Front disc brake dragging

Rustytruck · Jun 15, 2018

The caliper is on the knuckle side of the rotor. The piston of the caliper pushes the pad against the rotor. The rotor has no place to go so the hydraulic fluid pushes the caliper back towards the knuckle. It pushes back and pulls the pad on the other side of the rotor into the rotor.

As the pads wear the piston pushes the knuckle side pad deeper into the rotor but on the other side of the rotor the pad needs to be pulled into the rotor further and that can only happen if the caliper can push closer to the knuckle.

Once the caliper hits the knuckle the caliper can no longer pull the pad on the other side of the rotor into the rotor. The piston will still push the knuckle side pad into the rotor since the piston in the caliper still moves towords the rotor. But with the caliper jambed into the knuckle you get no pull on the pad on the other side and you get no clamping force between the two pads against the rotor.

Just like taking off one brake pad from a bicycle rim you only get a little drag and no real braking force.

Now with a 4500 lb Bronco your only stopping with your rear brakes.

lbuch40 · Jun 16, 2018

OK. this makes sense. the bicycle brake is a good analogy. I see where you need the clearance to have good braking, but how might this clearance issue be related to the brakes dragging....not releasing far enough?

bronconut73 · Jun 16, 2018

Rustytruck said:
The caliper is on the knuckle side of the rotor. The piston of the caliper pushes the pad against the rotor. The rotor has no place to go so the hydraulic fluid pushes the caliper back towards the knuckle. It pushes back and pulls the pad on the other side of the rotor into the rotor.

As the pads wear the piston pushes the knuckle side pad deeper into the rotor but on the other side of the rotor the pad needs to be pulled into the rotor further and that can only happen if the caliper can push closer to the knuckle.

Once the caliper hits the knuckle the caliper can no longer pull the pad on the other side of the rotor into the rotor. The piston will still push the knuckle side pad into the rotor since the piston in the caliper still moves towords the rotor. But with the caliper jambed into the knuckle you get no pull on the pad on the other side and you get no clamping force between the two pads against the rotor.

Just like taking off one brake pad from a bicycle rim you only get a little drag and no real braking force.

Now with a 4500 lb Bronco your only stopping with your rear brakes.

As usual Rustytruck nails it,

Very good explanation for a newbie to follow so they understand just how much of the knuckle needs to be ground.

It should be saved as a sticky or archived in tech.

lbuch40 · Jun 16, 2018

I agree. Its a good explanation.

PDQ · Jun 17, 2018

Rusty, I've got what you mentioned earlier in the post, where my brake pedal doesn't fully return after everything warms up causing the brakes to drag. Its a very short distance to get it back as I can just nudge it up with my foot or my hand each time after application, but its driving me crazy. Is it a spring issue or a pressure issue, a rod adjustment issue or?
Everything seems tight and also works well when cold, but as it warms up it tends to 'stick' each time.
I have a Duff power brake setup and a disc conversion up front.

Thanks!

bronconut73 · Jun 17, 2018

When my FSB did that it ended up being the vacuum booster.

ntsqd · Jun 17, 2018

lbuch40 said:
I am not understanding the relationship between the clearance between the caliper / steering knuckle and a worn out pad. I probably need to take the wheel off and have someone apply the brakes while I watch the moving parts....especially the clearance between the caliper and the knuckle.

The worn-out outer pad represents the closest that the caliper body will ever get to the knuckle. By using just the pad backing plate only you are simulating the worst possible case, it should never get that far.

lbuch40 · Jun 17, 2018

Is the brake light warning supposed to come on with the ignition switch and then go out when running? Mine does not come on at all.

PDQ · Jun 17, 2018

Bronconut, how did you narrow it down to that? Is there a way to test the booster spring?

Sorry if this is a highjack, I can repost if needed, its just a different symptom of the same original thread issue

DirtDonk · Jun 18, 2018

lbuch40 said:
Is the brake light warning supposed to come on with the ignition switch and then go out when running? Mine does not come on at all.

Could be as simple as a burned out bulb. Could also be the switch is old and the "proof-out" function is no longer functioning as it should. That's when the body of the switch grounds out to the dash board and, in our case, completes the circuit for the brake warning lamp.

After all these years, a poor grounding scheme at the dash becomes even worse. Maybe the switch is working, but the dash just isn't grounded well enough for things to work at their best. Does everything else seem to work ok?

But has yours ever worked? You might check out down on the brake line distribution "H-block" to see if the switch is intact or maybe even the wires have been disconnected.
This is often done to turn out a light that is illuminated all the time and the usual methods to turn it back off don't work.
That, or a melted lens are the two most common reasons for the light not working.

Paul

lbuch40 · Jun 18, 2018

DirtDonk said:
Could be as simple as a burned out bulb. Could also be the switch is old and the "proof-out" function is no longer functioning as it should. That's when the body of the switch grounds out to the dash board and, in our case, completes the circuit for the brake warning lamp.

After all these years, a poor grounding scheme at the dash becomes even worse. Maybe the switch is working, but the dash just isn't grounded well enough for things to work at their best. Does everything else seem to work ok?

But has yours ever worked? You might check out down on the brake line distribution "H-block" to see if the switch is intact or maybe even the wires have been disconnected.
This is often done to turn out a light that is illuminated all the time and the usual methods to turn it back off don't work.
That, or a melted lens are the two most common reasons for the light not working.

Paul

Mine did work last year when I replaced the calipers on front. The light came on as it should have after flushing all the fluid, replacing with new, bleeding, etc. I went through the centering routine and got it to go out. But my key ring has a leather flap that covers the light (probably not smart) and so I don't know if/when it quit working. The wire is connected to the H block and looks fine. It could be a burned out bulb or grounding issue. But if I understand you correctly, it SHOULD come on when the switch is turned on, correct? If so, then the problem could be the bulb, the ignition switch, or the actual switch at the H-block? Which brings me back to my original question as to whether the H-block proportioning valve might have something to do with dragging brakes when they get hot.

This past weekend I ground on the steering knuckle on the drivers side to insure clearance and blew out the ports on the MC with compressed air. It sounded like the valves in the MC were opening and closing. Also checked the plunger between the booster and MC. it was already shortened as far as possible and there was even some spacers placed between the MC and booster. Still have to grind on the passenger side knuckle. Was only able to take it for a short test drive. There was no significant dragging, but it wasn't a real test either.

So many variables!!

DirtDonk · Jun 18, 2018

lbuch40 said:
But if I understand you correctly, it SHOULD come on when the switch is turned on, correct?

No. Not like a modern car.
The light will only illuminate when the key is in the START position. As soon as you release it to RUN, the light should go out.

lbuch40 said:
If so, then the problem could be the bulb, the ignition switch, or the actual switch at the H-block?

Still yes.
Or the H-block's shuttle could be offest, grounding the switch, and turning on the lamp to show that either there is a problem with the brakes, or a problem with the H-block.

lbuch40 said:
Which brings me back to my original question as to whether the H-block proportioning valve might have something to do with dragging brakes when they get hot.

No on the H-block (unless it's just clogged?)
But a proportioning valve can, as mentioned previously.

lbuch40 said:
...and blew out the ports on the MC with compressed air. It sounded like the valves in the MC were opening and closing.

Never done that myself. So don't know if it's a problem to test a master like that or not.
While the hoses are off however (are they still off?) poke a small paperclip or other poking device of some kind, to see if there is a spring loaded seat just under the cone seat.

lbuch40 said:
Also checked the plunger between the booster and MC. it was already shortened as far as possible and there was even some spacers placed between the MC and booster.

Well that's not a good sign. Did you put the spacers there, or did someone else? Were you able to measure the distance/gap between the tip of the rod and the back of the piston? Would be nice at this point to see just how far it is. Even with the spacers, it could still be too close.

The fact that it's down as far as it can go and someone still had to add spacers means that it was another poor match of parts. Not automatically a particular fault of someone specific. Just that these days many parts are not made to the same exacting specifications that they were 30 years ago and brake masters don't always fit up against vacuum boosters correctly.
Without reading back along the thread I don't know if you mentioned putting the spacers in as a test. But if it wasn't you, I would get to the bottom of the rod-vs-piston thing. Find out just how big the gap is between them.
One way to do this is to remove the shims and as you place the master up against the mating surface of the booster, see if the piston touches down before the master does. If it does, then get that measurement to make sure there is enough shim in there.

Paul

DirtDonk · Jun 18, 2018

You said single wire I think, when you mentioned the switch the first time?
If so, then you have an aftermarket proportioning valve perhaps? The original H-blocks used a 2-wire switch, and all the aftermarket ones I'm familiar with use a 1-wire switch.

Are you familiar with the two styles? If not show us a pic if you have not already.
Because while an H-block will not cause a caliper to drag (unless it's partially clogged) a malfunctioning proportioning valve will.
I don't know what the functional differences are, since both are just fancy ports to transfer fluid and even the prop valve only limits flow to the rear.
But the new-fangled prop valves typically available through the aftermarket (and yes, us too) can indeed have issues with dragging brakes and leaking delay valves.

Thanks

Paul

lbuch40 · Jun 19, 2018

DirtDonk said:
You said single wire I think, when you mentioned the switch the first time?
If so, then you have an aftermarket proportioning valve perhaps? The original H-blocks used a 2-wire switch, and all the aftermarket ones I'm familiar with use a 1-wire switch.

Are you familiar with the two styles? If not show us a pic if you have not already.
Because while an H-block will not cause a caliper to drag (unless it's partially clogged) a malfunctioning proportioning valve will.
I don't know what the functional differences are, since both are just fancy ports to transfer fluid and even the prop valve only limits flow to the rear.
But the new-fangled prop valves typically available through the aftermarket (and yes, us too) can indeed have issues with dragging brakes and leaking delay valves.

Thanks

Paul

I am learning so much. I thought H-block and proportioning valve were interchangeable terms. I definitely have an aftermarket with one wire that I assume was installed when the PO installed the power disc brakes. I don't have a photo of mine handy but can attach one from another website. I suppose if this proportioning valve is bad, it could explain why my passenger rear locks up so quickly. I just assumed it was because the pads were contaminated with axle grease. Its very possible that MULTIPLE problems are contributing to the various symptoms.

The spacers were installed by someone else. Not sure if it was the PO or the brake shop that last worked on my Bronco last year. Interestingly, a couple weeks ago I found one of those spacers laying on the frame and didn't know what it was....like one fell out. but the MC was still tight to the booster. Will add some more pictures tonight.

Good idea on how to check how much clearance there is. I will do that.

I read about blowing the MC out with compressed air somewhere. Possibly the Chilton's manual I have. But it said to not try to clear anything with a wire..

lbuch40 · Jun 19, 2018

Here is the valve I have...I believe.

ntsqd · Jun 19, 2018

Usually called a "Combination Valve" as it has the P-Valve function as well as the H block & Brake Warning light switch functions combined into one part.

DirtDonk · Jun 19, 2018

lbuch40 said:
...I suppose if this proportioning valve is bad, it could explain why my passenger rear locks up so quickly. I just assumed it was because the pads were contaminated with axle grease. Its very possible that MULTIPLE problems are contributing to the various symptoms.

Normally, no. Not by itself anyway.
If a single brake is dragging, it's usually something else since the prop valve is going to effect both equally from it's end of things.
Like you said then, it's probably multiple issues.

Paul

lbuch40 · Jun 19, 2018

I believe this is the Master cylinder that I have, based on this photo from Wild Horses.

DirtDonk · Jun 19, 2018

ntsqd said:
Usually called a "Combination Valve" as it has the P-Valve function as well as the H block & Brake Warning light switch functions combined into one part.

One other function added to them is the "delay valve" which is that little spring loaded thingy under the rubber boot.
Kind of a clever solution to a minor (but important) problem that can crop up from time to time. But with the aftermarket, this clever solution has turned into more headaches than not, in the form of leaks.

I used to recommend the 4-wheel disc version, instead of the disc/drum version because it did not have the delay valve. But from what I've seen, now they've all got them.

First thing I do then, when installing one of them is put a shield between the combo valve and any painted surface!
Make sure it doesn't leak first. Only then can you truly enjoy any of the benefits.;D

Or, if you're setting your system up from scratch, just go for the manually adjustable type of proportioning valve to begin with.

Paul

ntsqd · Jun 19, 2018

A well designed delay valve can eliminate the need for a proportioning valve entirely. We used one the LBS' that BCB now sells on a 1st Gen Tundra that has F/R 9"'s & Exploder RDB's without any p-valve at all. Different wheel base & weight distribution than an EB, so I wouldn't expect necessarily that it would work on an EB, but were I building from scratch that is what I'd start with & only add the p-valve if necessary.

Front disc brake dragging

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