proportioning valve

there's a few more parts than just a proportioning valve.

What I did was put a two pound residual pressure valve between where the rear line connects to the master cylinder and the master cylinder. Mine works like a champ with no problems. I've also got 4 wheel disc brakes. Dan

Sorry, like this one.

A proportioning valve would be put in the rear line. It's purpose is to reduce the pressure send to the rear brakes to keep the rear brakes from locking. In other words, it attempts to put the F/R braking action back in the same proportion as it was before you added the rear disk brakes.

Note that, except for bragging rights, this makes the rear disk upgrade pretty pointless, unless you improved the fronts, too.

Of course, if you also improved the fronts, and by approximately the same %, you wouldn't need the proportioning valve.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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What do you think this residual pressure valve does for you? It might have some valus in a car where the reservoir & master cylinder were lower than the wheel cylinders, but that's not true on VWs.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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The rear disc brakes require more fluid to compress that the drums did, the valve leaves two pounds of pressure on the rear circuit, thus requiring less brake peddle travel to operate the rear brakes. Dan

You're confusing pressure and volume. And there's a slight quibble here with the difference between "compress" and displace.

In general, residual pressure in a disk brake circuit is a bad thing because the pistons and pads don't retract well against even minor back pressure. More specifically:

Disk pistons have large areas for the pressure to work against and no springs to retract them.

Drum pistons have small areas for the pressure to work against, and also have retraction springs to pull the shoes back.

I would expect residual pressure valves on a disk system to cause the pads to drag, heating the rotors and causing rapid pad wear.

What's the diameter of your rear caliper pistons? Let's calculate the force this 2 psi puts on those pistons.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Jim, I suspect there's a difference between a modern proportioning valve used largely in the hot rod market and the proportioning valve VW used on the buses - am I correct? The one used on the buses was inertia reactive with the steel ball rolloing to partially block the outlet port to the rear brake circuit. That said, would a modern hot rod market proportioning valve work to balance the front disk / rear drum circuits if a person was to remove the original VW unit?

How would you recommend proceeding with such an installation, as far as adjusting for the correct balance? In my feeble mind, this seems overwhelming, but could it be as simple as some test skidding on hard gravel road?

I don't know that I want to remove the PV, but over the years there have been some peculiar things happen with my brakes. Years ago, I pulled the PV and replaced it with one I had bought at a wrecking yard and cleaned up inside. What I had perceived at that time to be a bias realated issue (don't remember the particulars) was dramatically improved with the cleaned up PV. I have flushed fluid about every 3-4 years (usually not intentionally!) so that should not be an issue.

It seems to me that sometimes in the past 14 years that in a panic or near-panic stop I had little or no braking from one end or the other whereas I'd try it again after the adreneline subsided and had all 4 working in unison, practically standing on end! The only thing in the circuit that is somewhat flexible in its manner of operation is the PV. Everything else is relative to a slight displacement of a column of fluid and the pressure behind that displacement. The PV is a rolling ball.

From a thread we had a year or more ago, I shortened the rod from pedal to MC and the braking became almost 100% predictable (thank you.) I had adjusted it before that but not enough. Apparently there is enough wear in all the linkage points and enough change in geometry from the front end collision back in '91 that the piston was not returning far enough part of the time, but returning far enough most of the time.

When the AH slams his brakes on directly in front of you, you don't have time to guess if this is a 2-pumper (related to return / not air in line!) or the first application will do it. So many considerations in that split second, like is the shoulder available? Is the guy behind me or the one behind him going to stop? If I swerve will I flip it? Should I try to pull UP on the column? (That trick NEVER works!)

Thanks for the discussion.

wondering where to put a

brake line?

purpose is

rear brakes

braking

added the rear

disk

too.

approximately the

I've never worked on a bus valve, so I'm not sure how it works or exactly what it does. Looking at the Bentley manual they call it a regulating valve, so it may well have a different function. Since they test it by tipping it downward 30 deg, thereby simulating a 1/2g decel, I suspect it is much more than a simple proportioning valve.

Proportioning valves just pass a certain % of the front pressure thru to the rear. This regulator seems to change the proportion in accordance with the deceleration, which could be ideal if done right, because the weight transfer to the front wheels is also proportional to the deceleration.

Skidding is one way to test this, but with a simple proportioning valve the ideal proportioning % varies depending on the amount of deceleration. It would be fair to set this up to be right for the most severe stop you could expect, but then you would have to guess whether this would be on dry pavement or on gravel. There would be a huge difference.

It's also true that the placement of a load in a vehicle will also affect the ideal F/R proportioning, but in a VW bus large loads are most likely to be placed near the center of the vehicle, so the center of gravity is not likely to move too much. This means that tests done with an unloaded bus are still likely to be valid for most loads.

The only proportioning valve I'm similarly familiar with is the one in the 914, which is a fixed % proportioning valve. I had one which had a problem with a tiny internal valve which must have had dirt in it, so it wouldn't close, and didn't proportion. 2 quarts of brake fluid flushed thru it finally fixed the problem, but this was only AFTER I had picked up a junkyard unit and rebuilt it.

I never used the the rebuilt valve, but taking it apart showed how it worked and made it clear what the problem might be. I'd love to see how the bus valve works....

Parts like this, of any design, are always likely to give problems in brake systems, because there is always much more dirt in there than we can imagine. This is just another reason that I like silicone brake fluid, since it seems to limit the corrosion and wear that is the source of all this "dirt."

It sounds like the OE regulating valve is a more sophisticated design and a better solution than a straight % proportioning valve,

So I guess my advice would be to possibly take your OE regulating valve out and clean it again, and then install silicone brake fluid to help keep it clean.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Just buzzing in with my $0.02

About 2 years ago I put front disc brakes in my 73 super. The master cylinder is stock for drum/drum meaning the fronts don't have the small restriction feeds like the one found in the ghia nor did it have the stock residuals.

Just recently, I put in 2psi residuals and a metering valve (where the T was) in the front circuit and a 10psi residual on the rear circuit.

I have yet to do some 70-0 panic stop tests, but I can feel the change in normal driving. Before it always felt like the fronts were doing most of the work initially (nose dive), now it feels even and balanced. The only drawback I can tell is stopping requires more initial peddle pressure, but after that it is very proportional.

Speaking of proportion... I have an adjustable proportioning valve, but it isn't in the rear circuit yet pending some wet pavement tests to see if I need it.

I've heard that wheel cylinders come in different diameters: 17.5mm is stock, but the fronts use 23.8mm or 22.2mm. I think a 19mm is available, too.

If I find I don't need a proportioning valve in my current setup, this means the rears just aren't contributing much, yes? Should I up the wheel cylinder diameter so I need a proportioning valve?

This won't help much. The larger diameter cylinder needs MORE fluid, in order to move the piston out as much as before. BUT, it is stronger. You win some, you lose some. And you are left with basically very marginal overall change.

Jan

Oh yea, I did this.

Jan

.................I understand and agree that a larger wheel cylinder or caliper piston travels less but with more force than a smaller area piston. This other stuff about proportioning valves in this thread, however, is going to make my head explode.

:-)

The stock valve is like a tilt mechanism on a pinball machine. There is a chamber with an inlet and outlet the ball rolls ahead to partially block the outlet. Seems to me (by memory) that there is one more opening... Guess I need to find the spare or look in Bentley - Naw, just in and out and a big steely! I'll have to dig up that spare and take a closer look. It is extremely simple, but there must be some grooves or something, so that some of the fluid can get by - or it doesn't get by until there is less nosedive. The inertia of the ball is the regulator.

Last I checked, they were available new for something like $500! Over the years I have recommended disassembling and cleaning to several people who were having unpredictable braking with their busses and all were kind enough to reply that it had solved their problem. When doing this, you need to open it carefully - in a tray - and watch where the parts are and the order they go back in! You also need to make your own gasket!

That's a good project for me - find the spare, take digital photos of the diassembly, clean-up and reassembly! I'll send you the photos when I "get around to it!"

- Dave

and

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proportioning valve.

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silicone brake

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sophisticated design

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brake fluid to

Can I assume that I'll require more pedal travel for a larger diameter wheel cylinder? I'm already needing about 1 1/2 inches of travel from onset to a panic. It looks like the ghia master cylinder has a wider bore and, I assume, wider bore means less pedal travel per volume.

What's the difference between the Type1 and Type3 rear drums? John Connelly mentions this @

contact area, wider shoes, larger diameter drum? wonder what those "super stoppers" would do?

never mind.. questions answered:

If everything is working correctly the two setups would be equivalent. Manufacturers choose the wheel cylinder diameter in order to get the F/R proportioning right. Only if they can't do this do they go to a proportioning valve. This is most likely in cars with rear wheel disks, where they just couldn't make the disk pistons any smaller.

There's no advantage to a proportioning valve unless it's adjustable, which allows you to adjust it until you get it right. You could also play with different sized wheel cylinders, but that's probably more trouble.

I still don't see any advantage in a residual pressure valve on a disk portion of a system in a VW. There are several disadvantages.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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The same exact thing happens with a proportioning valve when used on a circuit with large pistons (which is the reason you need the proportioning valve.) There's no free lunch.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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