1997 Mazda MX-5 Miata: Chasing The Squish
November 29, 2012
An upcoming (likely floody) track weekend lit a fire under my backside to re-tackle Project Miata's brake pedal feel.
As mentioned many times previously, its pedal is rather soft. Always has been. Moreso than in any of the countless Miatas I've driven over the years. It's not that scary squishy pedal you get when there's air in the lines, it just has a very gentle ramp into actual braking force. And that's despite numerous brake bleeds (including cycling the ABS unit) and several pad changes. The calipers are in good shape and function normally, there are no leaks, pedal free play is about one millimeter, and it has stainless braided lines. Full ABS braking capability is still available when called upon.
That leaves the master cylinder.
Now, normally when a master goes bad, the pedal gradually sinks as you apply a fixed force (leakage past the o-rings), and Project Miata isn't doing that. But with 140k miles on the original master and an existence predicated on spirited driving, replacing the master just on general principle isn't a bad idea. It's preventive maintenance on a piece of safety equipment,. So that's what we did.
Masters are easy to swap and inexpensive, anyway -- our Centric one cost like seventy bucks.
You have to bench-bleed a master cylinder prior to installing in order to purge air trapped inside it. This process makes a mess no matter what. Clamp the new master in a vise, fill the reservoir, have an assistant push the cylinder inwards, piss fluid everywhere. Once fully depressed, hold your fingers over the ports while the assistant lets the cylinder return to its original position. Repeat until no air comes out and you've got an inch of fluid on everything in the tri-state area.
Once completed with the installation and you've again bled the brakes, you'll hopefully find that it made a difference in the brake pedal feel. In our case, it didn't. The pedal feels the same as it ever did. I'm out of ideas on this one.
Jason Kavanagh, Engineering Editor