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Servicing a brake caliper and have found one or more pistons stuck? Compressed air is one option to free them but sometimes the pressure is not enough to free them or your compressor isn't big enough to do the trick. There is a quieter but messier option available - using a grease gun. The pressure exerted by the grease gun can be vastly more than a compressor - usually in the thousands of psi.
Keep in mind that the hydraulic force from a brake master cylinder can also be an effective way to push stuck pistons out - this isn't always an option but it's worth keeping in mind.
Unlike using compressed air the pistons are unlikely to exit the caliper violently, so this method may be more appealing for some from the outset - it is a good deal messier, though, and you will have to clean grease from the inside of the caliper once you're done.
The caliper we are using as an example today is from a Honda CX650ED motorcycle - it's a pretty common Honda caliper from the time and is a twin-piston model. The process is basically the same whether it's a single, dual or however-many piston caliper.
The basic premise is that we need to block up all of the holes leading to the space behind the pistons except for one, which we will attach a grease gun to. These particular calipers have three holes - one for the bleed screw which is blocked by said screw:
One is for the brake banjo, which we can block with a non-banjo bolt of the same thread size and pitch:
...and the third, which has a grub screw in it and isn't used otherwise:
We got lucky here and the thread for this hole is a match for the thread on our flexible grease gun extension:
If we hadn't found that we would have put a grease nipple on the bleed screw location or, failing that, found/made a threaded adapter to mate with one of the other available holes. If you have a multi-piston caliper, put a thin bar across the caliper like so:
This is done because one piston will usually come out first, and if it fully exits the caliper the grease will just flow through the hole it left and not push the remaining piston out of the caliper. We want to block it from fully exiting so that the other piston comes out as well. You'll see what we mean in the video later on. If you have a single piston caliper there's no need to do this.
Now start pumping! Video of this caliper's release process below - audio will be added down the track:
If you end up not being able to remove your bar simply unscrew one of the bolts or the grease gun to release the pressure. If the pistons are still too hard to remove once they have connected with the bar, try a thinner bar!
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