26 replies to this topic

[RacerDad]

The R/F brake caliper is binding but not enough to cause any noticeable pull or to overheat the rotors.  The other three wheels spin with normal friction.  To diagnose the problem I have completed the following:

 

1. Replace the R/F caliper with a new unit.  This did not make a change and the old caliper was new this season.

2. Replaced the front rotors.  I did this as part of routine maintenance as the rotors were starting to crack (4 weekends).

3. Replace all the braided steel brake lines with a new kit.  It didn't make sense to change just one.  Before installing the lines, I loosened the bleed screw on the R/F caliper and the wheel spun a little easier.  After installing new lines and doing a full bleed of the brake system, the R/F caliper was still binding.

4. If I back off the brake caliper piston, the wheel spins freely

 

At this point, I am leaning towards replacing the proportioning valve (or Valve, Dual; Part #: NC10-43-900A). The master cylinder has one line connected directly to the L/F caliper.  The other lines go through this valve.  As the other three wheels are not having this issue, I am assuming the valve is the cause.

 

Am I on the right track?

 

 

[FTodaro]

Do you run the brake hardware? the slides and springs? I and many others due,  but some will come on here and say they don't like to. I am not going to debate that just try the springs if your not running them. You can also check the clearance on the brake pad your running and grind or file them down to be sure they have good clearance. I am thinking the proportion valve would not be the issue but i would focus on that brake caliper first. When you replaced the Caliper I assume you replaced the bracket with it? if not you need to change that out also.

[callumhay]

I'm no expert but have a couple of suggestions in addition to what Frank said.

Try removing the clips that are used as spacers between the caliper bracket and the tabs on the pad. The pads will still stay in there, it's just they have a small bit of clearance. Run the springs.

Try or continue using the backing pad against the pad

Lube and clean the sliders frequently ( I'm assuming u have changed and they are not bent). As Frank said, did u change Caliper? And I guess it's possible the spindle could be an issue if that while assembly was bent..Any hits that the car took to the RF?

Let us know if you figure it out. The prop valve could be the issue but I would double check everything else before going there.

Cal

[Steve Scheifler]

Cal, like Frank I don’t want to rehash the old debate about brake hardware but eliminating the spacers at each end of the pads is a bad idea and only increases the risk that a pad gets misaligned or drags. I also wouldn’t run the backing plates, particularly with new pads.

I’m not ruling out the proportioning valve but it really just sounds like a tiny bit of piston drag when static. You call it binding, but to what degree exactly? If anything the prop valve, just by being in the circuit, may not allow that side to pull back slightly as pedal pressure is released. I’m not standing there fiddling with it but from what you describe it sounds perfectly normal. There is no real force pulling them back after all, but in the real-world just the spinning rotor of the car in motion would knock the piston back the 1mm you’re looking for.

[Steve Scheifler]

BTW, how did pad wear look on that side vs the other?

[RacerDad]

Thanks for the responses.

 

When I replaced the caliper, I replaced the bracket as well.  I greased the two slide pins even though they are brand new.

I do not run the shims, hardware, or springs.  Due to this, the pads fit loosely in the bracket  The brake pads are worn about 50% with only slight taper.  The pads on the L/F have more taper.

 

I do not believe the spindle is bent.  The car alignment is spot on and there are no abnormal vibrations.  Backing the piston off just slightly allows the wheel to spin freely.

[Bench Racer]

This is not the first I've heard/viewed of the R.F. having some drag. To test out without the proportioning valve, plumb around so line travels directly to the R.F. line. 

[Tom Hampton]

Cal, like Frank I don’t want to rehash the old debate about brake hardware but eliminating the spacers at each end of the pads is a bad idea and only increases the risk that a pad gets misaligned or drags. I also wouldn’t run the backing plates, particularly with new pads.

I’m not ruling out the proportioning valve but it really just sounds like a tiny bit of piston drag when static. You call it binding, but to what degree exactly? If anything the prop valve, just by being in the circuit, may not allow that side to pull back slightly as pedal pressure is released. I’m not standing there fiddling with it but from what you describe it sounds perfectly normal. There is no real force pulling them back after all, but in the real-world just the spinning rotor of the car in motion would knock the piston back the 1mm you’re looking for.

 
I'm with steve.  I have a slight amount of drag on the RF, also...when sitting still on jackstands after pressing and releasing the brake pedal.  On the LF, if I spin the tire I'll get 10ish revolutions before the tire comes to a stop.  On the RF, more like 3-4.  I went round and round on it back in 2009-10 or so.  Like you, I swapped everything except the prop-valve (master, calipers, bleed and rebleed, etc).  I rebuilt the calipers with new seals, polished the pistons and bore surfaces, everything to ensure the calipers could retract when pressure was released.  Nothing ever made a difference. 
 
I think I posted about it on here, and Saul pointed out that road-vibration -> pad knock-back would overcome the difference in pull-back in any real-world scenario.  I did a road test, since I have reasonably rough roads around my house.  I only used the e-brake to slow the car coming home.  I put the car up on stands and tested the "drag".  All gone---LF/RF tires both spun the 10ish revs. 
 
Also, as Steve noted...if there were any apprciable drag, you'd see it in the pad wear...replacing the RF pads more often than the LF. I admit that 10 years later it still nags at me, and I keep tabs on the LF/RF pad wear. I've even go so far as to measure pad thickness (with a micrometer, at 3 points...looking for taper) before/after each race weekend.  I was able to note a sticking caliper pin with this data, but nothing to infer that the RF was wearing overall any faster than the LF. 

[callumhay]

Well, the question is a nagging one because there is probably an answer.... I went and took a look at the car (1999) and the shop manual.  Firstly in the shop manual they show a bunch of pressure gauges on the prop valve and whatever else with SST numbers (that we know won't be available or cost a s##t ton to get)….I gave up after 5 minutes on reading that

 

but seriously, if you look at the routing of the brake hard lines, the LF comes right out of the front of the master cyl. The RF comes out of the prop valve and is much longer with a run taking it over the edge of the trans tunnel. It is not inconceivable that an issue in the prop valve (or damage to the line, say during engine install) could give an issue on the RF. Begs the question as to whether this issue is a "normal" miata issue, uncovered by observant racers or the sign of a failure in the prop valve. I for sure have no idea how the thing works. 

 

I googled proportioning valve issues in miatas and there is not much out there unless you want to replace them with the aftermarket ones....which we know everyone (except the SMMD guys) are not doing. (OK sorry if I offended the SMMD guys)

 

If you are going to replace the prop valve. presumably get a non ABS one? or is there a difference? I'm just asking questions and is there a difference between valves in ABS vs Non ABS

 

I'm pretty sure someone out there can explain how the thing works ..and I would like to know ...well just because. 

 

If you do find out what the issue is, again let us know. 

 

Steve, yeah I hear you on the little metal things that support the pads, I was just wondering if removing them and running a test with the car might make a difference in terms of trying to isolate the problem. But with what Tom says, it may come down to the way these cars are on the RF...for non racing purposes not a huge issue ...but for those of us who want answers it is going to churn in our minds...

 

Cal

[Caveman-kwebb99]

i think I would take a very high quality straight edge and go from one of the mounting surfaces of the caliper braket to the other and make sure one of those is not tweeked or twisted or something, I have seen that before looks good to the naked eye but its off.

[Steve Scheifler]

i think I would take a very high quality straight edge and go from one of the mounting surfaces of the caliper braket to the other and make sure one of those is not tweeked or twisted or something, I have seen that before looks good to the naked eye but its off.

I think you mean on the spindle where the caliper mounts but that isn’t entirely clear as worded. If they are not on the same plane or otherwise “bent”, the I would wager some $$ that it is one of the old spindles modified for more camber. That was an issue if not done correctly (with a steel block bolted to those ears). But I don’t think it would cause the perceived problem as described.

[Steve Scheifler]

Cal, I think our explanation IS the answer. As I said, if the existence of the valve in series with that side restricts back-flow even slightly compared to the other, and surely it must, then the simple law of path of least resistance allows the left side to get sucked back (or more accurately I think, relax) more than the right. No real mystery and no real harm. It is truly a non-issue.

[Bench Racer]

Cal, I think our explanation IS the answer. As I said, if the existence of the valve in series with that side restricts back-flow even slightly compared to the other, and surely it must, then the simple law of path of least resistance allows the left side to get sucked back (or more accurately I think, relax) more than the right. No real mystery and no real harm. It is truly a non-issue.

This issue has existed as far back as one cares to search. Can one truly say no real harm/non-issue without at minimum doing a heat test of RF and LF? I had the same issue on my 85 RX7 Spec7 race car.

[Bench Racer]

Cal, here's some info on the Miata OEM proportion valve.

 

 Following shows innards of the Miata proportion valve. When that internal spring loaded gizmo changes position that would be when the pressure to the rear lessens at the point called the knee. Scroll down until a picture on left hand side has a title, Miata Prop Valve Innards.

https://www.google.c...BWwHOaZM:&vet=1

 

Following shows the front/rear brake pressure knee.

https://www.flyinmia...h/stock_bpv.php

[callumhay]

Thanks Bench and Steve.

[Steve Scheifler]

This issue has existed as far back as one cares to search. Can one truly say no real harm/non-issue without at minimum doing a heat test of RF and LF?

Yes, I can.

[Bench Racer]

Yes, I can.

Have you completed a rotor heat test and what are the temp numbers?

[Steve Scheifler]

Nope, don’t need to, not sure why you think otherwise. But you do love to dream up overly complicated ways to prove the obvious.

[Bench Racer]

Nope, don’t need to, not sure why you think otherwise. But you do love to dream up overly complicated ways to prove the obvious.

Prove the obvious with some heat test numbers. Otherwise your getting pretty comfortable with assumptions.

 

What is so "overly complicated" about connecting the P valve front line to the MC front line with a union and plugging the P valve hole and the MC hole? This is a jack stand test, no track required. Spins easier may be the answer. Then on a test day being careful use the brakes and pyrometer the front rotors. 

[Steve Scheifler]

Knock yourself out Bench, but you’ve already changed the rules. First you want me to prove it’s a non issue (irrelevant regardless of cause) with some crazy temperature test “without at minimum doing a heat test” where the noise is likely greater than the data, and now you switch back to proving the cause of the observed difference in caliper behavior is due the plumbing, which I frankly don’t care about enough to waste time on because it’s irrelevant. (All assuming of course that we correctly understand the degree of “binding” and drag described).