My Rear Disc swap from drums observations

[John Cunningham]

Well I think I answered my own question with a quick test.........Drove the truck around the block and with the truck still running and with it not running in park tried it both ways I thought I might see some pressure even a small amount if I cracked the bleeders on the rear disc...........so I opened the bleeder and all it did was dribble out from maybe gravity and certainly not even 1 pound of pressure........I seen no residual pressure at all............so does this answer the question? I also have enough reservoir in the master even though the masters for 2000-2004 have larger reservoirs. I see no reason to change the master cyl. I bought this kit from Lug Nuts 4x4 and he said he has sold thousands of these kits for trucks and has not heard of anyone changing the Master cyl..He also said the tried a test where he added a 10psi residual pressure valve to the rear and said he seen no difference in any part of the braking. So I would say unless you have a bad master cyl leave it alone during this swap. I would think this would show up if you jacked the tire off the ground would you require any real resistance because of those 10psi...as it stands I am sure if any there is almost no residual pressure left in the line.............
This is all good because its one less thing you have to purchase to do this swap......

 

[John Cunningham]

Wheelman is referring to the proportioning valve, pretty sure it's only installed in vehicles without RWAL or 4W ABS.

My understanding of the residual pressure valve in the master cylinder on drum brakes was to prevent the springs from pulling the shoes to the fully retracted position. If the shoes were fully retracted, the "take up" for them to contact the drums would take more fluid volume than the m/c was capable of delivering on a single pedal pump. Couple that to any wear on the shoes/drums it could lead to accidents with less experienced or inattentive drivers. Probably not an ideal situation on disc brakes though as the piston seal provides the "pull" to retract the piston and inner pad from the disc surface.

Yes its to keep the wheel cyl engaged and not leak.....not much to do with moving the drums but talking up the slack in the wheel cyl which would have a benefit..but I see no residual pressure at this point in my system

 

[John Cunningham]

Well I think I answered my own question with a quick test.........Drove the truck around the block and with the truck still running and with it not running in park tried it both ways I thought I might see some pressure even a small amount if I cracked the bleeders on the rear disc...........so I opened the bleeder and all it did was dribble out from maybe gravity and certainly not even 1 pound of pressure........I seen no residual pressure at all............so does this answer the question? I also have enough reservoir in the master even though the masters for 2000-2004 have larger reservoirs. I see no reason to change the master cyl. I bought this kit from Lug Nuts 4x4 and he said he has sold thousands of these kits for trucks and has not heard of anyone changing the Master cyl..He also said the tried a test where he added a 10psi residual pressure valve to the rear and said he seen no difference in any part of the braking. So I would say unless you have a bad master cyl leave it alone during this swap. I would think this would show up if you jacked the tire off the ground would you require any real resistance because of those 10psi...as it stands I am sure if any there is almost no residual pressure left in the line.............
This is all good because its one less thing you have to purchase to do this swap......

I will do one more test and thats to hit the brakes mildly harder on gravel and see if the rear brakes lock up first. If they do I might want to consider a prop valve... Somebody should observe this

 

[John Cunningham]

John, here's a much easier experiment to show the operations of discs,, and you don't even need to drive a vehicle.
Take a piece of sheet metal and clamp a pair of vise-grip pliers to one end , so you can pull on it.
Now, lay the sheet metal down on a smooth surface, like a sheet of plywood.
On top of the sheet metal put a disc brake pad, with a couple of pounds of weight on it.
Now, holding the pad, see how easy it is to pull the sheet metal out from under the pad,, it's easy because there is only a couple of pounds of pressure between the pad and the metal.
Now, have your 200lb friend stand on the pad and try to pull the sheet metal out.
Big difference, right. OK, when your friend stood on the pad, how much did he move it,, the answer is almost nothing, he didn't move the pad, he just added pressure to it.
Discs work the same way, the pad is already touching the disc, you're just adding pressure.

No residual pressure observed see my post

 

[John Cunningham]

I know,, it's a hard sell to get people to really understand what's going on in brake systems, so many people believe that with discs you somehow "need to move more fluid" than in drums,, and it's totally false,, it's one of those falsehoods that keeps getting spread around by people who have no understanding of hydraulics, or fluid pressure transfer.
The disc/drum setups have a "step bore" master that uses the larger piston to "move" a lot fluid to the drums, and you have to,, you gotta pump a lot of fluid to expand-out the shoes before they even touch the drums enough to start any braking action.
You don't "move" fluid, except just a tiny bit, with discs,, the pad are already in contact, all's your doing is using a column of fluid to transfer pressure.
All that pedal travel you feel before the brakes start working is just getting the rear shoes to expand.
The masters for 4 wheel discs are not step bored, they move less fluid then the disc/drum masters, but do exert more pressure.
It's easy to prove what I am saying. take your vehicle that has disc/drum brakes and do a little test;
Get the vehicle coasting along say ~10>20mph in neutral on a hard level surface, and slowly push the brake pedal down and see how far you have to push it to stop
Now, do the same thing again,, Except, this time push the parking brake pedal just enough to start the rear drums to drag,, now push the brake pedal,, you'll see that the pedal hardly moves before you stop,,
That slack you took out of the drums with the parking brake is close to the same slack that the master cylinder has to take out pushing fluid to the rear.
Discs hardly "move" any fluid at all,, just enough to move the pads a few thousands of an inch,, it's hardly more than an eyedropper full.

Ha I dont have emergency brakes now as I elected not to use the kit with ebrakes. I have not used an E brake ever so I did not care. I did do one test...drove the truck and stopped and opened a bleeder......no pressure observed...just trickled out of the bleeder like gravity...no pressure not even a little......So I am wondering if because I do not have as much pressure to the rear discs now that this is acting like a prop valve? And if I change to a non step bore Master Cyl which gives will give me more pressure to the rear discs then I might need a prop valve because I have more pressure to the discs. Do you understand what I am saying? Am I getting enough pressure to work the discs correctly leaving that step bore master cyl in place or would you replace it?

 

[John Cunningham]

The residual presure valve keeps a small amount of pressure on the drums only, because the wheel cylinders tend to leak air in if the presure goes slightly negative like when the petal comes back up after a hard stop. With disk that negative pressure is what helps to fully disengage the brakes.
Or at least thats my understanding after too much reading on the subject.
I have read (but have not verified it) that gm started mounting the master higher on the fire wall and so that gravity would do the job.

am not getting any pressure to the bleeder after cracking and the car in neautral......I cracked it within 5 seconds of stopping.......just trickeled out

 

[John Cunningham]

Yeah, I did.
A 2-13/32 dia piston has 4.547 sq in area,, times 10psi=45.47 lbs of force, then divide that by 2, the pressure is only on one pad, but the caliper slides to equalize the pressure between the both of them.
That 10lb residual is holding 10lbs in each rear caliper, so each one of the 4 pads is pressing on its rotor side with ~22.235 lbs of force.
Ideally you want very little residual on discs, they have no retractor springs, the only retracting force is a tiny bit of "spring-back" in the rubber boot, they always drag a little, and the column height of the fluid above the calipers is plenty of pressure to keep the pads in light contact.
With discs the difference between "relaxed" and "hard braking" is only a few thousands of an inch.

So that would be easy to check by cracking the bleeder open and seeing if there was any pressure left in the system after a stop and leaving the truck running......just trickled out...I do believe I may be giving up some pressure if its a step bore master cyl but adding pressure may lock up the brakes.... The right thing is to put a pressure guage on the rear brakes and compare that to the front.

 

[John Cunningham]

a good read
One of the most important yet often misunderstood elements of any racecar is the braking system. Racers employ a disc brake system, which is made up of calipers at each wheel, brake lines, a master cylinder and a brake pedal or lever. The key ingredient to any brake system is line pressure, which is a function of the master cylinder size and how much force is applied to the M/C via brake pedal or lever. It also determines the clamping force of the calipers. If the line pressure is too low, which is an all-too-common occurance, clamping force is reduced (much the same as putting a block under the brake pedal of your street car). The obvious consequence of this reduced clamping force is not being able to slow the car down quickly.

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But this also leads to excessive heat being generated in the brake rotors —which can cause warping and/or cupping, creating a whole different set of problems. What is the proper system fluid volume (a door car with 4-piston calipers on all corners will require more than a dragster with only rear brakes) and how much line pressure should you have? M-W Enterprises recommends that at least 1200 psi system line pressure is available under severe braking conditions. And the master cylinder bore size (which determines how much fluid the piston can move) can be determined by using a special Brake System Pressure Calculator on this website.

Now, how do you generate 1200 psi line pressure? This is a function of the force being applied to the push rod in the master cylinder. Because this force is greater than a human could apply directly to the M/C, a system of mechanical advantage (pedal or lever) is employed. The pedal ratio is determined by dividing the length of the pedal/lever to its pivot point, and from there to the attachment point of the linkage (see diagram). Using the interactive calculator on the website you can determine the ratio required to generate 1200 psi. Example: A dragster with two 4-piston calipers requires a 7/8" bore master cylinder, and minimum pedal ratio of 4.8 to 1 (7.3 to 1 for a lever, since you can’t generate as much force with your arm as you can with your leg). To verify line pressure, the most reliable method is to employ a gauge. M-W offers a premium grade liquid-filled gauge (p/n 81105) that can easily be attached to the caliper in place of a bleed screw in the caliper and measure pressure right where it counts.

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Once sufficient brake line pressure can be obtained, it’s important to set the front/rear balance so that braking forces are applied commensurate to the capabilities of the disc/caliper (or drum) and weight transfer so one doesn’t “lock up” before the other.

Setting the proportioning valve will require some experimenting to get the proper balance. Also, if the master cylinder is mounted below an imaginary line connecting the front and rear calipers it is recommended to use an RPV (residual pressure valve) to help maintain line pressure.

The most expensive brakes you can buy —even carbon fiber—cannot stop a car without having ample line pressure. It’s as simple as that.

 

[John Cunningham]

So I ordered a Brake Pressure guage to check the rear discs....If its 1000 psi or less it means the larger step bore is for the drums which it should be and your not taking advantage of the rear disc install. You should see around 1500 psi discs....having said that you would then need a prop valve to stop the rear from locking up to soon and it would considering you would not have as much weight in the rear unless your loaded.
I think leaving the guage on and driving it under certain conditions might tell you alot.
You could leave the step bore in place but not getting the advanatge of those brakes especially in a towing situation when you need the added pressure.
In a way not if it has only 1000 psi to the rear its kindal like a prop valve without having one. However you have cut back 33% of the pressure to the rear that you could have if needed. Its much better to give full pressure to the back and cut it back as required for your application..

Stay tune for the results and this will be put to bed.

 

[Gibson]

Setting the proportioning valve will require some experimenting to get the proper balance.

Don't overthink it,, that Hydro-Boost system will easily develop more than enough pressure to lock-up any of the brakes, disc or drum,, that's why you have ABS.
A disc/disc proportion valve is what you want, and let the ABS modulate the brakes,, that's what it's for.
Actually, the term "proportioning valve", is not totally correct, Chevy calls it a "combination valve",
What a disc/drum valve does, is allow fluid to flow to the rear brakes until they expand-out and start to apply some pressure against the drums, then, after the rear shoes start to do some work, the front discs start seeing some real pressure.
In the disc/disc valve all 4 wheels start building pressure at the same time,, you're not waiting for the rear shoes to expand.
If you want to go to the disc/disc master, hey, go for it,, nothing to lose.
When I did the squarebody I kept the old master, and could easily lock-up the rear discs,, that's why I used the Willwood adjustable valve,, but I didn't have ABS to keep the rears, (or fronts,) from locking.
Ya know,, none of this stuff is easy is it, I want rear discs on my '97, but with the way I use the rig, I don't really "need" rear discs,, and I'am kinda lazy in the wintertime,
Good Luck, sounds like you'll have a nice rig.