bumps on rear pads?

[markmakeitso]

Good day eh.
I was throwing new rotors and pads on the rear of my 91 AWD the other day, and I couldn't swing the caliper back over the new pads, turns out the piston was hitting a little bump on the back of the pad backing. If I turned the piston out a bit the bump would fit into the slots on the piston face, but then it seemed like the piston couldn't turn any more once the pads started to wear. Finally I ended up pounding the bumps flush with the shim so they could close right.

I double checked the parts number at Napa and asked them about it, but they had no idea what I was talking about. Anybody else had this problem before?

 

[Defiant]

Put the pads in the caliper, then slide the assembly over the disk. The rear pistons retract by being screwed into the caliper, those "bumps" keep them from retracting (or, extending) during use. Throw away those nice new ones you've just ruined, buy another set, and get a manual that'll tell you all this for $15.

 

[markmakeitso]

Allright, what parts do you think I've ruined? How does a litle bump keep the piston from retracting? I figured it was to keep the shims on, since the shims have a hole in them that fits over the bump.

I did say "If I turned the piston out a bit" , which should indicate I turned the pistons.

 

[Defiant]

Okay, I'll play. The rear pistons retract by being screwed in. The bumps index in the slots on the piston faces and keep them from turning. The parts you ruined were your new pads. How is this not translating for you?

 

[markmakeitso]

Okay, I reread my last post and it sounds kinda confrontational. I'm not saying I think you're wrong, just wondering why you would want to keep the pistons from turning if that's how they will keep the pads by the rotor as they both wear. It seems the bump would keep the piston from retracting and extending, when it should only keep it from retracting.

 

[Defiant]

Apologies for my attitude as well, but for me it's congenital (I'm a ****).
The rear pistons don't rotate as the pads wear, they just slide straight out. They have to have an anti-retract function, because they're also your parking brake. So to retract them when it's service time, the gizmo is set up to retract them by screwing. With normal driving (let alone what DSMs go through), they could get screwed back in by varying pressures and you would wind up with enough pad-to-rotor clearance that you'd not have a parking brake (it doens't take much).
You're not supposed to assemble brakes by putting the pads on the rotor and sliding the caliper over them, you're supposed to put the pads in the calipers and slide the assembly over the disk.
Again, I regret my former tone, it's my fault for logging in while sober.

 

[markmakeitso]

Okay, i guess that clears up my confusion. My Haynes manual just says to screw them in, so I assumed they screwed out too. You know what they say about assume though.
Anyway, I might just try to flip the pads around on each rotor. I only have about 40 easy miles on these pads, so I hope it'll work out okay. Thanks for the help.

 

[Defiant]

If you left any of the bumps on there, it should be enough. Or, find a pal who can braze a couple of lumps on there, they only need to be about 1/16" high, about 3mm. Or give them a quick zap with an arc welder, just make sure the bumps are trimmed down enough that the cylinder face will fit flat on the pad backing.

 

[AWDPetmitsu]

Defiant,

What tool do you use to screw the rear caliper piston back in?

 

[Defiant]

As best I remember, I just use the points on some needlenosed pliers. They make (probably Lisle, figure $15- if you have an Autozone near you, they'll probably have a loaner for free) a tool to do it with, but it shouldn't be necessary. The pistons are plenty tough, and even if you nick them a little, it won't matter. Don't tear the dust seal around the edge, though. And don't gouge the crap outta your knuckles, nor pinch a finger in the pliers.

 

[AWDPetmitsu]

Thanks for the reply, Defiant. Another question. How easy or hard is it to screw the piston back in? I think my piston is seized because it won't budge one bit when I screw it in. I used a special tool and tried turning it both clockwise and counterclockwise, just to be sure, but it won't move at all. I sanded down the pads to prevent them from rubbing and heating up the rotor for now, till I get a new caliper, but when I applied the brakes a few times, it seems the piston applies because the wheel gets tight to turn again. So I'm wondering is it possible for the piston to be seized going back in only??? (compressing with tool), yet it can still move out with brake application? And to compress the piston, do you turn clockwise or counterclockwise? Thanks.

 

[Defiant]

The turn clockwise to retract, and yes, yours sound seized. Do they use chemicals on the roads for snow there? Could be the problem, corrosion. Those calipers have been around for a long time. Not hideously expensive to replace, and not really a tough job if you have someone to help you bleed. If not, get some speedbleeders when you get the calipers. Replace them both, regardless.