RX-8 Spongy Brake Help

Since last season the brakes on the RX-8 have gotten pretty spongy and I'm trying to figure the best way to remedy this. The brakes used to be great. When you applied pressure to the pedal there was very very little motion, you just applied more pressure and there was more braking, but not a lot of pedal travel, which made it very easy to modulate the braking input. Now, when I apply the brakes hard, there's an inch or two of travel as I apply more and more pressure to the pedal. It doesn't go to the floor and I can still lock up the brakes just fine, but it's crappy for racing, especially when I'm using left-foot braking.

I bled all four corners earlier today and it didn't make any difference so I'm thinking about replacing the master cylinder. I had to replace the clutch master a few thousand miles ago, so I wouldn't be surprised if the brake MC was on its way out. The brake system isn't loosing any fluid as far as I can tell and other than this extra travel in the pedal which has developed, I think everything is working fine.

Does replacing the MC sound like the right place to start? Should I try bleeding the brakes a bunch more first?

Also, I've heard it's really hard to get all the air out of the ABS modules so are there any precautions I should take to avoid getting air in there when I change the MC? (Out of the dozens of cars I've owned I haven't bled brakes on one with ABS)

Any thoughts on this? I'd like to get some feedback before ordering the MC, looks like it'll be about $130 from RockAuto...

Thanks!

Maybe the rubber lines are getting a bit soft? That's the only other thing i could think of, I'm definitely no expert on brakes.

I don't know what style of caliper the RX8 uses, but if it's a floating style make sure your sliders are not seized.

are the calipers sliding freely? they can use up pedal travel if they are sticking

the basic check for MC is to apply the brakes ... hold the pedal firm... sorta relax your leg muscles and then push on the pedal again ... if it goes a bit farther then repeat ... if each time it seems to go a bit farther then the MC is leaking around the "o" rings and is time for rebuild / replacement

That's interesting about the sliders - I'll check that and see if it might be a problem.

As far as feeling the leakage in the MC, it's definitely not bad enough that I can keep pushing the pedal further and further as the fluid leaks past the seals. I had that issue in my Datsun a year or two ago and remember what that felt like.

I was inspecting around the MC a little more and did find a little trail of etched paint leading down from the MC-booster interface, so I think it might have a very slow leak.

I'll check out the calipers and if they look good (which I expect they will), then I'll order the new MC.

Thanks for the help!

The correct answer is to get a compression test...lol

Actually, did that today too. Came back with great numbers 110 psi on all 6 rotor faces. Factory spec is 120-90 psi if I remember correctly. The engine has ~68k on it, and it's been great so far. Does need a new set of plugs though, and man are they pricey...

Wanted to give an update on this since I see other people with chronic brake trouble similar to this.

Yesterday I hooked up the master cylinder with a bench bleeding kit (looping the lines from the MC back up to the reservoir). Since this is a '06 RX-8 it doesn't have the bleeder on the MC like the earlier cars. I left the MC in the car, bolted to the firewall, and spent about 20 minutes pumping the pedal in various fashions. After that I hooked the brake lines back up the MC and bled all 4 calipers again. I only noticed bubbles in the passenger rear (1st one I bled).

Result? The brakes are definitely much better. There's less travel in the pedal and it's more solid. However, it's still not as good as when we got the car. So, I'm going to replace the booster as well, since it might have been messed up by having brake fluid in it for a while.

I'll update again after I replace the booster.

In reply to JohnyHachi6:

Check your pad thickness too. Not sure about the RX-8 since I'm new to it, but in my experience with Miata's, pedal travel would increase with pad wear. I could only run pads to 1/2 thickness before having a hard time heel-toe-ing. Hopefully you can find a booster faster than the dealer. They told me 3-4 weeks, but are trying to expedite it. Hopefully it's a different booster than mine.