No rear brakes after 4 wheel disc swap?

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Justin Angle

Justin Angle

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So I swapped from drums to wilwood disc on the front of my Duster and swapped to a ford 8.8 rear also with disc on the back. Im getting no brakes at all at the rear calipers. But the front ones are good. The petal feels decent. I vacuum bled the rear calipers and they are getting fluid.. Car has a newer master cylinder but I'm pretty sure its the wrong one now that I have 4 wheel disc but wanted to make sure before I buy and wait on a new and still have the same problem. Seems like I should still get some brake pressure at the rear even with the wrong MC but maybe not. Below is the master cylinder I have currently and the other is the one I was going to go for. Could it be anything else? Thanks

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Maybe the combination valve some call it a proportioning valve. I am headed this way with an 8.8 disc brake rear and cordoba big brakes on the front. I am planning on using a GM style 4 wheel disc brake proportioning valve. Not sure if this is the route to go, as I have not gotten that far yet.
 
Well if neither of the rear disks is working then you know the issue has to be between the master cylinder and where the brake lines split for the rear calipers.

I assume the master cylinder worked before with the rear drums. It should still work with the rear disks, if anything it would have a residual pressure valve for the drums that could potentially keep the disks locked up. So I wouldn’t suspect the master cylinder since it worked before.

Was the car disk/drum or drum/drum originally? If it was drum/drum you should have a distribution block, not a combination valve. If it was disk/drum or if it has a combination valve it may need to be “adjusted”, which is done by bleeding the brakes in order- right rear, left rear, right front, left front. There’s a procedure in the FSM. That can recenter the bias adjuster in the combo valve.If it was drum/drum and you have a distribution block it’s unlikely to be the block. Not impossible but unlikely, especially if it worked before.

It could be the brake hose from the frame to the rear axle. In fact that would be my first suspicion. With an 8.8 swap that’s probably a brand new part, but, that doesn’t mean a thing. The first thing I ALWAYS suspect if both rear brakes don’t work is the frame to axle hose.
 
Your photo shows a disk drum master cyl.

The front most cyl outlet has a valve in it behind the seat. That should be removed for the rear disc to work correctly.

As for the rest of the system if you get fluid to the rear calipers than there is no good reason why the rears do not apply.

When you say the rears do not apply, how are you determining this?

Are you sure you have bled all the air from the line? Maybe try the old fashioned method of 2 people and a bottle of brake fluid.
 
72bluNblu said:
Well if neither of the rear disks is working then you know the issue has to be between the master cylinder and where the brake lines split for the rear calipers.

I assume the master cylinder worked before with the rear drums. It should still work with the rear disks, if anything it would have a residual pressure valve for the drums that could potentially keep the disks locked up. So I wouldn’t suspect the master cylinder since it worked before.

Was the car disk/drum or drum/drum originally? If it was drum/drum you should have a distribution block, not a combination valve. If it was disk/drum or if it has a combination valve it may need to be “adjusted”, which is done by bleeding the brakes in order- right rear, left rear, right front, left front. There’s a procedure in the FSM. That can recenter the bias adjuster in the combo valve.If it was drum/drum and you have a distribution block it’s unlikely to be the block. Not impossible but unlikely, especially if it worked before.

It could be the brake hose from the frame to the rear axle. In fact that would be my first suspicion. With an 8.8 swap that’s probably a brand new part, but, that doesn’t mean a thing. The first thing I ALWAYS suspect if both rear brakes don’t work is the frame to axle hose.
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Ok ill try that that bleeding method. The car was originally a drum/drum car. And I did move the car around a bit with drums on the front and 8.8 on the rear and the calipers are very loose feeling on the rotor which tells me it probably never was working in the rear. What threw me is getting fluid by the vacuum bleeding but not by pumping the brakes.
 
Justin Angle said:
What threw me is getting fluid by the vacuum bleeding but not by pumping the brakes
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2 options, one the MC has an air bubble in it between the rear piston and the front piston. Or you have a lot of air in the lines.

Or the mc is bad but if it worked with drum drum I doubt that is the isdue
 
Dana67Dart said:
Your photo shows a disk drum master cyl.

The front most cyl outlet has a valve in it behind the seat. That should be removed for the rear disc to work correctly.

As for the rest of the system if you get fluid to the rear calipers than there is no good reason why the rears do not apply.

When you say the rears do not apply, how are you determining this?

Are you sure you have bled all the air from the line? Maybe try the old fashioned method of 2 people and a bottle of brake fluid.
Click to expand...

The piston for the calipers are all the way pressed and haven't moved since I have messed with them. I don't think I'm following you on the valve in the master cylinder if it was the master cylinder your referring to. I pulled the line off the front outlet and I don't see anything removable.
 
Justin Angle said:
The piston for the calipers are all the way pressed and haven't moved since I have messed with them. I don't think I'm following you on the valve in the master cylinder if it was the master cylinder your referring to. I pulled the line off the front outlet and I don't see anything removable.
Click to expand...

He’s talking about the residual pressure valve I mentioned earlier. Drum master cylinders typically have a residual valve, so after you step on the brakes the valve holds a little bit of pressure in the lines to “stage” the brake shoes so the brake application is faster. If it was there it wouldn’t keep the brakes from applying, although it might cause the calipers to drag. Since that doesn’t seem to be the immediate problem we can worry about that later, the brakes have to work first.

Tell me about this 8.8. Where did you get it? Have you rebuilt the brakes and calipers? Or did it go straight from the junkyard to your car?

And is the frame to axle brake hose new?
 
The valve he is referring to is called a "residual pressure valve" used on drums. It has nothing to do with the problem. You should NOT use a residual valve on discs, but it will not stop them from applying. It WILL stop them from releasing. As others have said, you either have something wrong with the master, or it is not bled, or a prop/ dist valve is effe'd up

Or the caliper pistons are rusted "in place"

Also some of those rear disc setups have a WEIRD parking brake setup. I don't know how they work, and I'd maybe suspect that "in your mind"
 
67Dart273 said:
The valve he is referring to is called a "residual pressure valve" used on drums. It has nothing to do with the problem. You should NOT use a residual valve on discs, but it will not stop them from applying. It WILL stop them from releasing. As others have said, you either have something wrong with the master, or it is not bled, or a prop/ dist valve is effe'd up

Or the caliper pistons are rusted "in place"
Click to expand...

Ha! Jinx!

great minds think alike! :lol:
 
Cool ill check into those possibilities and keep messing with it and change the calipers...they are out of a junkyard and let you know what I come up with thanks for the help so far
 
Justin Angle said:
Cool ill check into those possibilities and keep messing with it and change the calipers...they are out of a junkyard and let you know what I come up with thanks for the help so far
Click to expand...

Ok, if they're junkyard calipers then I would suspect them first. Pistons are probably frozen.
 
Justin Angle said:
Cool ill check into those possibilities and keep messing with it and change the calipers...they are out of a junkyard and let you know what I come up with thanks for the help so far
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E-brake cable attatched to caliper,then e-brake needs to be cycled to pring piston to rotors. Thats the adjuster.
E-brake with seperate shoes inside rotor hat then thats a different setup.
When bleeding,whoever is on the bleeder screw needs to open it more than just cracking it,like 1/2 turn. Slow movement of fluid will retain air in the line.
Residual valve for drums is 10 psi, disc is 3psi.
Check prop valve, it could be shuttled over. I check with key on, and if brake warning lamp is on,then it needs recentering.
 
E brake on these exploder 8.8s are small shoe brakes inside the rotor hat. They do nothing with the calipers. They operate Independently of them.
 
Tooljunkie said:
E-brake cable attatched to caliper,then e-brake needs to be cycled to pring piston to rotors. Thats the adjuster.
E-brake with seperate shoes inside rotor hat then thats a different setup.
When bleeding,whoever is on the bleeder screw needs to open it more than just cracking it,like 1/2 turn. Slow movement of fluid will retain air in the line.
Residual valve for drums is 10 psi, disc is 3psi.
Check prop valve, it could be shuttled over. I check with key on, and if brake warning lamp is on,then it needs recentering.
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Ok how do you tell if its shuttled over? And I'm assuming the prop valve is whats down low below the master cylinder? Thought that was just a distribution block and I didn't have a prop valve. But ill check it out tomorrow. Thanks
 
Tooljunkie said:
E-brake cable attatched to caliper,then e-brake needs to be cycled to pring piston to rotors. Thats the adjuster
Click to expand...
Can't speak to 8.8 rear disks but back in the day, ford rear discs used a screw on to the caliper piston. And as others have said you had to apply the parking brake to force the piston out to the disk, then it was adjusted. When you had to change the pads you had to get a special tool, and screw the piston back into the caliper.
 
Justin Angle said:
Ok how do you tell if its shuttled over? And I'm assuming the prop valve is whats down low below the master cylinder? Thought that was just a distribution block and I didn't have a prop valve. But ill check it out tomorrow. Thanks
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The switch on the proportioning valve goes to ground. As i mentioned, the brake light would come on if it has moved.
 
moparmat2000 said:
E brake on these exploder 8.8s are small shoe brakes inside the rotor hat. They do nothing with the calipers. They operate Independently of them.
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Thats what i thought, gm and some dodge vehicles use e-brake on calipers.
 
Have you replaced your rear brake lines? If you haven’t, have you checked them for blockage?
 
Put plug on front brake line connection on MC. See rear brake action when pedal pressed. and or bleed rear brake line system by itself.
Im guessing need different distribution block, (Drum/Drum) Had similar PITA issue myself.
 
Justin Angle said:
Ok how do you tell if its shuttled over? And I'm assuming the prop valve is whats down low below the master cylinder? Thought that was just a distribution block and I didn't have a prop valve. But ill check it out tomorrow. Thanks
Click to expand...

If it was originally a drum/drum car it should just have a distribution block unless someone installed a combination valve later.

Distribution block should look like this
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Combination valve should look like this
rsd-pv12_xl.jpg
 
Appreciate all the feedback.
So today I plugged the reservoir for the front brakes. Then the pedal had no resistance at all until 3/4 the way to the floor then it stopped with no give. I was thinking maybe the line going to the rear was blocked up with debris so I pulled one of the banjo bolts out of the rear caliper and had someone push the brake pedal and there was fluid pushing so I am getting fluid to the rear calipers. But at the same time no fluid is purging out of the bleeder. So then I put new rear calipers on since the old ones were out of a junkyard and no fluid was purging. But saw no change.

. As far as the 8.8 ebrake goes I opted to leave it off and pitched it back when the swap was done. I don't really see how it could have had any effect on the caliper but may be wrong. And the 8.8 is out of a 1998 explorer with disc rear if that helps.

I'm thinking of ordering the Dr. Diff kit with all new prebent brake lines and dist block and the correct master cylinder just to eliminate these out of the equation but seem like itd be a pita fitting all the new lines since the motors in the car and all.
Just my thoughts and where Im at currently.. Thanks for the help
 
Update:

Alright so here's where I stand now.

I put a brand new master cylinder on the car from dr. Diff. The disc/disc 15/16 bore master cylinder and the car and the rear calipers are now engaging.

The pedal now pumps up and Im getting a streamline of pressure when the rear caliper bleeder are cracked but as soon as you quit pumping and hit the brake again 2 seconds it sinks to the floor. Even after calipers are bled.. I capped where the brake line splices into the hose and the pedal feels firm like it should. This told me that its either one of the hoses leaking or the caliper.

So I pulled the banjo bolt off the drivers side caliper and replaced it with a regular bolt and nut just to try and put hose back into the equation but leave out the caliper. And even with the bolt and nut replacing where the banjo bolt threads into the caliper leaking of course, I still have a firm pedal. This tells me it HAS to be something with the caliper.

. So is there a way to adjust these calipers? I deleted the whole ebrake setup as well. Does anyone know for sure if somehow the ebrake assembly needs to be on inorder for the caliper to work properly?
 
Justin, I am installing a new Ford 9 inch rear end from PEM racing in Missouri. They use a GM metric disc brake system on their "Hot Rod '' rear end they sell and YES the E brake has to be up and running and in place which is what Tool man was referring to.
Nothing else on my 72 Duster project has got baffled but my Manual brake system. I feel your pain brother. Some Great info has been brought forth by tried and proven methods. Gary
 
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