2YellowMazdas

Hi guys,

I'm having an issue with my brakes. After braking from highway speeds, I get a slight vibration in the car under braking and/or cruising and it feels like the brakes are on slightly when going to roll from a stop. (When the car is cold, I can have the car on a slight incline and it will roll freely when off the brake pedal. Once the car have been driven and warmed up, the car will hold itself.) When I parked the car, the LF wheel/brakes seemed very hot and had that "brake smell".

So I assumed it was a sticking caliper. But a friend said it could be the master cylinder going out, applying pressure when it shouldn't be. He also said this should affect the LF and RR (cross braking system). Next time I parked the car, the LF and RR seemed warmer than the RF and LR.

So now I'm thinking master cylinder. However, looking for a BMC for a 2004 Manual w DSC is tricky. I'd rather rebuild it if possible, but I can hardly find a fully rebuilt one, let alone a kit. It seems the part number changes halfway through 05, and I'm not sure what the difference is and if there's any cross compatibility (will an 06 Manual w/ DSC fit an 04?)

So I have two questions:

Has anyone had a BMC go bad on their 8?
Does anyone know of a good place to find a BMC?


Thanks,

Zac

BigCajun

iTrader: (1)

I use Advance Auto for most of my aftermarket stuff.
I've never heard of a bad MC giving those problems, but that doesn't mean it isn't.
Have you tried troubleshooting the problem?
Jacking the wheel up, turning the wheel while someone presses & releases the brake?
Taking the calipers off and lubing the slide pins?
Sometimes the rubber hoses can collapse, acting like a check valve.

9krpmrx8

iTrader: (46)

The MC can be found plenty of places, I replaced mine recently (Advance Auto parts), I have a 2004 6 speed manual with DSC. Yes my MC was bad and I just replaced the booster for good measure since it was 10+ years old and aesthetically unpleasing to me.

But I suggest testing the MC and booster rather than diagnosing by parts replacement.

2YellowMazdas

What I want to do this weekend is get the calipers removed, make sure the slides and shims are nice and clean/lubed, and make sure none of the caliper pistons are sticking anywhere before replacing anything. I did find a BMC at Oreilly for ~$75 so if I have to go that route, it's right around the corner.

I did another "temperature test" this morning when I pulled in to work and the LF again is scorching hot, but the RF is so cool I can rest my hand on it indefinitely. So that doesn't seem right to me. I'd think any working caliper after 20 mins of driving would get a rotor hot enough, under normal conditions, to be untouchable upon immediately parking the car.

So my thought is, since the 8s are diagonal split brake systems (LF-RR, RF-LR braking done by same chamber of BMC) if there is an air bubble in the BMC for the RF-LR channel, they may not be working right, causing the LF-RR to do all of the braking for the car. And I've got that bleeder screw on the BMC so I can bleed it right there.

But if that doesn't work and all of the calipers seem to be in good shape, I'll probably be getting myself a BMC.

Thanks guys.

200.mph

iTrader: (9)

pretty sure i have a mc at home when i stripped my wrecked 8 but it sounds more like a frozen caliper pin to me

BigCajun

iTrader: (1)

I think you can rule out the air bubble theory.
You would have a spongy pedal since air is compressable, not seizing.
Jack the car up and put it on jackstands or a lift.
Have someone sit in the car and press the pedal while you try to spin the wheels.They should stop instantly, then let up on the pedal, it should spin easily.
When you get one that's seized, loosen the brake line at the caliper and see if it releases.
If so, replace the brake line section with the rubber hose.

2YellowMazdas

Well, I solved it, guys.

Got home Friday and got the caliper off ot the bracket and stood on the brake pedal. SQUEEEEEAAALLLL as the piston pushed outward. Definitely a bad LF Caliper.

So, being cheap and time pressed, I pushed the caliper piston all the way out until it was basically held on by the seal/boot and I cleaned off a bit of surface rust with a wire brush, doused the piston in white lithium grease, pressed it back into the bore, bled the entire system (all 4 corners + MC), and for the past 3 days, it hasn't re-seized itself.

(Full Disclosure: I am a technician for a different Japanese automaker, and do not recommend this method of "repair" in any way, shape, or form.)

It will probably seize itself again at some point (probably when the winter is at its coldest, because Murphy says so) but for now, feels like a million bucks.

Thanks again guys,

Zac

BigCajun

iTrader: (1)

If you pushed the piston out that far, then pushed it back in without honing or polishing the cylinder, there's a good chance you may have damaged the seal.
You should replace/rebuild it ASAP before it starts leaking.
I would.