Brake problem? - PeachParts Mercedes-Benz Forum

Wasuchi · #1 01-31-2003, 05:34 PM

I'm sure this is a problem but what do I need to do? I'm putting in new pads/rotors on my fronts. Everything worked fine until I came to the inside right (front). The piston wouldn't push in, I figured well I'm pulling the whole wheel off so I have to take the caliper off anyway..... I finally managed to get the pad out once I took the retaining bolts out. That pad was hardly worn at all. The left pads seemed to drag some so I'm assuming that those two pistons need a rebuild. Is the right one going to be rebuildable or am I most likely going to need a new one?

Are the rebuild kits on Fastlane one per side or do I need two per side? I figure I'll do new hoses too as long as I have to order stuff.

The Warden · #2 01-31-2003, 06:02 PM

The rebuild kit I just got from Fastlane was enough to do one caliper. If you want to do the left and right units, you'll need to get two...but the one kit will do the whole caliper, not just half of it.

You might want to get the pistons out before getting the rebuild kit, though; if the pistons or bores are too rusty it would be better to get another caliper. In fact, I should have replaced the caliper, and I probably will do both next time I do the brakes...

You say the pad was hardly worn at all? If that's the case, it would make sense that you couldn't really push the piston in; there is only a tiny bit of travel left when the pad's almost full...

Just some thoughts...good luck!

Wasuchi · #3 01-31-2003, 06:49 PM

Well, after looking at my new pads the one pad was pretty worn but it still had almost twice as much as the others. The old ones only have about as much material left as the steel backing is thick, so they are pretty worn. I guess my next step is to get the pistons out. Thanks Warden.

psfred · #4 01-31-2003, 08:25 PM

The piston is definitely stuck if you didn't get pad wear -- usually they stick out rather than in, eating the pad faster.

You can remove the piston by using a C-clamp to hold the other one in (other caliper installed with pads in place!) by pumping the brake pedal until it pops out. It must be attached to the brake line, obviously.

This is safer than compressed air although it is messy, and allows you to use LOTS of pressure to drive it out without shooting the piston around the garage when it finally lets go.

If the chrome plating on the piston is scratched or pitted, or the bore isn't clean or is rusty below the seal, get a new caliper. You can clean the bore above the seal with fine sandpaper or Scotchbite, but not the piston or the bore below the seal -- they will either leak or seize if you do. If at all in doubt, replace the caliper!

Peter

Wasuchi · #5 02-01-2003, 09:40 PM

Anybody have any ideas how to get a stuck piston out? I got all except my stuck one out with air... I hate to put everything back together on both calipers to hook back up to the line so I can use the brake pedal to try to force it out..... at this point I'm not sure it will come out anyway. I did manage to force it in with a clamp in hopes of breaking it loose..... soaked it in WD-40 first but it won't budge back out now. I don't have much hopes of it being good but I would like to look at it at least. There was plenty of crud behind the other three pistons but they are nice and shiny once you wipe them off. I'm tempted to use one of the old brake hoses to rig up to my air compressor so I can put 125psi behind it, about the most I can hold on there is 50 which got the others out but then I loose my other option of hooking it back to the car. Maybe I can cap the other side (brake line) so I don't have to put it together? As you can probably tell I'm running a bunch of ideas through my head before trying them.

rwthomas1 · #6 02-02-2003, 02:25 AM

Try this,
Reassemble the good side with the reworked caliper and new pads. C-clamp moving piston opposite stuck piston, bleed system and use pedal pressure to push out stuck piston. Not really much work as you have to assemble the good one anyway. RT

psfred · #7 02-02-2003, 04:59 PM

PLEASE DON'T USE 125 PSI ON THAT CALIPER!!!!!!!!!

The piston, if it does come out, will bounce off the other side and come flying out of the caliper, likely killing you if it hits you. In effect, a small canon!

re-assemble (you can use the old seal for the other piston), and attempt to force it out with hydraulic pressure. Air simply isn't safe here, it's going to take quite a bit of force to move it. You can also generate several hundred psi safely, as the hydraulic fluid won't explosively expand if the piston pops out.

An even better option is to replace the caliper. Very likely the piston or bore is corroded if it is that stuck, anyway.

You may also have some luck using a pair of small prybars -- the green handled ones at AutoZone about the size of a large screwdriver. Hook them under the dust boot lip and pry agains the housing on all sides, alternating directions and positions. This may force the piston back into alignment with the bore so you can get it out. At least it won't blast out of the caliper this way!

Peter

Wasuchi · #8 02-02-2003, 09:31 PM

Don't worry, the clamp holding the other side won't let it come all the way out (only about half way actually)..... I already made sure of that. (It wouldn't come out anyway at that pressure.) I would reassemble and use the brake system but I don't have the rebuild kits yet (this wasn't a planned part of the project until the frozen piston was discovered). I think that you're right, it's very doubtful that the frozen piston is any good anyway, it's just a want to see project now. Looking at the gunk/crud in the front calipers that makes me want to do the back ones too. Thanks for the safety advice...... I'm pretty aware of dangerous procedures having grown up and living on a farm all my life (lots of good ways to kill yourself if you aren't careful). I had a piece of old tire tube in between the others when I popped them out (low psi) to slow them down, worked well. Once again thanks, many people wouldn't even think of the cannon issue.

#9 02-03-2003, 12:20 PM

back in the middle '60s, when ATe disk brakes first began appearing on Porsches, we discovered a pressureless technique for stuck pistons, provided you have good hand/eye coordination: hold the caliper horizontally in front of you at eye level with the stuck piston on top, facing downwards, using something on the opposite wall as a guideline to get it very close to level; then let it drop on the concrete, being careful to release both hands at once; it's surprising how often inertia will cause the stuck piston to come free and move outward when reasonable amounts of air pressure had failed. Often I have been reluctant to use the brake line pressure method because I was helping a friend with a spotless garage and lots of nicely painted items standing about, and not enough rags to absorb all the possible leakage.

(Bear in mind that this comes from the same folks whose shop manuals told us to remove the 356 series transaxles by going underneath on a mechanic's creeper, undoing all the necessary bolts, wires, etc., and then lowering the transaxle unto one's belly, after which the two assistants are supposed to each grab an axle and an ankle and roll the whole mess out towards the rear of the car.)