CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Master Cylinder Shot?

I’m changing the pads on an 07 Mazda 3 and can’t seem to get the piston pushed back in even with the help of a piston compression tool. I’ve removed the (rear passenger) caliper from the car and the bleeder so resistance from the fluid negligible. The piston isn’t frozen as I’ve spun it all the way out and removed it from the caliper. Spun it back in to the point where piston and shaft are turning. Any suggestions?

Is it a spin type piston?

It is. The threads I spun the piston back onto were quite large and led me to expect spinning the unit back into the caliper wouldn’t take long. But now after continuous turning it seems to be retracting back into caliper…

It’s very fine thread…so it’ll take a while.

you might need something like this:
https://www.amazon.com/gp/product/B0122Q83T4/ref=ox_sc_saved_title_6?smid=ARHTXJNP710LD&psc=1

your local AutoZone might have it in rental tools

I did get ahold of the piston tool, and got the piston turned all the way in. I put the pads into the bracket and the caliper back on, brake line and e-brake reattached and started bleeding the brakes.
I have nothing but what seems to be endless amounts of air bubbling out of my bleeder hose, pedal goes straight to the floor; no brakes, front, back or e-brake. Any suggestions / ideas would be much, appreciated.

You didn’t mention putting a nurse bottle on the master cylinder, so unless you kept an eye on the level while you were working on the brake with the bleeder nipple open, I’m gonna guess you dropped it below the line and sucked in a big gulp of air.

The other bad news is that if your pedal went to the floor it’s very possible your master cylinder’s gonna die soon, because the piston would have traveled beyond its normal reach and picked up all the gunk that’s been collecting on the cylinder wall for the last 12 years.

No nurse bottle on the master, not sure exactly how that works, but next time I’ll put a hose clamp on the brake line before I take it off. The reservoir didn’t run completely empty as I was topping it up as I was trying to bleed the line into a half fluid-filled bottle.
The pedal only started hitting the floor after I took the caliper off, worked fine before that; as my wife is sure to soon remind me.

Yeah, if I’m reading what you said right, when you took the caliper off you detached it from the brake line. That’s where at least some of the air came from.

Nurse bottles aren’t necessary as long as you make sure to keep the MC topped off while you’re bleeding - they just make one-man bleeding sessions easier because you don’t have to keep going back to the master cylinder to check it. It’s basically an upside down bottle full of brake fluid. As the level drops and exposes the bottom of the bottle, brake fluid flows from the bottle into the cylinder.

it’s the one on the left - you’d flip it upside down from the picture and stick that brass end into the reservoir.

Did you remove the lid/cover from the master cylinder? That’s all I usually do to relieve the pressure so I can compress the caliper.

Ahh thanks shadowfax, good to know. Ill have to pick up a nurse bottle somewhere. So you’re suggesting the lines are just filled with a ton of air and to just keep bleeding them out while a buddy keeps the reservoir filled even if it takes 10-15 minutes.
I did pull off that cap, thanks whitey.

Yeah, if you’re absolutely sure the MC fluid never dropped below a line, then that’s what I’d do. Or if you have an air compressor, Harbor Freight sells a vacuum bleeder kit (which comes with that nurse bottle) that makes it very easy and fast to do it yourself without all the pedal pumping.