks for the ideas, all.

How does running two ports to one tube before the prop valve an improvement over just running one line from the MC and plugging the other port?

K


Kirk,

It's not necessarily an improvement, just the way I'd do it.


------------------
MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608

Greg Amy
04-22-2005, 08:54 AM
Something to consider as you're designing your system: volume displacement.

Since the Golf uses an cross-braking system, on a stock car each MC chamber displaces fluid for one front and one rear caliper. Thus, equal requirements.

However, when you re-plumb the system front-and-rear each (equal) chamber now displaces either two front or two rear calipers. These calipers use significnatly different piston sizes, thus displace significantly different fluid volumes. The rear caliper pistons will reach full extension well before the fronts.

The significance of this is that the rear hydraulic system will "hydrolock" (I can't think of a better term) before the fronts, and any further pedal displacement to fully extend the front pistons will result in much higher rear hydraulic pressure, thus much higher force. In a stock system this isn't a problem, as both MC chambers displace the same amount of fluid, and engineers size the pistons and pads to provide the appropriate baseline delta forces on the rotor. They then add a pressure regulator to reduce rear pressures at extreme performance envelopes.

If you are going to replumb your system, you will need to *significantly* restrict your rear pressure and/or volume; you will never be able to run the system fully open without experiencing rear brake lockup. I would encourage you to use a prop valve that bypasses flow if possible, rather than one that reduces pressure.

In my particular case, I continue to run the factory cross-braking system and factory prop valve, and I adjust rear brake bias via pad compounds/coefficients. To date it has served me well. - GA

racer14itc
04-22-2005, 10:13 AM
Originally posted by GregAmy:
Something to consider as you're designing your system: volume displacement.

Since the Golf uses an cross-braking system, on a stock car each MC chamber displaces fluid for one front and one rear caliper. Thus, equal requirements.

However, when you re-plumb the system front-and-rear each (equal) chamber now displaces either two front or two rear calipers. These calipers use significnatly different piston sizes, thus displace significantly different fluid volumes. The rear caliper pistons will reach full extension well before the fronts.

The significance of this is that the rear hydraulic system will "hydrolock" (I can't think of a better term) before the fronts, and any further pedal displacement to fully extend the front pistons will result in much higher rear hydraulic pressure, thus much higher force. In a stock system this isn't a problem, as both MC chambers displace the same amount of fluid, and engineers size the pistons and pads to provide the appropriate baseline delta forces on the rotor. They then add a pressure regulator to reduce rear pressures at extreme performance envelopes.

If you are going to replumb your system, you will need to *significantly* restrict your rear pressure and/or volume; you will never be able to run the system fully open without experiencing rear brake lockup. I would encourage you to use a prop valve that bypasses flow if possible, rather than one that reduces pressure.

In my particular case, I continue to run the factory cross-braking system and factory prop valve, and I adjust rear brake bias via pad compounds/coefficients. To date it has served me well. - GA


Greg's on the right track. When I tried this on my ITC Scirocco, and ran a single outlet to the rears off of one piston the M/C, I didn't experience hydro lock on the rear. Instead when I got a little taper wear on the pads, there wasn't enough fluid displacement on one piston to fully extend the front caliper pistons! The pedal would go to the floor in a hurry before any pedal pressure would build up. Add that on top of the aluminum foil firewall the VW master cylinder is bolted to, you got a scary pedal. If you think about it, the rear brake cylinders are quite small in diameter compared to the front caliper pistons, and stock M/C is designed for a mix of caliper piston/brake cylinder. When you put both caliper pistons on one circuit the fluid displacement necessary is quite a bit more than the stock setup.

My only cure was installing fresh pads when they wore more than 50% and constantly swapping the pads from left to right to even up the taper wear. (If you have an A1 Scirocco/Rabbit, it works!).

I propose allowing bolt-on Tilton/Wilwood dual master cylinder setups on IT VW's because this is obviously a safety issue specific to these cars. http://ITForum.ImprovedTouring.com/wink.gif

MC


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Mark Coffin
#14 GP BSI Racing/Action Digital/Airborn Coatings/Krispy Kreme VW Scirocco
http://pages.prodigy.net/Scirocco14gp

[This message has been edited by racer14itc (edited April 22, 2005).]

Knestis
04-22-2005, 11:16 AM
Good considerations - thanks again, guys. Greg's explanation was reflected in our experiences with the Golf II in rallying conditions.

Thinking continues...

K

Joe Camilleri
04-22-2005, 03:11 PM
Why not take the two lines running to the rears (keeping the X split), T them, run one line into a prop valve then T it again at the rear beam?

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Joe Camilleri
HP Scirocco
http://ca.f2.pg.briefcase.yahoo.com/bc/joe...&.src=bc&.view= (http://ca.f2.pg.briefcase.yahoo.com/bc/[email protected]/lst?.dir=/Joe+Camilleri+race+cars&.src=bc&.view=)

Greg Amy
04-22-2005, 03:21 PM
While at first glance that would seem a reasonable solution, to do so would create a single point of failure. Any breach in any brake line (front or rear) would expose both chambers of the master cylinder to loss of pressure.

shwah
04-22-2005, 06:47 PM
Never thought of the volumetric differences before. DUH.

Running the current pressure reducing type prop valve is probably putting a whole lot of strain on my single line before the prop valve. The brakes have performed OK though - although I have been running drums in back. I guess its time to look into flow reduction to the rear....

Chris

Eric Parham
04-22-2005, 10:06 PM
As a point of reference, I kept the original 20mm (or 21mm ?) tandem Kelsey-Hayes ("K-H") master cylinder ("MC") in my 83 Scirocco, plumbed a K-H knob-type infinitely adjustable prop valve between the secondary (or frontmost) part of the MC and a line to the rear drums, plugged the other port on the secondary, and used the ports on the primary (or rearmost) part of the MC for the front K-H calipers. I've found that a 1/4 turn can make a real difference, and prefer this to the much coarser adjustments of the lever types.

I believe that the fronts should be plumbed to the rearmost section of the MC for better brake feel, due to the inertial damping of the secondary section. Fred Puhn's brake handbook indicates fronts to primary section as well, but doesn't seem to say why.

I've worn the pads down to paper-thin on occasion (and even had a few delaminate and drop the friction material) without any problem with MC volume. I do use stainless teflon lines. I assume that Mark's problem was because he may have been using a smaller 17mm MC (as was available without brake booster on some very early VWs). Perhaps the thinner solid rotors would draw more fluid volume as well.

I don't understand why Greg's "hydro lock" concept would present any problem when using a tandem MC (as opposed to a balance bar, perhaps), since it's only the pressure that controls the braking (volume is just for clearance take-up). Perhaps there's some nuance of MC operation that I'm missing... In a tandem, even if one section were "hydro locked", I think the other section should still move well.

[This message has been edited by Eric Parham (edited April 22, 2005).]

924Guy
04-23-2005, 09:05 AM
Oh my, some scary lack of knowledge being expressed here. I wish the links to James Walker's lectures on brake system design (WRT competition cars) were still active...

I'm totally with Greg Amy on this one, using the stock system and adjusting grip with friction material selection. One less thing to screw up - volumetric considerations. Besides which these cars are not sufficiently modified from stock, especially considering the rules for brake system mods, to warrant a need for major replumbing, IMO (Prod cars aside). Take it from an ABS Engineer.

As an aside, 924's and 931's came in both flavours, both TT and X split, so I'm quite familiar in the variances and considerations of both setups.

------------------
Vaughan Scott
Detroit Region #280052
'79 924 #77 ITB/GTS1
www.vaughanscott.com

Eric Parham
04-24-2005, 02:31 PM
Just to be clearer, earlier I said:
<font face="Verdana, Arial" size="2">...In a tandem, even if one section were "hydro locked", I think the other section should still move well.</font>

To explain a little better, both the primary and secondary circuits of a tandem master cylinder that had originally been configured in an X (e.g., not "stepped") see substantially the same maximum pressures (before the proportioning valve). When reconfigured TT, if one circuit (say the rear brakes) is at the point where any additional volume would otherwise cause a huge increase in line pressure, that circuit *will not receive* any more volume because the line pressures in the tandem are automatically equalized between the two circuits (except at the lower limit such as when one circuit loses pressure). That's what a tandem is designed for.

For a given set of piston diameters, the ultimate front-to-rear brake balance for any car is directly proportional to friction at the rotors, which, in turn, is directly proportional to the line pressures and not volumes. The volume displacements are only for clearance take-up, and it doesn't really matter whether the clearances in the rear are taken up faster than in the fronts since no real braking occurs until all clearances are taken up (except in single circuit safety mode due to either insufficient MC volume or fluid loss).

Along these lines, although I had sufficient MC volume in each of my MC chambers for the switch from X to TT, I realize from the above posts that this would not be the case for every car. I guess it should at least be tested with completely worn out pads and rotors (worst case?, consider taper as well or simply push caliper pistons all the way in to start) that there is still sufficient displacement to fully actuate all four brakes.

Therefore, it seems to me that an allowance for larger MC sizes should be made in the ITCS (I don't find it there now). This is truely a safety issue!

[This message has been edited by Eric Parham (edited April 24, 2005).]

Mark LaBarre
04-25-2005, 06:35 PM
I had done this on my ITC Scirocco, using both front ports to the front calipers, plugged 1 rear, used the other through a knob type and tee'd to the rear. Ended up cranking the thing all the way up anyway, still no lock up.

And I still contend this had absolutly nothing to do with me hitting the wall at T7 at Road America.
For that, I blame Global Warming.

Eric Parham
04-26-2005, 02:45 PM
Strange, I can very easily lock the rears somewhere near the center of the prop valve adjusting range. Do you also use the Kelsey-Hayes front calipers (I think some early ITC VWs might have had Girling or Teves, as the K-H came in 1980 or so). Also, I wonder if the rear brakes work the same way (mine have the self-adjusting wedges, which I think was also a circa 1980 change). Note that I use stock rear shoes with mildly ventilated backing plates.

I guess it's possible that not all prop valves are created equal... In fact, now that I think about it, when I bought my Kelsey-Hayes knob-adjustable proportioning valve (a long time ago, don't remember where), they asked specifically whether it was for FWD or RWD. Mine looks like the one at the top of page 56 in Fred Puhn's Brake Handbook (HPBooks), although I'm not sure if the adjusting range graph is the same.

Oh, and I assume that you already removed all stock proportioning AND limiting valves. Note that some MCs had smaller orifices in some same-threaded ports that were used for sensors (e.g., hydraulic brake light and/or warning switches) rather than brake lines. If someone accidently used one of those small-orificed ports for any actual brake lines, it could certainly cause problems...

[This message has been edited by Eric Parham (edited April 26, 2005).]