My Rear Disc swap from drums observations

[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.

I tend to agree with you that I am getting 10 psi to the rear either thru the master or abs. I am not sure how to know without taking the master cyl off and seeing if it has a residual pressure valve in it. I could take it our or just purchase an 01 to 07 Master Cyl that had 4 wheel discs...

Here is something another guy brought up...not sure how true this is but the master cyl on a 4 wheel disc set up require more brake fluid than a drum system and he claims that is another reason to change out the master cyl....they have different fluid capacitys which becomes important in a 4 wheel disc system. Funny thing is a dont feel any drag nor to the rotors look like that have been abused from draging...I am going to jack it up and run it and watch because if the caliper has that kind of residual pressure you would surly see it coming to a stop pretty fast after free wheeling...throw it in neutral and hit the brakes and the let off of them and see if the tire freely spins..or is draging...kinda crude but maybe a way to check for drag.

 

[R422b]

I would definitely check like you sed.
As they say better safe than replacing pads and rotors..... Or something like that. ☺

 

[Gibson]

Here is something another guy brought up...not sure how true this is but the master cyl on a 4 wheel disc set up require more brake fluid than a drum system and he claims that is another reason to change out the master cyl....they have different fluid capacitys which becomes important in a 4 wheel disc system.

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.

 

[Gibson]

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.

 

[1BigCarrot]

Interesting read on the technical stuff fullas. I do strongly agree on one thing . . . drums suck! I actually remembered to adjust them while I had my rear wheels off, now my parking brake holds more betterer but for how long, who knows. Shoes are new and the drums were re-faced, springs and the like replaced as well but still won't self adjust like they should. I do see a rear disk conversion in my future.

 

[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.

So the question remains and by the way I agree that what you say makes sense..but do I have 10 psi or so of residual pressure on the rear disc brakes now??..and or should I change the MC over to a disc brake MC?..In the past reading of this swap nobody talks about this for some reason?...right now I cannot discern if the discs have constant pressure on them while driving?...second thiught is with a true disc MC would I need a prop valve?
Thanks

 

[wheelman]

You are supposed to change the brake.thing.under the brake booster

From front disc and rear drum brakes
To front disc and rear disc.they sell them on ebay

 

[John Cunningham]

You are supposed to change the brake.thing.under the brake booster

From front disc and rear drum brakes
To front disc and rear disc.they sell them on ebay

whats a a brake thing?...there is the hydra boost and then comes the master cyl...then there is a prop valve that is inline and then of course there is the residual pressure valve in the master cyl...have no clue about a " Brake Thing"

 

[stratton]

You are supposed to change the brake.thing.under the brake booster

From front disc and rear drum brakes
To front disc and rear disc.they sell them on ebay

e-bay link ?

 

[Steve A]

whats a a brake thing?...there is the hydra boost and then comes the master cyl...then there is a prop valve that is inline and then of course there is the residual pressure valve in the master cyl...have no clue about a " Brake Thing"

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.