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New Front Disc Brakes Drag

MT_Mopar

MT_Mopar

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67 Coronet 500

I did a rightstuff front disc upgrade recently, when I did that I installed a Willwood proportioning valve on the line going to the rear with the OEM distribution block. Drove the car and seemed like the front discs were dragging, like the pedal was slightly pushed down.

Got back to the shop and the front brakes were hot as hell and they were dragging.

Then I bought a distribution block made for front disc/rear drum conversions from Summit here

Got the brakes all bled down and damned if I still have the same problem. Without even starting the car (power brakes with an electric vacuum booster)

Car is on jack stands and after bleeding the brakes I can turn the front wheels and the disc brakes are not free, they are not on fully, but I can turn the tire and wheel and feel that the brakes have slight pressure on them.

What the heck is going on here? Any help appreciated.
 
Shut the electric vacuum booster off and see if the brakes release. If they do replace the vacuum valve in the power brake booster.
 
Hey John,

What booster & master are you running? Did you happen to check the pre-gap clearance between the rod coming out the booster and into the back of the master? The Service Manual gives you a total length from the face of the booster to nut end. You can also verify by looking down in the bore of the M/C, and view down in the weep hole to see if the piston is all the way seated when the brake pedal is at rest. I use mechanic's wire or a fine diameter tig rod to put down in the rear weep hole to ensure the piston is all the way back. Trick is to have a very, very small amount of play with the pedal before it engages. Last thing you want is a preload.

br.jpg


Just throwing it out there, but what happens with a poorly adjusted rod (be it manual or power), is if there is a preload, your brakes will never fully disengage. Your brakes will drag, heat up and possibly lock up (until cooled down). Too much of a gap you will have too much freeplay in the pedal before braking actually occurs, and can actually hamper the full braking capability of your car.
 
using a strong flashlight shining in the master cylinder "air bleed" hole, when you loosen the nuts holding the master cylinder to the booster, you should not see that valve move. If it does then the brakes are applying and the rod is too long. This is a common issue and effects all makes of car. Common with replacement parts and just a little grinding will fix.

further on what Propwash was saying
 
Dang it, I measured and shortened the booster rod, completely cleaned and reassembeled the calipers, they are not stuck and slide good.

When the caliper is under drag, if I open the caliper bleeder the drag is removed (caliper relaxes and the hub spins free)

When I bleed the brakes everything is free, when I close the last bleeder (left front) apply the brakes the front calipers are tight.

:angryfire:
 
After the front brakes are tight, loosen the nuts for the master cylinder and see what happens. If nothing changes then the problem can not be the master cylinder. If brakes release then it just might have a issue.
 
did your factory hoses collapse?i have seen them work like a residual valve if they collapse inside.
 
It has PTFE stainless flex lines, I am pretty sure its not the flex lines.

I think my wheel cylinder seals are bad.

Thanks for all the suggestions. I am going to hunt around for a rebuild kit for the wheel cylinders.
 
MT_Mopar said:
It has PTFE stainless flex lines, I am pretty sure its not the flex lines.

I think my wheel cylinder seals are bad.

Thanks for all the suggestions. I am going to hunt around for a rebuild kit for the wheel cylinders.
Click to expand...
?????how are the rear brakes the issue?????
What makes you think the wheel cylinders are bad
 
Hemi Rebel said:
?????how are the rear brakes the issue?????
What makes you think the wheel cylinders are bad
Click to expand...

It is the fronts that are the problem, they got overheated really bad. The fronts are new aftermarket discs, the rears are oem drums.

Like a dummy I did not recognize the preload of the longer vacuum rod that Propwash bought to my attention until they overheated...

Live and learn, and bang head against wall in-between times.
 
You are not the first nor the last to have this issue. I will even now bet that someone else is having the exact same issues.
 
AMX364 said:
You are not the first nor the last to have this issue. I will even now bet that someone else is having the exact same issues.
Click to expand...

AMX, you are correct. Been chasing this on my A body for awhile now. Have something new to check now!
 
Hey John, may be a silly question, but you didn't use the original drum/drum master for your disc upgrade did you? Sorry, I can't remember if you had your disc upgrade in your resto thread or not.
 
Propwash said:
Hey John, may be a silly question, but you didn't use the original drum/drum master for your disc upgrade did you? Sorry, I can't remember if you had your disc upgrade in your resto thread or not.
Click to expand...

Prop: No, I have an after market master and booster. I used the OEM distribution block with a Willwood proportioning valve to the rear drums.

Another one of my problems was the oem distribution block was right next to my headers on the frame rail, which did not help with the heat problem one bit.

I moved the replacement block up near the master cylinder, next step is to install a front line lock.

Thanks for your help.
 
Ahhhh, makes sense now. I'd bet a 12 pack that the M/C you're running has residual valves in both ports of the master. Those residual valves hold 10lbs of pressure in the lines, and that would cause your issue with your discs dragging. With the drums, the residual valve is needed, but the 10lbs of pressure does not cause them to drag because the return spring force is several times as powerful.

You need a master like the one you depicted from pirate jack. You're adjustable proportioning valve in conjunction with your factory distribution valve should work just fine. If you want to balance out the braking a bit more, install a metering valve for the lines to the front brakes. What that does is hold off on supplying pressure to the disc's, for literally a split second or so (engage around 40psi). It helps with a smoother braking in hard/quick braking situations. Like mentioned, not 100% necessary (previous models and some other brands/makes didn't even have them). Up to you. Factory disc/drum B-bodies from our car's era did. Anyways....I'd be almost 100% certain, that the master you're running now is your issue.

You could remove the residual valve from the port for your front brakes and see what happens for troubleshooting. I'd recommend you step up to the master from pirate anyways. It probably has a bit bigger bore (around 1 to 1 1/8") and a bigger capacity.

Here's a disc/drum master from the FSM showing the residual valve and removal on the drum side. Yours would have them in both ports.

View attachment 246550
 
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