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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@paulster2 has very salient points
I believe through experience there is a very high likelihood that the piston is cocked in the bore of the caliper.
My suggestion is that you pop it out of the caliper and inspect the seal and the bore. Many times we do more damage than we undo. A cocked piston is not worth uncocking. Push on your brakes with a rag or some wood in the caliper to prevent it from shooting across the room. I've done that in the process of learning the hard way
Once it's out, do the following.
** Of note, getting the piston over the the inner seal of the caliper is the crux.
*** do not use grease or petroleum jelly to reduce the friction of insertion of the piston into the caliper, brake fluid is not petroleum product. They will not mix. Use brake fluid as a friction reduction medium to get the piston back in the bore and over the seal. Wash with water, brake fluid is miscible with water so water will get the brake fluid; off the caliper, off your paint, off your hands, off your shirt.
Once you get it over the seal in the caliper you are pretty much home free.
Bleed your brakes and use a handheld vacuum pump if your need.
Good luck, you'll be able to fix this.
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.
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.
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.
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.