OptionXIII

Hi all,

I'm hoping for some help figuring out a soft pedal on my 2001 Miata with the larger Sport brakes, without ABS.

I got in the car for the first time in about a month and at some point I noticed the brakes felt like crap. I'd been driving my Jeep for a while, so a soft pedal was what I was used to. I pretty much only use the Miata for track days and road trips, so I can't say with certainty when this started. The pedal is soft with longer travel than usual, and bottoms out on the travel stop before I can get lock up at any wheel, though I can still brake fairly hard. The braking I do get is smooth and even, it does not pull to either side. The brake fluid was low enough to trigger the brake warning light (thought I had an issue with the hand brake wiring... oops), but certainly not at the point of uncovering the fill hole in the reservoir. I don't believe this is due to a leak, I probably just left the fluid too low after the last brake bleed and the pads were at the end of their life, so they wore down quick at my last track day.

Rebuilt rear calipers, stainless steel flex lines, and a master cylinder brace were installed 5 years ago. The front calipers were rebuilt by yours truly last year. I've been to multiple track days since this work including two earlier this year, and had no issues with the brakes, other than running out of talent and getting the occasional front wheel lockup. I bleed them and grease the slides regularly as part of track day maintenance, and I've never had an issue like this.

Things I've done trying to fix it:

1. Adjusted the handbrake (tighten adjuster until it stops, back off 1/4 turn).
2. Bled the brakes
3. Bled the master cylinder by cracking open the brake line fittings
4. Bled the brakes again with a different friend
5. Replaced the worn out pads with fresh pads. Obviously this means the pistons had to be pushed back in. The fronts I could push in by hand (with a lot of effort!), and the rears were pulled back in with the screw adjusters. All caliper slides were smooth and slid/rotated freely. Hand brake readjusted at the end of this.
6. Completely drained and refilled the system - vacuumed all the fluid out of the calipers, lines, and master cylinder, then refilled the system.
7. Inspected the lines, master cylinder, and calipers for signs of leaks or other damage. Nothing found. There was no fluid behind the sealed parking brake adjuster cover bolts.

We bled the brakes with both the classic two man system, and a vacuum bleeder, including greasing the bleed nipples to prevent air from getting sucked in. More than a quart of fluid has been used for this. We followed the usual order from the drivers rear corner, passenger rear, passenger front, drivers front. Other than when the system was bled dry and refilled, no air bubbles came through the lines, and it was solid after fluid reached the caliper. We put the lug nuts back on and tightened them down on the rotor so that piston knockback wouldn't be a concern with the wheels off the car. The result of all this is the pedal feels exactly the same as it did before.

The system seems to hold pressure fine - if the pedal is held at half travel, even for a full minute, there's no change in effort to hold it steady and it doesn't bleed down.

I've completely replaced brake systems before and never had this much trouble with a soft pedal. The last time the pedal felt like this was due to a failing wheel bearing creating caliper piston knockback. I checked all the wheel bearings - they're solid and tight.


Does anyone have any recommendations for things I should do before I start playing parts cannon and throwing $60+ at a new master cylinder?

yossi126

iTrader: (6)

I feel your pain and been through everything you've been through with a 2005 Sport w ABS.
Like you the only thing that has not been replaced is the master cylinder.
Please update if you end up buying one and it solved your problem.

HarryB

I have had the same issue on an FSAE car a few years back. In that case, it was an internal leak between the master cylinder compartments.

OptionXIII

I've replaced the master cylinder with a Raybestos MC390944 and installed Russell speed bleeders on each corner. No bench bleeding, I just installed it and snugged the fittings, filled with fluid, backed off the fittings slightly, and pushed the pedal until fluid leaked from all three outlet ports. Then I did a standard caliper one man brake bleed with the new speed bleeders.

The pedal feels exactly the same on the jackstands. I'm going to give it a drive when I can, but I'm not optimistic. I can try bleeding again, but I'm not optimistic. I got a lot of fluid out of the Drivers Rear and Passenger front.

Any thoughts on if it could be a prop valve gone bad? None are available on Rockauto, I haven't done much of a search otherwise. I like the idea of an adjustable prop valve, but I have read the Wilwood one ends up getting set to full rear bias anyways on a non-ABS Sport brake Miata, so I am not sure I'd be gaining any adjustability.

rdb138

iTrader: (1)

Hey 13…

Soft pedal and no fluid leak almost always means air in the system. You need to bench bleed the master. The new master has same exact problem as old master…air bubble trapped in it.

This started when you had low fluid in the reservoir…and I would guess you sucked some air into the system while braking with it that low. Running a little fluid (1 quart isn’t much when we are talking about getting air out of a master) through the system won’t get the bubble out. You would need to run lots of fluid through it. Thus why you bench bleed. FYI…Bench bleeding takes a while…it’s not 10 pumps and done. It’s more like 10 minutes worth of pumping the master, while it’s out of the car.

Good Luck

OptionXIII

Sometimes you got to admit when you've been too stubborn.

I pulled the master cylinder off and gave it a bench bleed. The Dorman kit I had bought was absolute junk and totally worthless - it had a single size of each fitting and no plugs. So I made my own setup.




Some M10x1 fittings, 3/16 brake line, and 3/16 ID clear tubing are all you need. I filled the master with fluid, then pumped until a decent amount of fluid was in the lines. Whenever air bubbles came through and couldn't make it out the end of the hose before I ran out of travel, I would slowly let out to half travel to keep the bubbles at the top of the clear tubing arc, quickly raise the ends out of fluid, let the bubbles rise to the top, then return to max travel and return the line ends back into the reservior. Repeat until no air bubbles come out.

Then I brought it back to the car as a complete assembly, removed the bleeder lines one at the time and replaced them with the vehicles lines as fast as I could. Lastly, a full bleed of the brake system, I went most of the way through a quart.

On the test drive, I still couldn't get lockup, just a little bit of barking type noise of what would be a really good threshold brake. I went ahead and bedded in the pads anyways, figuring it wouldn't hurt. That seemed to do the trick, and I can now lock up the front tires again.

I'm not certain that my brakes are back to 100% of what they were before, but I also am paying a lot more attention to the pedal. It feels softer than I remember, but at least it's no longer to the point of unsafe. I'm calling this solved.

Thanks for the help guys.

OptionXIII

Saw another post about people not liking it when OP never updated on the actual fix and I realize that really bothered me too.

I believe this was a massive contributing factor in my brake pedal feel complaints.



The rear hub was not sufficiently torqued and eventually developed play and slop. Yes, I used a torque wrench, but targeted the lower end of OEM spec for some reason. Since I had installed the Brofab MR2 conversion hubs, I reached out to Bronson to get some wisdom. He is now recommending 250 ftlbs for the rear axle torque spec and says customers are getting much longer life out of bearings as a result. The stock range is 170-230, and the theory is that new tires are grippy enough to stretch the axle and remove the preload from the bearings. The hub is then able to pivot within the bore, and eventually the continued movement leads to the galling damage you see above.

Every time I have had a hub/bearing that was failing, the first sign has been a soft brake pedal. If you have an issue with your pedal that no amount of bleeding will solve, check the hubs, and check them well. Do a torque check at least once a year if you're tracking. I had to really put a lot of strength and weight to feel the rocking, clicking of a bad bearing on this one

This also happened when an E30 front hub failed on me in shortly after install. That was due to an issue with the cheap bearing I bought, not the torque on it. The irony that I upgraded hubs to avoid problems, and ended up creating more problems than I solved is not lost on me. I think it's all sorted now though.