Need help with Miata Brakes

I’m using a complete 99 Miata brake system in my LS6 Lotus Europa build. I’m having trouble with the brakes dragging. Seems to be all 4, all 4 corners have similar temps after driving. All 4 wheels turn fine when cold, but seem to stick when they get some temperature in them.

l’m thinking that the pressure might not be releasing in the master, as the pedal seems very hard and close to the top when this happens. The other possibility could be the push rod binding going through the firewall that I fabricated.

Any educated / experienced opinions before I start throwing new parts at the problem?

Sounds likely that the master cylinder pushrod is too long. You may be using the complete Miata system, but it's in a new home, and I'm guessing something isn't exactly the same. Heck, having thinner sheetmetal between the pedal assembly and master could close that gap.

I thought about that.  I have adjusted the pedal as much as possible. Maybe I’m going  the wrong direction?! I have shortened the adjustment to the point where the brake light switch just barely touches the pedal. I could definitely shim the master more forward, but it seems like shortening the adjustment would have done that. 

I think you need to pull the master, measure from the mounting face down to the point the point the pushrod presses against, and then also measure from the firewall to the tip of the pushrod. There's going to be a spec for free play there, though I don't know what it is on a Miata. Or at least you could get it warm enough that the wheels won't turn, and then back off the MC mount hardware to see whether that relaxes everything.

I haven't been through these parts on a Miata, but I think it's pretty common that pedal adjustment != master cylinder pushrod adjustment. Since I don't know on a Miata which way the switch works, I don't know which direction you adjusted it, but a quick google suggests Miatas do have a separate adjustment for pushrod length, so I think pedal height isn't what you're after here.

Hopefully you can get some Miata-specific input here, but I'm about ready to bet a pint that it's MC pushrod length. The symptoms are right, and just to slam-dunk it, you changed out the Mazda-made spacer (the car itself) that holds the bits at a known distance for one made by Lotus.

I can't vouch for quality, but there's some brake setup info that encompasses some of this here: http://www.mellens.net/mazda/Mazda-Miata-1997/brakes.pdf

I agree, pushrod adjustment. It's too long.

There's a bleed hole on the master that allows fluid to return to the reservoir as it heats up and expands. If the piston is pushed too far into the master, the bleed hole is blocked. When the fluid gets hot, the only thing it can do is push against the pistons and apply the brakes. 

Add in some free play, see if the problem goes away.

Thanks! That makes perfect sense given my situation! 

From what I’ve read, the free play adjustment is via the brake light switch and I have tried it with that completely removed, with no help. I’ll try to shim the master forward. Kieth thanks for the insight on how the master bleeds. I assumed that it must work some way similar, but I wasn’t clear on how. I think that is the piece of information that I was missing!

Ransom, thanks for pointing me in what must be the right direction. I’ll let you know how it goes!

In reply to Sofa King :

Progress!

Peek at that PDF I linked. The pushrod itself is adjustable for length, which is the correct way to make this adjustment. You could certainly sanity-check by shimming, though.

Just got done reading that! Much easier than what I was planning! Thanks for the link! I’ll make the adjustment tomorrow, but I won’t be able to test it for a couple of days. We have some bad weather coming!

Yeah, mazda doesn’t consider it adjustable but it is. 

I can say from experience that if you take the car on track as-is, it will pretty much lock its wheels solid. I was lucky, I was able to get into the pits...

In reply to Keith Tanner :

Luckily I’ve been teaching it to walk before I let it run. So I haven’t had any drama. But I’m really glad to have the potential solution!

thank you!

In reply to Keith Tanner :

I’ve been playing with the push rod adjustment, and done the measurements to determine its length. It doesn’t seem to be all that much too long. Perhaps 1/16”, which was all the adjustment that the rod seemed to have. Is this enough length to cause the problem? How much did you have to shorten yours?

i have made the change but won’t be able to test it until later today. (Weather permitting)

That is totally enough adjustment to make a difference.

EDIT: I don't know whether it's enough in this case, but it is a meaningful adjustment.

In reply to Ransom :

Great! I won’t jump of the ledge just yet!

thanks!

My problem was not with a Miata master cylinder. The hole is not big, though.

If you want to be sure this is the problem, make a big adjustment and see if it changes. Then you can zoom in on exactly where the threshold is. You can test by removing the pads from one front caliper and attempting to compress the piston. If you can, that means the fluid is going back into the reservoir. If you cannot, that means the hole is covered. This is assuming you're using sliding calipers with a single piston. It won't work with multi-piston calipers.

Yes it is a single piston. 

I will see if I’ve made any difference, and if not I’ll try compressing the piston!

Thank you again!

Weather cleared and I was able to go on an extended test drive. Brakes worked perfectly! I wouldn’t have guessed  that 1/16” would make that big a difference!

Many thanks to Ransom and Kieth for all of their help!! I could have floundered for quite a while without it!