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Disc brake conversion questions

mikkelstuff

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So I did the Mustang rear disc brake conversion on my '02 Ranger 4WD XLT which I love. However, now working a brake issue with my '75 Ranchero, I'm wondering if the Ranger master cylinder and proportioning valve should have been changed out as well? For the Ranchero at least, the master cylinder and proportioning valves need to be changed out when upgrading the rear drum brakes with Lincoln Continental Mark IV disc brakes.
 


RonD

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Proportioning is a driver's call

trucks use 70/30 from factory, because there can be very little weight on the rear axle
cars use 60/40, because there is always some weight on the rear axle

With disc brakes on the rear its less likely to get a lock up, so you can switch to 60/40 in a truck when you switch to disc brakes, or use an adjustable proportioning valve, go to a gravel road and find what you like


If you converted from 4 drum brakes to 4 disc brakes then yes, you need to change the Master cylinder
If you have front disc brakes and just change the rear drums to disc then it will work OK, but a 4 wheel disc brake Master would be better, you won't notice a difference but rear pads should last a bit longer, but that also depends on proportioning, lol
 
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mikkelstuff

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Thanks Ron. I was wondering.
 

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The Mustang disc brake conversion on the 28 spline works just fine with the factory master cylinder/proportioning valve. There is no issue with the rear braking being to strong or too weak. I had that on my 03 for years and drove a lot of miles so experienced pretty much every condition with no issues.

Now I have the 05-09 Mustang rear brakes on my 31 spline Torsen rear axle on the 09. That is what you get when you buy the Ford Performance 9" disc brake kit, which also fits on the 31 spline 8.8" axle. I still have the factory master cylinder/proportioning valve, and this has a bit too much rear braking. If I jump on the brakes hard (some idiot cuts me off while going 15 mph slower) I get a little chirp out of the rear tires. If its wet, I can get the rear abs to kick on with a quick stab of the brake pedal. Some day I will address that, but for now its tolerable.
 

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So I did the Mustang rear disc brake conversion on my '02 Ranger 4WD XLT which I love. However, now working a brake issue with my '75 Ranchero, I'm wondering if the Ranger master cylinder and proportioning valve should have been changed out as well? For the Ranchero at least, the master cylinder and proportioning valves need to be changed out when upgrading the rear drum brakes with Lincoln Continental Mark IV disc brakes.
I'm assuming you meant Explorer master cylinder and proportioning valve as well.

If comes down to how often you like to change rear brake pads.
Drum brakes hold 10 psi pressure on lines to ensure shoes are still close to drum for next application​
Disc brakes hold 2 psi as there aren't the springs pulling the pistons back in on discs.​
Jumping on brakes got little chirp our of rear tires with my '98 and 9" rear drums and rear abs to kick in when wet and/or icy when empty.
 

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@don4331

I thought residual check valves were a thing of the past. At least that seems to be what I remember from trade school many... many moons ago.

Also if the thought that this 10 psi residual pressure holds the brake shoes close to the drum by overcoming the return spring pressure... how come the pedal travel will increase if the rear shoes are misadjusted?

This must also mean the the caliper square cut seal on the piston has greater then 2 psi force to return the piston so the brakes don't drag.

Have I honestly be thinking wrong for like... ever? Seriously... I'm confused.
 

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I put explorer brakes on mine 10 years and haven't had to do a thing to them until I added dual piston front brakes three years ago.

Then I swapped to a 80's fullsize master and then I haven't touched the hydraulic brakes since.
 
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don4331

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@don4331

I thought residual check valves were a thing of the past. At least that seems to be what I remember from trade school many... many moons ago.

Also if the thought that this 10 psi residual pressure holds the brake shoes close to the drum by overcoming the return spring pressure... how come the pedal travel will increase if the rear shoes are misadjusted?

This must also mean the the caliper square cut seal on the piston has greater then 2 psi force to return the piston so the brakes don't drag.

Have I honestly be thinking wrong for like... ever? Seriously... I'm confused.
The easy one - the run out of the discs overcomes the residual pressure and forces the piston back just that amount that the pads hardly drag. But drag they do - so if you wanted ultimate fuel economy you want drum brakes. for the Shell super mileage car we build in university, used an older coaster bike brake not disc as it had less drag.

Aside: We tired steel bands over the tires and found that increase resistance as we were crushing the stones in the concrete...who would have though it...

I know the RABS module has a spring in it to hold brake pressure (one of those parts I had to pull apart to get working again).

I'm thinking on the explanation on the increased pedal travel...
 

pjtoledo

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if there is 10 psi residual on the rear brakes why don't the bleeders squirt when opened?
 

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I remember installing residual valves in some of our buggies. We would have problems with the addition of a steering brake staying at the top of its travel. My supplier of the components said it had to do with applications that the master cylinder was below the the height of the actual wheel cylinder or caliper. It fixed the issue and I never questioned it further.

I also remember using a pressure bleeder years ago. It was pressured to 10 to 15 psi... it would squirt fluid halfway to the other side of the vehicle.

I also remember my instructor saying they were originally intended to prevent air from entering the hydraulic system through the seals. But if you think about it... 14.7 psi atmospheric pressure has never pushed air to the bottom of a glass of water... that's why they went away.

Perhaps they're used in certain applications but I honestly thought most more modern systems don't require them.
 

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If there is a check valve in the system, how can you put your piston compressor on and just push the piston back in?
 

Ranger850

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If there is a check valve in the system, how can you put your piston compressor on and just push the piston back in?
In the brake shop I worked at, this^^ process is frowned upon. We were told to crack the bleeder to push pistons back, to keep from pushing the seals out. Dot 3 isn't that expensive and every job has to get bled anyways.
 

Uncle Gump

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In the brake shop I worked at, this^^ process is frowned upon. We were told to crack the bleeder to push pistons back, to keep from pushing the seals out. Dot 3 isn't that expensive and every job has to get bled anyways.
That never worked well in the rust belt. I've also been just slowly pushing caliper pistons into their bores for longer then I care to admit.... literally thousands of them. Never had a problem pushing a seal out.

Edit... Also doing a standard brake job should never require bleeding... unless you open the system... which apparently you folks always did.
 

Ranger850

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That never worked well in the rust belt. I've also been just slowly pushing caliper pistons into their bores for longer then I care to admit.... literally thousands of them. Never had a problem pushing a seal out.
Key word ; "slowly"

We would do it slowly sometimes, if the bleeder looked suspiciously round or rusty. Nothing worse than a broken bleeder on a beater and the customer is trying to cut corners save money.
 

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I've never cracked a bleeder to compress pistons either. Just use the clamp to press it in and reinstall the caliper.
 

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