proportioning valve

.......................Thanks to you and the others in this thread for a very educational thread David. I have the opposite situation that you're experiencing. Let me see if you and the others who have been following this thread think that I'm 'up to speed' on what to do with my system: I have a '77 std. bug with a stock type1 drum system front & rear, 185 width tires up front and 205's in the rear. Right now, I have premature front lock up that I'm assuming is because I have more rubber on the road in back than up front. From what I've learned from the mpbrakes.com site that you provided a link for, I should get the 10 psi residual pressure valves for both the front two lines from the master cylinder along with a third one for the line to the rear brakes. However, how do I increase the front-rear bias to the rear to compensate for the larger tires that are on the backend? A proportioning valve would only be able to decrease the pressure in the line to the rear, right? Should I get two metering valves for the two lines from the MC to the front drums even though they're described as being for front discs? Would it instead be a better idea to install the larger size wheel cylinders that are up front on the rear brakes as well and then try to tweak the front-rear bias with an adjustable proportioning valve on the line from the MC to the rear?

BTW...........Eventually.........I'm planning to install front discs and type3 drums in the rear but I don't think that it'll happen until next winter. At that point, Should I consider getting a disc-drum combination valve along with an adjustable proportioning valve to the rear?

BTW#2...............Right now, I do my brake adjustments when I sense that there is increased pedal travel. Also, the hand brake requires more travel when I'm overdue on adjusting these drums. With 10 psi residual valves, how will I be able to tell that it's time to turn in those brake adjusters? Questions.....questions..........lol

TIA

Yes. 10 psi residuals are the ones to match drums.

Yes, it only decreases the rear. I would think a larger wheel cylinder in the rear would improve it. But how much larger I don't know.

They're designed for discs. I can't recommend it. With drums all around everyone should be contacting at the same time. The metering valve is there to match the behavior of front discs wanting to contact first.

If I was committed to drums all around, that's what I'd try. Again, I don't know by how much to increase the wheel cylinders to get the affect.

Yes, please, get discs. It has to be the best mod anyone can do to a bug. I wrecked my prior bug due to a brake failure with one of the front drums. The stopping power you get with discs is amazing. And best of all, no side pull with fighting to steer while you stop. They're very balanced. The more you press, the more you stop. With drums up front, I always felt a limit.

My local boneyard has Type3 rears from a squareback.. $45 a side with hub and drums.. I'll post a report when I get them in.

I'd get the 10 psi residuals now, and pass on the other valving. You can get the prop, but you might not need it until you up the rear wheel cylinders. Or maybe add it now so you're ready and dial it off..

When you add discs, get a metering valve, too. None of the disc kits mention or come with one, but I recommend it. I added a metering valve because after a 70mph panic stop I had to do, I was questioning the safety of my brakes with discs. I locked up my fronts when I punched it (yes, at

Do realize that all this valving means you need to press harder to get the onset of braking; it isn't for free. Discs require more volume of fluid which means the pedal will have greater travel. Using larger bore rear wheel cylinders will most likely cause even greater pedal travel over the braking range. To get less pedal travel, you would need a larger bore MC. What choices there are for that, I don't know... should it be an issue. I have yet find-out myself.

I'm not sure. I haven't been living with residuals long enough. They won't help the ebrake travel, I would think.

.................I already have the parts that I need for the type3 drum conversion but I'm not going to install them without putting discs on the front at the same time as well.

..................I already have some new front drum cylinders that I can install on the back but if I have to open up the hydraulic system anyway, maybe I'll put in the 10 psi residuals and the proportioning valve at the same time so that I only have to purge the bubbles once.

..................I think that the disc-drum combination valve that mpbrakes.com sells already has the correct metering valve for the front discs. It also has built in residual pressure valves that are configured for front discs and rear drums. I'm guessing that the only thing that you might add to it is a proportioning valve to the rear brakes.

Thanks, I'd very much like to see them, although sometimes there's just no substitute for holding the parts in your hands.

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

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I envy you. I'm getting myself some of that soon. I expect to try a few wheel cylinder bore sizes and see what I like.

I'm confused with what their combo has:

....................hmm............I forgot that the combination valve had a proportioning valve built into it. OK, forget the combination valve and instead how's this sound for a disc & type3 drum system: A pair of metering valves for the two lines from the MC to the front discs. Like you say......maybe there's no need for the 2 psi residual pressure valves in those two lines. A 10 psi residual pressure valve and then an adjustable proportioning valve in the single line that exits the MC to the rear drums. Maybe you'd have to bleed the system without the 10 psi valve and then put it in. A messy procedure I'd bet but not impossible as far as preventing any air getting introduced while installing it. With this configuration, all that I'd need to do when switching from stock drums to discs & type3 drums is switch out the two front 10 psi residual pressure valves for the stock drums with disc metering valves. BTW...........Is a silicone type brake fluid recommended when using these valves?

Well That's it, then! I send it to you, you clean it up all spanky new and send it back to me! :o)

digital

send

there's

I'm going for the same setup, too :)

You need just one metering valve. You might need to plug one of the two front exits on your MC. My Super Beetle MC just has one front exit and I put the metering valve where the T was, but the std. beetle has 2 exits instead of the one, so you'll need to mount the metering valve somewhere and redo the front brake lines. I'm looking (will look) like this: _ --------------------| | Master Cylinder | | --------------------|_| I I | | | | 2 psi residual | | | | 10 psi residual (blue) | | | | (red) I I Rt. front rear line Metering Valve | | | | (hold-off) | | |-----| mpbrakes VL3526 | | Adjustable Proportioning ------- Valve -- Wilwood 260-2220 I | Lt. front Like you

I'm just using one right on the MC.. but then again maybe not having the

Also, I went with teflon flex lines, too, while I was at it:

Naww.. I just sat down and pressed to help it out. No air is drawn back in, really. At low pressure, the residuals are one-way valves.

I went with the cheapy DOT-3. Is it recommended? Oh no... :( Good luck with your project. Post results :)

Thanks for your help David. I'm saving this last post from you in a folder. My garage is about 34 deg right now so I won't be spending too much time out there for awhile. When this freaking winter weather lets up in a month or two, I'm going to get to work. My bug stays off the road during the winter months here in NY state.

Thanks. Keep her away from the salt. The more I improve my brakes, the safer I feel. I like feeling safer.

My brother-in-law keeps kneading me about owning such an unsafe car then I retort by bothering him about being a conformist for owning an SUV. I need more arguing fuel for those barbecue parties.

You're into a handling question I don't know much about, so I won't even speculate about this. Good food for thought, though.

I'd have to agree.

I assume you're in a Beetle since you mention Ghia calipers. In that case the reservoir is quite a ways above the MC, and it's the vertical difference between the two end points of the system that we are concerned with, so your calipers are about 3 feet below the reservoir.

That's a possibility that's worth considering, but a caliper piston probably only has to move .010" while a drum piston may easily move .100". Of course the fluid displaced is equal to the motion times the piston area, so this is often a "wash."

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

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If this is really what you observe now, then I would do something to increase the power of the rear brakes. The simplest way to do this would be to find a slightly larger rear wheel cylinder that you could substitute for your originals. If you overshoot, then you could add an adjustable proportioning valve for the rear circuit.

Under no conditions would I add two proportioning valve to the 2 sides of the front circuit, as that would add in the possibility of the 2 sides not being balanced. If you really wanted to do something like this you should add ONE proportioning valve and a Tee and plug the unused front circuit port in the MC.

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

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.............I think that I'm going to install the larger sized front cylinders on the rear and maybe an adjustable proportioning valve.

..................Agreed. I'd rather fool around with adjusting the rears to match up with the front brakes.

Good call. I hadn't thought of it that way. Yeah, reservoir is up a couple feet. Yes, a super beetle.

We'll see how my disc/drum matching game goes. Parts are on order for the T3 rear-end swap. I'll try to take good notes to make sure I can measure any changes, both good and bad. We're up for lots of rain here in the next week. I want to make sure I don't miss our yearly monsoon season here in northern California. That and I can afford to do it.

I thought about this some more in terms fluid dynamics and here's an analogy to electronics. The metering (hold-off) valve is acting just like a zener diode. The metering valve holds-off pressure (thus fluid) going to the calipers until the rear shoes are contacting their drums. Let's say the set mark is 50psi (I don't know what it really is). And, I'm assuming, the drain-back is a simple unrestricted one-way valve back to the master cylinder.

Assuming fluid in equals fluid out (in the short term, please), the metering valve would result in letting the piston pull back further then if the metering valve didn't exist in the line. Or another view, is the initial snap action of fluid when the set point is reached puts the rest point further from the contact. Think about it... fluid out is easier than fluid in. It must rest further from the contact due to the snap.

In this case, the residuals don't do anything except make sure the piston's seals don't every see a vacuum to ambient (plus the weight of the fluid in the reservoir.. if you can count that as pressure).

Now if the metering valve didn't exist, I can see the situation where the discs would be rubbing with only 2psi residuals in the line. And in that, I agree.

marijuana is good for something ;) pass the bong.

Okay, I had to completely rewrite this, because I didn't understand what you meant at first. Now I gotcha.

A hold-off valve is just a proportioning valve with an output pressure curve (vs. input pressure) that passes below the origin. Actually, I assume that it outputs zero when this linear "curve" is negative. In this case I'm guessing that you're thinking of the "proportion" to be

While this makes some sense, when you look at the pressure that it actually takes to move the rear shoes out to the drums, you'll see that this is virtually zero. Thus the amount of holdoff pressure you would need would be almost nothing.

In fact, the system you need already exists in your master cylinder. Note that you can't build up any more pressure in your front circuit than you have in your rear, and you don't get any in the rear until the shoes contact the drums.

I gather that you're completely comfortable here with the difference between displacement and pressure, because it's clear that we have to displace some significant fluid before we can build up any pressure.

The only correction to this would be that there would be a certain amount of pressure drop in the line during the time when we are actually displacing fluid into the rear cylinders. This figure will be larger the faster you jam down on the pedal, so maybe there would be some need for some front holdoff here. Still, we have to keep in mind that the same pressure drop concept applies to the fronts, too, although the line to the rear is longer and serves both wheels.

Yes, short term, agreed. ;;)

I'm afraid I don't get what you're trying to say here. Care to try again?

Yes, this pressure, referred to as "head" in hydraulics, counts. The MC just adds to this pressure. With your foot off the pedal, there is a clear fluid path from one end of the system to the other, via the compensating port in the MC.

I'm missing something here. Isn't this just a question of whether or not it's a good idea to leave some residual pressure in your calipers or not?

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

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Assuming it's no harder to get apart that a typical VW MC, that sounds fair to me. ;-)

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

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Just got my Type3 rear components :)

The drum is almost an inch larger in diameter from 9.059" to 9.8xx" and wider pads from 1.575" to 1.75". I like the way the drums can come off the hub (two piece, not one) without having to undo the nasty 250 ft/lb axle nut.

What a rust pot these are. I need spend time prepping these for the swap.. wire brush, paint, clean, new pads, springs, turn the drums, etc..

I'm still undecided where to mount the prop valve. The convenient spot is the indent where the brake pedal top could contact the floor. Just above the throttle pedal would seem to be the better choice...

..................I have a pair of new type3 drums that I bought from a place that occasionally advertises in the magazines. They're called Bill & Steve's and I bought these drums maybe four years ago. They also were my source, I think, for shoes, slave cylinders & hardware kits.