Never ending front brake problems

[79Vette]

I'm having a terrible time trying to get the front brakes working on my m1009. Both front calipers won't fully release and are constantly dragging.

I just replaced the calipers thinking they may have had corrosion I the bores or something. Also replaced the pads and had the rotors turned and replaced the flex lines while I was at it. When I put everything together prior to bleeding the new brakes the hubs spin freely.

As soon as I bled the system, now the calipers won't release. I can spin the hubs but it takes both hands and significant effort to get it to move.

It seems like there is a restriction in the system somewhere that is preventing the return spring in the caliper from retracting the piston, but I have no idea how to troubleshoot further without just throwing parts at it. Is there a way I can test if it's the proportioning valve, master cylinder, brake booster? Any advice would be appreciated.

Loosening the bolts between the master cylinder and booster does not cause the brakes to release. Opening the bleeder screws on the calipers allows fluid to begin seeping out, but also does not release the brakes. With the bleeder open it seems like any residual pressure should escape and the caliper should release, but they do not.



Also when the rotors came back from being "turned", one now has 0.015" runout and the other has 0.040". I need to check the TM for the spec, but I doubt it's more than 0.002"... I didn't measure before I took them off, but I can't imagine they could have been that bad before. The truck drove and stopped fine other than the constantly dragging brakes and I'd expect rotors that badly out of true to cause the pedal to pulse pretty badly.

Is my next step to press the rotor back off the hub and measure the runout on the back flange of the hub?

 

[cucvrus]

Lost here from afar. Are the slide pins and slide pin seal clean and free of rust? Not sure if they don't release with the hydraulic pressure released.

 

[Barrman]

There are no springs inside the caliper to pull the piston back. If you can open the bleeder and not have fluid come out under pressure. Then it seems to me your hydraulic part of the system is correct. Yes, the "new" calipers could have scored or rusty bores that are limiting the pistons moving because you probably got rebuilt calipers if you bought them from a normal chain parts store.

That leaves slide pins as already mentioned above. Since there is only 1 piston the caliper has to move side to side for both brake pads to make contact. That little bit of runout you have on the rotors should allow the rotors to self clearance the pads back a pretty good bit while driving.

 

[79Vette]

Slide pins are brand new, and greased. Before I filled and bled the hydraulic system there was plenty of space between the pads and rotor and the hubs spin freely, and I could move the caliper back and forth on the slide pins with 1 finger.

Since the first time I pressed the pedal, the calipers have clamped down on the rotor and won't let go.

i guess I'll pull the calipers off again tonight after work and see if I can move the piston back and forth or if it's somehow sticking in the bore. I'm sure they're remanufactured, but it's strange that both old calipers stuck like this and now both new ones do the same thing...

 

[nyoffroad]

Sounds stupid, but are you sure the calipers are sticking? I haven't had a rotor turned in years and never measured runout but could that be the problem? Remember the piston only moves back a few thousands when pressure is released, if the rotor is rough or has essive runout then it's possible the brakes feel like there dragging but aren't.
My money is on the rotors or crud making them 'wobble' .

 

[Keith_J]

Brake hoses can collapse and cause dragging brakes.

 

[79Vette]

Sounds stupid, but are you sure the calipers are sticking? I haven't had a rotor turned in years and never measured runout but could that be the problem? Remember the piston only moves back a few thousands when pressure is released, if the rotor is rough or has essive runout then it's possible the brakes feel like there dragging but aren't.
My money is on the rotors or crud making them 'wobble' .

At this point, nothing sounds stupid to me... Even if the rotors had massive runout, wouldn't it just push the piston back into the caliper until the point of highest runout on the rotor clears the pads? When the caliper was first installed (dry) there was over 0.060" clearance between the pads and rotor surface, so the piston can definitely retract that far. It just isn't for some reason...

Brake hoses can collapse and cause dragging brakes.

You're definitely right. Hoses are just a couple years old, and are the expensive ones from Off Road Design. Best hoses I've ever put on a truck.

If the hose was collapsed internally, would you expect loosening the bleeder to release the pressure and get the caliper to release?

 

[MrMikey4026]

Loosen the master cylinder nuts to see if the drag goes away. Also pull back on the brake pedal, make sure it is coming back to the stop.
I have seen more than one with pedal linkage problems.

 

[79Vette]

Loosen the master cylinder nuts to see if the drag goes away. Also pull back on the brake pedal, make sure it is coming back to the stop.
I have seen more than one with pedal linkage problems.

Good idea. I did try that last night. Unbolted the master cylinder completely and hug it from the hood. The calipers were still stuck.

I didn't look at the brake pedal itself yet, but with the master cylinder unbolted I would expect the caliper to release if the pedal/linkage was the problem.


All I can think to do now is disassemble the "new" calipers tonight and ensure there is no debris or internal damage in them that is preventing the pistons from retracting. I'll post back when I get home tonight and do that, but until then I appreciate any additional suggestions

 

[Barrman]

Hold on. Unbolt the master cylinder again and look at the piston in the bore. Is it all the way back to the snap ring or in just a bit? Push the piston in with your finger or a screw driver and see if it pops right back to the snap ring or slowly makes it way back there.

I agree that non squirting fluid at the bleeder says it isn't a hung master cylinder or brake hose. But, just some of the hole in the master cylinder being blocked could cause the fluid to release very, very slowly and by the time you get to opening the bleeder it is all bled back already. Just a wild thought.

 

[Keith_J]

If there is a brake line restriction, the volume of brake fluid required to cause drag is just a few drops. It won't be easily noticed.

The caliper piston dust seals should provide enough force to slightly retract the piston. All it takes is 0.001" per pad.

 

[79Vette]

Hold on. Unbolt the master cylinder again and look at the piston in the bore. Is it all the way back to the snap ring or in just a bit? Push the piston in with your finger or a screw driver and see if it pops right back to the snap ring or slowly makes it way back there.

Good suggestion. I didn't think of that, and I'll check it out tonight. If it's just a bad master cylinder that would be an easy fix.

 

[cucvrus]

I am lost. You said you removed the bleeders from the calipers and they are still stuck. Brake hoses, master cylinder, what ever. If you open the bleeders the caliper is the issue because with the bleeders open all hydraulic pressure is gone. Something is not right here. Time for some pictures of these new calipers. New calipers, rebuilt calipers, new to you calipers, new that someone sold you and said they are new. I never seen calipers hold with no hydraulic pressure unless the brake system is not assembled correctly. Picture time. Please.

 

[nyoffroad]

Don't over think this!
If you are opening the bleeders and the calipers don't release, then it's got to be the calipers. There is no spring inside the calipers, the only thing that draws the piston back is the lack of pressure hyd. pushing it out (vacuum?)

 

[cucvrus]

Where are the pictures? I gotta see this. I gots to know.