TalkingPie

I came across a couple of posts on Miata.net about adjusting the free play on the brake pedal. Does anyone have any experience with this?

The reason I'm thinking about messing with it is because there's just too much dead travel in my brakes. It's been this way for the three years I've had the car, despite pad, rotor, and multiple fluid changes. It's completely consistent, and the pedal becomes rock-hard about mid-stroke. I would suspect knockback, except that nothing changes when you pump the pedal while standing still. The car brakes quite well; the feel just sucks with all that travel which does nothing, especially during DD duty when you're just *****-footing the pedal. Even with the engine off and no boost vacuum, the first couple of inches of travel are like stepping in pudding.

Parking brake is properly adjusted, calipers move freely and are freshly lubed, tons of meat on the pads, fresh rotors, no obvious play in the bearings when I wiggle-jiggle the wheels, etc. Rubber lines are original, but seem to be in fine shape at 75k miles. When I bleed, it's with the car level on jackstands, old-school method of cracking the bleeder open and closed while a helper pumps the pedal, going from left rear to right front. I don't have ABS to worry about.

[NA] Changing Brake Pedal Feel - MX-5 Miata Forum

Brake Pedal Free Play - MX-5 Miata Forum

Failure

Well, sounds like you've tried everything except adjusting the pedal. I guess that's next.

matthewdesigns

iTrader: (8)

Mine were like this until I replaced the master, now they are bueno. I also did pads/rotors/fluids prior to this with no resolution of the issue.

olderguy

iTrader: (19)

Remember if you heel-toe it is going to move the brake pedal in relation to the gas pedal and may throw you off.

TalkingPie

Bumping this because I played with the free play adjustments, and seem to have positive results.

I only turned it a few turns and noticed no adverse effects. The brakes don't seem to drag, and it took up a bit of the free, empty stroke that was bothering me. Doing this doesn't change the resting height of the brake pedal; it sits on a stopper anyway. Result is a pedal that starts actually slowing the car sooner into its stroke.

The total time commitment is about 5 minutes (maybe a little more the first time because the nut can be on tight and orienting yourself can be tricky) with a flashlight and a 15 mm wrench upside down under your dash. Just undo the lock nut and give the shaft a spin by hand or with some pliers.

I may try turning it a little more to see what I can get away with before bad things happen, but I'm also still playing with the idea of doing a master and booster out of a 2002+ Sport.

chewy

I forgot that I wanted to do this; thank you for reminding me.

Also, I had a pretty severe form of this because of my bias being thrown way off.
Bias valve cured it if that helps anybody.

cyotani

iTrader: (8)

Could you simply adjust the set point of the brake switch which act as a pedal stop to remove free play. So essentially your brake pedal starts lower and takes out the free play part of the stroke.

This of course assumes it really is only free play and not a mushy pedal which will load the master cylinder with force.

TalkingPie

Turning the threaded rod does exactly what you describe, except that the pedal stays in the same position against the stopper/brake switch. You could achieve the same result by moving the stopper, except that you might reduce the available pedal stroke. Judging by how close I can get my brake to the firewall when I stand hard on it, that might not be advisable.

Chewy, are you saying you had a defective stock bias valve, or did you replace a good stock valve with an adjustable one?

I want to get an adjustable valve to get more use out of the rear brakes, but wasn't expecting that improvement to improve on the pedal feel.

matthewdesigns

iTrader: (8)

I would like to hear more about this, too. I just did a trackday, burned through my front pads and left me with a LOT of pedal travel. Rears look fine. Figured I'd need a valve to adjust bias, but if it helps with this too it'll be a bonus.

chewy

Sorry I wasn't clear. I'm browsing MT while doing P-Chem homework and everyone says that I have to re-learn how to talk afterwards.

I changed the front pads only, on the stock valve (bad idea, but desperate). This caused the pedal to be mushy and nearly dangerous. The pedal traveled way too far to engage.

I swapped it out with the adjustable one from either FM, TSE, or 949 and dialed it in. The pedal was way stiffer. I couldn't lock the brakes on the old dysfunctional one no matter how hard I hit them, and now it locks the fronts at ~80% pedal. The travel, feel, and stiffness problems went away completely.

I hope this helps

matthewdesigns

iTrader: (8)

Thanks for the followup. Changing the prop valve also necessitated a bleed I'd assume? That may have been part or all of the fix. If you didn't do a full bleed then I'll give the valve the credit.

chewy

When the Brakes were borderline scary, I bled them at least 3 times. The first time I used new and clean DOT3, the second and third times I used ATE DOT4 just to see if there was a difference.

It "helped", I didn't die or anything. But after the adjustable valve was installed the car felt like it had around twice the overall brake force.

I must have really screwed up the bias on the stock valve.

Also, just for controlled variable sake; the brakes were bled by the same partner, doing the same job with the same type of pads and rotors for each of the 4 brake bleeds.

OGRacing

iTrader: (1)

i would like to before and after track temps. normally any pressure on the rods at rest causes drag. Maybe with the booster it's not 100% true, but i would like to see rotor temps to be sure.

matthewdesigns

iTrader: (8)

Thanks a bunch, that pretty much settles it for me.

chewy

No problem.

I hope it works out for you as well as it did for me.