Disc Conversion... I want my drums back!!!!

dfjeepster · #11 08-09-2002, 10:51 PM

The last time I did a conversion I had read the same thing but I couldnt get them to work until I borrowed the valves off our sprint car(off season) Wilwood makes a red and blue one with different holding pressures for each and I had to try both to find the one that worked best for me. Usually there is a valve in the factory tee fitting but if using one from a drum brake application it doesnt work properly. I hope this helps you cause it is an easy fix and cost under $20.

RRich · #12 08-10-2002, 02:30 AM

This is gonna be long.

Let's look at a few basic principles. Hydraulics can multiply effort.

Disc brake pistons reposition themselves outward as the pad wears. As the piston moves out more fluid is needed to fill the void behind it. So you could say it "consumes" or "uses" fluid as it wears.
On the other hand, drum brake linings wear too, but as they wear, the self adjusters take up the slack, repositioning the pistons back to where it started each click. So you could say they don't "consume" fluid.
That's why the disc's resavoirs are larger - they have to be to accomodate wear. The size of the resavoir has nothing to do with the cylinder's diameter or ratio of effort.

The dia of the master cylinder compared to the dia of the slave (either disc or drum) determines how much pressure is applied to the mechanical portion - linings or pads.
Usually the disc's pistons are about 2" in dia, where the master is more like 1" in dia.
Calculate: pi R squared
M/C's piston 1" --- 3.1416 x .5 = 1.57 sq"
Disc's piston (2") ---- 3.1416 x 4" = 12.57 sq" About an 8:1 advantage.
Drum's piston (7/8") ---- 3.1416 x .875" = 2.75 Not quite 2:1
You can see the disc's pads push harder against the disc, but move less.
Drum brakes are somewhat self activating, once they touch the drum, the drum pulls them into it harder, that's why the difference.

Braking effort is a function of the pad surface area, and pressure applied to it (as well as lots of other factors.)
Arbitrarily mixing and matching parts can create all kinds of imbalances. Some master cylinders have different sized pistons for the discs and drums, some the same sizes.

As for the check valves, there are lots and lots of posts on the subject. To completely release disc brakes the fluid pressure has to drop to 0, ziltch, nada. Else the pads drag on the discs all the time - brakes stay on. There is no spring to pull them back. Normally on an operating system, the fluid does just that, drops to 0, then the slight runout of the disc itself pushes the pad back a tiny bit, loosening it's grip. If a check valve was used in the line to hold residual pressure, the brake would be constantly engaged. Make sense?

Drum brakes need a little residual pressure kept in them, as the piston's cup is shaped different. If there's no slight pressure kept, the cup's edge collapses slightly, letting air in past the cups. It has to have a check valve to keep slight pressure in the line. But, since heavy springs pull the shoes back, the shoes are not in contact with the drums - no drag. Make sense again?

That's why you TAKE OUT the check valve when it's feeding disc brakes, and PUT IT IN when it's feeding drums. There are lots of posts where the complaint was the discs would not unlock due to NOT removing the check valve, and lots of posts where problems arose where the check valve was NOT used in conjunction with drum brakes - complaint was air kept getting in.

Now yours - the larger resavoir HAS TO BE FOR THE DISCS - the smaller one does not create any more line pressure, nor does the big one, unless the piston sizes are different! The resavoir is simply a storage area. The larger one accomodates the consumption requirements of discs.

Now again yours - the reason you don't have brakes is the master HAS TO HAVE PRESSURE IN BOTH HALVES OF THE CYLINDER when you push it (both sections.) Otherwise as you create pressure for the discs, the pressure in the MC bypasses into the other chamber where no pressure is created because it's as if it's open.

---- You have to get the rear brakes working, adjusted properly, and holding pressure before you'll ever be able to stop! ---

Whether you use the 10's or the 11's, it's the same. You have to get them working in order for the front to work.
The two systems are NOT INDEPENDANT of each other like one would think.

You can replace stuff till you are broke, but until you do that it will be broke too.

I suspect the ones talked about on the sprint car are hold-off valves, same idea as a proportioning valve but separate. If there were check valves holding the disc brakes on - well, was it always at the back of the field?

Just kiddin on that, I ran Sprint's for a few years, but that was before wings were legal -- uh, years ago, many years. Too many.

Let us know how it comes out.

DanStew · #13 08-10-2002, 07:08 AM

What Rrich??? A long post.. fron you?? [img]images/icons/crazy.gif[/img]

Thank you very much, this was the shot i needed. I will slap on the rear brakes and then go from there. I will have to go out and get a proportioning valve for the setup since I am now putting the bigger brakes in the rear, but hopefullyi won't need it.

That was very informing and this post should really be archived.

The only odd thing i will have to think about onw is. I went to the dual resovior MC from my single resovior early CJ5 brakes for the safety aspect, because if the MC died I had no brakes at all.
Given I will be found at fault for driving with only front drum brakes and non-functional rear drums.
If I can get the system to work with the 74 MC what happens when say i get a failure in the rear brakes again, then i won't have anybrakes again because the fronts wont have the stabalizing affect of the rear brakes.
Potentially i will have no brakes while driving. So I guess I have to get an emergency brake for the jeep installed.

I really did think that the MC dual resoviors were seperate from each other and acted independently. I guess that is a big misconception around, i bet alot of people thought the same way that theyare independent.

Well thanks for the help.. I will post later.

RRich · #14 08-10-2002, 10:00 AM

The resavoirs are separate, like 2 cans of brake fluid. But that's just where "spare" fluid is kept.
The fluid goes down throuh holes in the bottom into a long hollow tube, - the cylinder itself.
Inside that cylinder is the piston. It has sealing cups to create the pressure as it moves. When everything's working fine, the two systems are separate, sealing cups are between the systems. Fluid isolated from each other due to the cups on the piston.

A minor failure in one system, like air getting in, causes the portion that's still working to push fluid into the other side, hopefully enough to get some braking. The effect is a much lower pedal - low but it still has some braking. That's for a minor problem, like letting one resavoir get low on fluid and it took a gulp of air.

But - in your case where nothing works back there, it's a catastrophic failure it can't push enough fluid into the rear system to make it work partially, so it can't develop it's own pressure enough to operate the discs enough. They move, but until there's resistance on both systems, no brakes - it can't develop any pressure.

It's a common misconception that they are two totally independant systems that operate totally independant of each other.

Don't bother with the proportioning valve until you know you need one.
Get it all working first.
Then get on flat dirt, stop hard. See which end tries to lock up. They feel fairly even braking.
Actually it should be about 60/40, 60 in front, since the rear has less weight and it tends to lighten up during braking due to weight transfer, especially in SOA's or lifted vehicles.
Some folks are running that MC with no proportioning valve and claim it's fine, some need the valve.

If you do use a proportioning valve, get an adjustable one, since your system is a mismatched system with different sized pistons there won't be a stock one that will be right.

Funny, while I was writing this this morning I got a phone call from a friend. He just did the brakes on his Ford PU and replaced the MC, can't get any pedal now. He's been bleeding the front until he's blue in the face, hasn't bled the rears yet. He didn't bench bleed the MC either. -- Sounds like he's got an airlock in the rears. We'll bench bleed it on the car, then get the air out of the rears, - bet it'll be fine. He doesn't believe me - yet.
Almost the same problem as yours.

Funny how they come in 3's.

dfjeepster · #15 08-10-2002, 10:55 AM

Rich, I'm not looking to start an argument here (you have enough of those with TR) and I do agree with most of what you said especially concerning the master cylinder and volume of fluid but I do stand by my statement that you do need some pressure in the front lines for discs. I admit it is not as much as with drums but without out it the pistons will back off and the brakes will lose their "pedal". (see attached link) I have seen numerous articles and posts where people are misinformed into removing all pressure valves including for the drums which causes even more problems. It does sound like there may be some pushrod length issues with his conversion as well but he cant tell that for sure unless he disconnects both lines to the master cylinder and hooks up lines for bench bleeding so that he can bench bleed using the pedal to see if the rear brakes should even be working off the MC. Oh and by the way the sprint I borrowed the residual pressure valve off of was a 60's CAE used for antique racing (I hate wings) and with the built 327 it was never in the back of the pack. I do agree that too much pressure will make the brakes drag but not enough will cause the caliper's pistons to retract. I know there are some calipers that do not need this; like 4 piston calipers for 67 Mustangs; but most large single piston calipers need some pressure.
<a target="_blank" href=http://www.speedwaymotors.com/product.asp?strBase%5FList=&hilt=&source=& amp;base%5Fno=8352601874&str%5Fbase%5Fno=83526 01874%2C&header%5Ftitle=Keyword+Search&pag e%5Fname=search%2Fkeyword%5Fresults%2Easp&sear ch%5Ftype=residual+pressure+valve&search%5Fopt ion=Exact&deptsearch=&dept%5Fid=2&dept %5Fid%5Fp=&dept%5Fname=Race+Products&dept% 5Fname%5Fp=&size1=&size2=&gender=& ShowImages=yes&sq=0&cont=1&intPgNo=1&a mp;mscssid=49AX7GFNRMNP8JUSC318FGSQT0VFD024>http://www.speedwaymotors.com/product.asp?strBase%5FList=&hilt=&source=& amp;base%5Fno=8352601874&str%5Fbase%5Fno=83526 01874%2C&header%5Ftitle=Keyword+Search&pag e%5Fname=search%2Fkeyword%5Fresults%2Easp&sear ch%5Ftype=residual+pressure+valve&search%5Fopt ion=Exact&deptsearch=&dept%5Fid=2&dept %5Fid%5Fp=&dept%5Fname=Race+Products&dept% 5Fname%5Fp=&size1=&size2=&gender=& ShowImages=yes&sq=0&cont=1&intPgNo=1&a mp;mscssid=49AX7GFNRMNP8JUSC318FGSQT0VFD024</a>

DanStew · #16 08-10-2002, 12:06 PM

I am not going to get a proportion valve until i get it all together. I have one side done already. Real easy with the Warn FF kit, but now i am on the other side and it looks like a bearing doesnt want to come off the spindle. it still rolls, but i cant get the hub to come off the spindle. so i have to take it all part to get it off.

As for the pushrod length, it should be just fine if it was the stock rod for the pedal setup. I bought a set of hanging pedals from a 74 jeep, and i am using the matching MC for it, and it has worked well before this. But I will go one step at a time and get the rear brakes hooked up and bled, then try the brakes.

Those old 10 inch brakes in the rear are in bad shape, they have pretty much no lining in the rear and everything is pretty messy with rust.

Interesting note about the Warn FF kit. I pulled off the hub assembly that they have and measured it to my extra CJ hub and they are the same from the rear to the back of the plate that the studs press into. This means that you dont have to buy the $115 warn brackets for disc brakes, you can bolt on the chevy calipers and use the same set up from the front onto the rear. I am going to try this in a month, I have to get my jeep in shape for a big campout/jeep ride in Ocala for labor day weekend. If this works then i can have the same parts at all fours and that makes a trip to the store easier when i dont have to memorize every different combination i have.

RRich · #17 08-10-2002, 12:12 PM

DF - please set your column width to 60 - otherwise it causes this super wide text that makes us have to scroll back and forth to read it.

Thanks for the site on the pressure valves. Never saw one before.

The piston in the disc brake systems don't have a spring to push it back away from the rotor. So when you let up the pedal, the slight runout of the rotor pushes the pad back slightly (unless the rotor was turned badly on a lathe with excessive runout - another problem altogether - chucking the rotor on cones creates the runout.)

The pad stays in slight contact with the rotor all the time "at the ready" - without any residual pressure. I'm sure you've tried to push the piston back during a brake job, it doesn't move easy does it - even with the lines open.
At least on every one I've worked on, singles, duals, and 4 piston types.
I notice the check valve on that website for discs is only 2 lbs, the drum type is 10 lbs.
I doubt 2 lbs would cause much of a drag, so it's OK. 10 lbs for sure causes way too much.
The 2lbs would tend to keep the pad in better contact with the rotor, even allow it to follow the runout, so it would feel "tighter." I wonder how much it actually makes the system "tighter" on one that was working properly without a check valve.

That also solves another problem that's come up here. Some rebuilders apparently aren't re-installing the check valves when they rebuild it. That's caused some problems for some folks here with the drum brakes (seems like drum brake repairs are rapidly becoming a lost art.) Now we've got a source for the check valves,- and now we know what the residual pressure in a drum system should be. I've asked "experts" and got that stare.

Until now the only check valve we used was in the MC itself, not an external thing. Our source was either switching it from the old one to the rebuild or a junk yard. Good to have a source, thanks.

The "no fuid" to the rears could be caused by the MC piston not going back enough to uncover the hiole and "take another gulp" of fluid. But I doubt it. Since both MC pistons are linked together, they would, or should, uncover both holes at the same time. If the rod was too long, neither could take a gulp, another problem altogether.

That's the beauty of a site like this, sharing info.

By the way, I remember when we were all putting on the "newfangled" discs on the sprint cars, along about the same time we started using real helmets, not leather! (I never did wear a leather one - I'm still here because of that.)
No "sissy" bars (roll bars/cages) either. That was when racing was really racing.
I saw one on display at a car show the other day, brought tears to my eyes.