NA6 Brake Calipers Failing to Release = Smoke!

[Scoota]

Hi Folks.

Has anyone ever had a problem with the front and rear calipers failing to release? Here's what happens to me...

1. Go for a drive.
2. Brakes start to drag a little.
3. Brakes start to drag a lot.
4. Crawl home in 2nd gear and arrive home in a stinky cloud of brake pad smoke.

After the brakes have cooled they seem to have released their grip.

The master cylinder was over-hauled and re-sleeved about 12 months ago, just installed this month. The booster hasn't had any attention. A Wilwood proportioning valve was installed and the factory one removed. The car's only recently back on the road after spending about 16 months up on jacks. The recent test drives have revealed the problem, which has obviously developed since last time I drove it. I've stripped all four calipers and they're like brand new. The subsequent test drive yielded the same result - smoke.

The lead theory at the moment is that the brake booster is applying the brakes of it's own accord. It's next on my hit list for an overhaul.

Any thoughts?

Ps. I know about the issues with corrosion in the back of the rear calipers. That's not the problem in this case.

[manga_blue]

I had the same problem once in my MkII Jag, so it's going back a while I have a photo somehwere of the car sitting on a ridge near Menangle at dawn with the discs glowing yellow through chrome wire spokes and smoke rolling off the calipers down the hillside.

As you suspected with yours, a faulty valve in the brake master booster assembly was quietly sucking the piston rod down into the brake master cylinder. It would happen so slowly you couldn't notice anything but a gradual loss of power. It took months to find it. Tapping the brakes hard would often release it for a few Ks.

The Willwood unit, if properly installed, only operates on the rear brake lines. Obviously the master cylinder and booster operate on both fronts and rears. So just a quick check that the Willwood is only plumbed to the rear lines, then focus on the master booster. It shouldn't be hard to find and fit a used one.

BTW, the line of enquiry we initially followed then was that the brake master cylinder wasn't quite releasing the pressure. So a little bit of drag was heating the fluid, expanding it and progressively locking the brakes on harder. It's still worth a thought but in our case that was how we wasted months tracing it.

[mazdatenfive]

I have a used brake booster if that helps.
Just send me an e-mail.

[tbro]

Scoota,
Try this.
Pedal height 171-181mm with carpet
Pedal freeplay 4-7mm

1/ Depress pedal a few times to eliminate vacuum in the booster.. then slowly try and move the push rod to the booster from the pedal. You MUST have clearance if not guess what will happen!!!

If that fails pm me your email and I'll send through the paper work.

Terry

[zossy1]

Very suspicious that the rears are the only ones having the issue. Sounds like there may be a fault in the prop valve. Have you experimented with different bias settings? Shutting the rears down?

[Okibi]

Sounds to me like slider pins are sticking, these are something you need to regrease quite often (more often than you change pads) and a non MX-5 specialist might miss when servicing your car.

They're easy to regrease, sorry can't find a good link to a "how to" but i'm sure there's a few out there.

[hks_kansei]

I'd go with what okibi said, it's a common thing and an easy fix.

[Okibi]

Even if it isn't the problem as least you know they've been done.

For other members reading this thread wondering what they are then make sure you get them done next time your car is serviced.

[zossy1]

Sounds to me like slider pins are sticking

OP, I was assuming you did this when you rebuilt your calipers... If not, this is the obvious solution.

Still mighty suspicious that it would be occurring equally on both rear wheels...

EDIT: just reread the OP, all calipers are sticking... Even more suspicious that it would not be the sliders, but it is also unlikely to be the prop valve either, right? The fronts are isolated from the prop valve, so I think you're right. Sounds suspiciously like a booster or brake master. I'd try swapping the whole assembly and test fit one known to be good. That will rule out any doubt. Do you have any good friends with mx5s?

[Okibi]

I think you're correct zossy, but someone rebuilding the calliper might not have realised the slider needed to be greased so I thought i'd throw in the easy fix for good measure.

[manga_blue]

x2 with tbro on checking pedal free play. Cheap, easy and obvious first step.

[Scoota]

Hi Folks.

I'm so glad that I have access to a well infomed group of enthusiasts. Thanks for the advice, and here are some follow-up comments...

1. The original post refers to trouble with both front and rear calipers, not just rears. In fact, the fronts are suffering worst because I have the proportioning valve dialled in to give me some front bias.
2. I personally rebuilt all four calipers, and I did grease and exercise the slider pins, as I know these cause trouble if left alone (more on that below).
3. The proportioning valve is only connected to the rear lines, so it won't directly affect the operation of the front calipers (except for the obvious front/rear bias relationship).
4. It turns out I have heaps of friends with MX-5's... Thank you forumites!

The throries offered by manga_blue and tbro seem to be the most relevant for me. In the recent rebuild I have had the pedal assembly, booster, and master cylinder all of the car and all separated, so it's quite likely that I have rooted the pedal-to-booster free play. I did check pedal free play, but I wasn't looking for an air gap between the booster rod and the pedal rod - I reckon that's where my problem is. If I don't find anything suspicious I will then consider a booster overhaul, based on manga_blue's theory of a dicky valve in the booster. I'll let you know how I go.

Big thanks to manga_blue and tbro for sharing your expreience. I'd be keen to see that photo in Menangle if you can find it, just for a giggle.

In response to the many comments that focus on the rear calipers, I want to share a recent discovery... The rear calipers have an adjusting screw in the back that screws the caliper piston in and out to allow you to adjust the park-brake and pad offset. To access that screw you remove a plug on the back of the caliper. I recently learnt a few things about this assembly, thanks to my OEM Mazda manual.

There is a cavity inside the caliper that accommodates this adjusting screw and also the cam assembly for the park-brake. This cavity is not fluid-filled, and is kept clean by the aforementioned plug and the seal on the shaft of the park-brake actuating arm (where it goes into the caliper). If water or dirt get into this cavity, the mechanism won't be able to operate freely and the caliper piston will seize. This problem normally manifests as a park-brake that won't release. Very common, and mentioned often on this forum.

My recent discovery was to find that the adjusting screw in the back of the piston is not the screw rod that goes all the way through to the piston. Instead, the adjusting screw is just a short splined grub screw that meshes with the end of the main screw that goes all the way through to the piston. An oil seal on the main rod keeps the brake fluid out of this cavity.

The reason I am telling you all this is so that you can more successfully overhaul your calipers, if you intend to do so. It's very important to remove the grub screw and lube it up so that it can operate freely.

I've learnt a lot about MX-5 brakes in the last couple of weeks. Thanks again!

[zossy1]

Re the rears - yep, they're fun. Rule Of thumb is to screw the adjuster in to the initial friction point, then back out 1/3 of a turn for proper initial adjustment. From that point onwards, they self-adjust

Good luck!

[Okibi]

Photos of the rear brakes here:
http://www.miata.net/garage/ebrake/index.html

[Scoota]

Legend!! Thanks Okibi. That says it all much more concisely than I did.