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I was changing brake pads on a Nissan Almera 98 and I accidentally pumped the brake piston a bit out. I tried putting it back with a C clamp and the old pad but it won't budge. It's completely stuck! What should I do?

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Have you tried pushing it back in with the bleeder screw open? –  mikes Mar 3 at 23:27
    
no, do i need something to bleed into, like a bucket or? –  JohnD Mar 3 at 23:50
    
Open the bleeder screw and attach a small length of hose to the bleeder so that the fluid can be collected and disposed of properly. –  mikes Mar 4 at 11:45
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4 Answers 4

I do not believe opening the bleeder will help in your endeavor. The problem is not that there is overpressure keeping the piston out, it's more than likely when your piston came out as far as it has and you applied the c-clamp, it is now cocked inside the bore and will not re-collapse or the piston seal has come out of the bore and is not allowing the piston to collapse. You need to discover what the actual problem is (I'd lean towards the "cocked inside the bore" premise). Continuing to apply the c-clamp onto the piston without getting it straightened out might cause damage to the bore, piston, or seal, which will incur you purchasing a new caliper.

If the piston is cocked, you need to get it straightened out prior to trying to push it back in. The easiest way to do this is to take it off the car and put it into a bench vise. If you do this, ensure you have pieces of wood or something on either side to protect your caliper from damage. Gently screw the vise in to apply pressure to the piston to get it to straighten, release, then collapse.

If the piston seals have come out of the bore, it's a lot more problematic - one which I really don't have an answer for you. You should be able to tell if this is the problem, though, because you'll probably see brake fluid leakage on the piston side of the caliper.

One last thing, under normal circumstances, applying pressure to the piston will not cause an over pressure within the master cylinder or reservoir. They are designed to work this way (reservoir holds fluid until it's needed, then re-accepts it upon collapse of the calipers/wheel cylinders). Also, unless you have added fluid to a low reservoir, when you collapse the pistons, it will not overflow. Fluid out/fluid in.

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Thank you for your answer. I took the rubber cap off the bleed valve and I unscrewed the reservoir...I tried jerking it around a bit and then applied the C-clamp again and it did the trick. I changed both brake pads and the car is braking fine and everything seems to work. However, I would like to know if there could be something wrong even though it seems fine, judging from my circumstances and my past trouble with the brake pads? I don't want to endanger myself or anyone else with my vehicle. The car was still topped up on brake fluid the morning after so it´s not leaking. –  Bjarni Jóhannsson Mar 4 at 15:47
    
You say that if the caliper was leaking out of the sides of the brake piston, then the piston seals have come out of the bore...? It DID leak out of the sides of the piston seal when it extended almost all the way out (was sort of loose) and again when I successfully clamped it back in. Is that normal or did it come off? If it came off, would it still brake normally? –  Bjarni Jóhannsson Mar 4 at 15:50
    
If it leaked when it came apart, you popped the piston out of the bore. I would suggest if it's working without leaking, you've got it back together. Since you're stopping properly and not leaking at the piston, you're probably good to go. It should be safe to use as long as it isn't leaking. I think you actually dodged a bullet here. I personally don't know how your specific caliper is put together, but sounds like it finally went back together correctly. I'd keep an eye on it (look for puddles under your car at that wheel point/etc.) for the next while to ensure you aren't having any issues. –  Paulster2 Mar 4 at 17:57
    
Great. Guess I'm lucky, huh! Thanks a lot for your help. I will keep checking for leaks. –  JohnD Mar 4 at 20:16
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Take the screwcap off your brake master cylinder (where the brake fluid goes), and put a rag around the fluid reservoir. This will stop you breaking the reservoir from overpressure.

Using the C-clamp, very slowly wind the piston back. When I say slowly, I do mean slowly.

Try not to get any brake fluid on you or the paintwork - it will eat through everything, including skin, if given enough time.

As Mikes said, you can use the bleeder valve, collect this fluid in a container of some sort and dispose of it properly... NOT down the kitchen sink! I'd give the winding back another shot as brake fluid is a pain in the ass to deal with.

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The reason for opening the bleeder is two fold. If the brake hose has failed internally it can act as a check valve preventing the piston from retracting into the caliper. The second reason is that you have pushed the piston passed the area it has been operating in. There is a good chance that rust or other contaminates have been disturbed. Pushing the brake fluid back into the master cylinder will introduce these contaminates into the Anti Lock Brake Module. The ABS module contains very small ports that can become clogged by these contaminates. By opening the bleeder you are flushing the caliper. In your case I would raise the wheels off the ground, firmly apply the brakes several times and see if the wheels spin freely without the brakes applied. Any excessive drag would indicate a sticking caliper or a defective brake hose.

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Front or Rear caliper? With the rears, the pistons need to be wound in, not pressed.

If that is the case, you'll find either two or four semicircular indents at the edges of the face of the piston. With a pair of needle-nose pliers, use these to rotate the piston clockwise while applying a gentle pressure inwards. You'll also need to make sure the handbrake is off. Once wound in, check whether there is a pin on the back of the pad that engages with these indents, and if so, make sure they are aligned by continuing to rotate the piston.

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