Calling all physicist ???’s about fluid dynamics

[xumFJ40]

All right, I am ready to test out the new 4-wheel disk system, but before I coat my garage floor in brake fluid (drohweder) or shell out for a proportioning valve, I wanted to pose a question and have my “logic” reviewed.

I have a ’75 master so it has a dedicated reservoir for each brake circuit. I am/will be running ’79 disks up front and ’85 Eldarado’s in the rear.

Observations:
To date, I have not removed either of my residual valves from the master. With the ’79 disks up front and ’75 drums in the rear (leaking badly thus the upgrade) the truck stops well, no locking up, and the only side effect is that the reservoir for the front brake circuit overflows very slightly…

Hearsay:
I have read/heard from a number of people who have done the 4-wheel disk conversion and the consensus has been that about 40% of them did not need the proportioning valve, and the other 50% desperately needed it(10% not reporting) to make the truck stop correctly. Some with the proportioning valve say that they leave it “wide open” thus not serving any purpose.

Hunch:
I am thinking that the large disparity between those who needed it and those who did not could possibly stem from those who overlooked the removal of the residual valves in the master.

Theory:
Based on what a residual valve does, and the current behavior of my brake setup, it should be possible to eliminate the need for a proportioning valve by simply removing the residual valve from the front circuit and leaving it in place for the rear circuit. If the resistance created by the residual valve were great enough, the only drawback would be reservoir overflow on the rear circuit.

Variables:
NO...I have not compaired types of master...I know there are a lot of different ones being used.

 

[Rice]

Observations:
To date, I have not removed either of my residual valves from the master. With the ’79 disks up front and ’75 drums in the rear (leaking badly thus the upgrade) the truck stops well, no locking up, and the only side effect is that the reservoir for the front brake circuit overflows very slightly…

Hearsay:
I have read/heard from a number of people who have done the 4-wheel disk conversion and the consensus has been that about 40% of them did not need the proportioning valve, and the other 50% desperately needed it(10% not reporting) to make the truck stop correctly. Some with the proportioning valve say that they leave it “wide open” thus not serving any purpose.

Hunch:
I am thinking that the large disparity between those who needed it and those who did not could possibly stem from those who overlooked the removal of the residual valves in the master.

Theory:
Based on what a residual valve does, and the current behavior of my brake setup, it should be possible to eliminate the need for a proportioning valve by simply removing the residual valve from the front circuit and leaving it in place for the rear circuit. If the resistance created by the residual valve were great enough, the only drawback would be reservoir overflow on the rear circuit.

Variables:
NO...I have not compaired types of master...I know there are a lot of different ones being used.

Not having removed the residual valve for the front leads me to believe that either your front brakes are dragging or the residual valve itself is not holding pressure. Regardless I would remove it.

Keep in mind the residual valve and a proportioning valve are not designed to restrict flow in the same direction ... The residual valve is really only there to prevent fluid from flowing back into the reservoir and, thereby, prevent you from having to pump up the brakes every time you use them. Their average resistance hovers around 10 psi.

The true issue of determining if a proportioning valve is needed or not lies in master cylinder to slave cylinder piston ratios. With so many possible combinations some will luck out and get a perfect balance

I'm still running drums in the rear and, without a proportioning valve, have a nice balance. However, I'm convinced that if the day comes to put disks in the rear a proportioning valve will have to be included.

Last note ... the proportioning valve we run on our race car still offers some resistance even when wide open ... so its purpose is still in effect.

Rice

 

[Pin_Head]

You don't have enough quantitative data about the hydraulics and physics of the situation to do a meaningful analysis.

Without data, it is an empirical problem that you just try and see if it works or not.

 

[Rice]

You don't have enough quantitative data about the hydraulics and physics of the situation to do a meaningful analysis.

Without data, it is an empirical problem that you just try and see if it works or not.

That's what I sai ... no, wait ... analy ... quanti ... dang.

What does the empire have to do it?

 

[xumFJ40]

Rice, Pin_Head,

pretty funny ph.

Thanks for the info. I never realized that the residual valve was restricting flow back into the res. Why is it recomended to remove them when converting to disk? I have always heard that it was required in order to increase the amount/flow of fluid that was requried for disk calipers.

It will be tested one way or anohter, I just wanted to see if anyone had tried it already.

 

[Rice]

Rice, Pin_Head,

pretty funny ph.

Thanks for the info. I never realized that the residual valve was restricting flow back into the res. Why is it recomended to remove them when converting to disk? I have always heard that it was required in order to increase the amount/flow of fluid that was requried for disk calipers.

It will be tested one way or anohter, I just wanted to see if anyone had tried it already.

With drums the residual valve is there to hold the shoes up against the drum, keeping in mind there are springs there trying to pull the shoes back. Disks have no such springs and the pressure needed to hold them up to the rotor is much less (they do make a 2 PSI valve for some disks). So, in most cases, leaving the residual valve in a disk system can lead to the brakes dragging.

 

[toddslater]

Rice and Pin_Head have definitely done a fine job and raised all the relevent issues corncering what you need to know and where your lacking info to accomplish the contemplated conversion. Me, I'll go out on a limb...you say "With the ’79 disks up front and ’75 drums in the rear (leaking badly thus the upgrade) the truck stops well, no locking up, and the only side effect is that the reservoir for the front brake circuit overflows very slightly…" Well call me a crazy bassturd, but I would give serious consideration to replacing the rear wheel cyls and brake master and calling it good. Purely from a cost/benefit/uncertainty point of view.

 

[xumFJ40]

Todd,

There is nothing wrong with the master. The "leak" come from fluid coming out of the top of cap for the front circut where there are little "breathers"

There is not a lot of uncertainty, with this conversion...in fact it is complete, and I was just asking a followup before filling it with fluid.

As far as the cost/benefit goes I tend to disagree with you. Replacing all four wheel cyls in the rear of the truck w/toyot parts would be only $100 or so cheaper than the conversion to disk. Plus there is less maintance down the road and several other benefits as well.

so, that said "crazy bassturd".

Thanks for the input though

 

[Gumby]

Just pulled my residual valves today. Man, them suckers were in tight. We'll see if it makes a difference.

 

[toddslater]

Just a for instance...the master cyl on my 73 began to leak fluid out of the top of the front reservoir also very minor at first. I thought it odd that brake fluid would leak up hill. None the less, it progressively worsened to the point where it became obvious, even to me, that the problem causing the fluid to overflow the front reservoir was also depleting the rear reservoir in strangley proportional amounts. The rubber seal on the piston that seperates the functionality of the dual cyl had shiited the bed. I guess I just assumed that was probably the problem that was causing your fluid to evacuate out of the top of your front reservoir also....and hence a new or rebuild your master ...at least to me was a foregone conclusion. Yea go ahead...call me a crazy assuming bassturd. Good luck with your upgrade.

 

[xumFJ40]

Todd, Ok..I got you now. I guess my problem at this point is that I can't confirm that the fluid was draining out of the rear res and into the front because the rear was leaking so badly that I believe that is where the fluid was going.

Gumby, let me know how it goes

I took a look at the residual valve for the rear, and it does not look like the pictures I have seen of them. The ones that I have seen were flat disks with 4 small holes in them. The one that I have is more akin to a cup in shape with a small hole in the bottom and four on the sides of the cup... I don't know if this makes any difference at all....

 

[Pin_Head]

Just a for instance...the master cyl on my 73 began to leak fluid out of the top of the front reservoir also very minor at first. I thought it odd that brake fluid would leak up hill. None the less, it progressively worsened to the point where it became obvious, even to me, that the problem causing the fluid to overflow the front reservoir was also depleting the rear reservoir in strangley proportional amounts. . Good luck with your upgrade.

This is an indication that the piston seals are leaking internally and that the master needs to be rebuilt or replaced. If it leaks bad enough it will trip the "check brake" sensor switche.

 

[xumFJ40]

Update.

I finished up the Install and inical testing last night. I pulled the residual valves out of both the front an rear circuts....The truck will put you through the windshield if you romp on the brakes....man...huge difference. Gumby, what about you?

Put in the proportioning valve for the rear circut...it turns out I think that I needed it. May have a bit of fine tuneing left.

So far, I have not seen the fluid level in the rear decrease, nor have I seen the front circut overflow. I think I need a bit more testing, but I belive that it(the overflow) was due to the residual valve. As I stated before, the residual valves that I pulled out were not shaped like a disk as I had previously been told...these look more like cups.

Anyway, everything seems to be great, she stopps nice and smooth, with little brake pressure.

 

[t-o]

more of a fluid mechanics kinda thing, and i'm really late weighing-in on this, but anyway....

Hearsay:
I have read/heard from a number of people who have done the 4-wheel disk conversion and the consensus has been that about 40% of them did not need the proportioning valve, and the other 50% desperately needed it(10% not reporting) to make the truck stop correctly. Some with the proportioning valve say that they leave it “wide open” thus not serving any purpose.

I've heard the same things, and unfortunately the HEARSAY never has enough details.

Everything above needs to be qualified by tire size...compare 37" tires to 33" tires or to stock 28" tires. 37" tires take more friction force at the discs to slow the big tires for each revolution. at the same time, bigger tires travel farther ground distance during each revolution (increase stopping distance). By comparison, smaller tires (33") require less friction on the disc for an equivalent reduction of wheel speed (even less for 28"). ....so, would you rather hold your Cruiser off the ground at the end of an 18" stick or a 16.5" stick? an inch and a half of leverage is a lot of difference when you're talking about 1,000 lbs (each tire), especially if you have to carry the longer stick 9.5" farther distance than the shorter stick.

So the proportioning valve keeps smaller tires from locking in the rear because it keeps the friction at the disc from overpowering the tire patch friction on the ground. but with larger tires the disc force might not be enough to overpower the tire patch, so you can remove the valve. and here's the other half of the tire story....how wide are your tires? 9.5" will lock up before 12.5" or 15.5".

I'm happy with my '76 front discs, '75 rear drums and stock '75 MC. stops pretty well, i can lock up the 32x11.5s even with rear cylinders that haven't been adjusted in 5k miles.

Tim