By Ab Tiedemann, Geoff Fleming, Ken Daugherty and Jim Breazeale
Ab Tiedemann: In my opinion and it is shared by many, grease would be the preferred medium. 10,000 psi can be safely generated with a grease gun on the bench. Shop air at 85 psi is at the other endpoint of the safety scale and should not be recommended even though the manual suggests the technique. The pedal cluster that was suggested with brake fluid has a mechanical ratio of about 5 and is capable of generating on the order of 1000 lbs force [with the foot] and considerably less with the hand if such a set up is used on the bench with the cluster restrained [method free—like in racing]. With a 48mm caliper size about 6400 lbs force would be available for a stuck piston. With the grease gun, 28,000 lbs force. More than 4 times the force and much, much safer.
I think that AL Zim sells a kit with fittings just for this purpose.
I would rather clean up some grease than risk getting brake fluid everywhere.
I once removed a stuck piston in a brake cylinder. I used the hydraulic facility at the place I formerly worked. It required 2500 psi to break it free and it was done immersed in a fluid tank dedicated for the purpose. It was quite a sound when it broke free.
Certainly, air is to be avoided for oldies and newbies as well. It is far too dangerous. Grease is the safest although it does have to be cleaned off the surfaces well. A foot pedal cluster with mounted reservoir and lines appropriately block works as well if the pistons are not that "stuck". However, you cannot just run down to your local FLAPS and pick-up a 356 pedal cluster and these are not likely to be lying around in most folk's workplace. If they are really stuck, considerable hand/foot pressure will be required and they will "blow" just like if air had been used as. Grease will not. Another benefit of using the grease, the grease gun and the kit that Al offers is that your hands [both on the grease gun] are kept away from the work and injury to fingers is avoided. [Two button lockout analogy often seen on manually operated punch press]
Geoff Fleming: A grease gun can be used to push out a stuck piston...but then you have a caliper full of grease! The best method is to use an old master cylinder with two of the three outlets plugged. Ther remaining outlet is used with a short length of brake line. Now you simply rig a reservoir to hold fluid and after attaching the caliper , simply activate the master, either by hand or even by foot power, ( if you attach it to an old pedal cluster). The caliper will submit to this pressure. It will be necessary to secure one piston with a C clamp or strapping, in order to prevent both from trying to pop out simultaneously. If the caliper is still on the car, you can remove the two large bolts that hold it to the axle, allowing the caliper to hang downward. After securing one piston with a clamp, simply activate the brake pedal...this will pop one piston. The other is removed the same way, but the first is reinserted and seated, clamped etc. It pays to have sufficient towelling and perhaps a large pan in order to catch the brake fluid that will be used.
Using the master cylinder set up to free a stuck piston is easy, sure and mess-free. The pressure is easily controlled, unlike using compressed air. I have an old pedal cluster, attached to a wood base and the master bolted to the cluster ( I eliminated all but the brake pedal). With this outfit, I can push the pedal with the heel of my hand quite easily and even the most stubborn pistons free up...and without drama. The brake fluid runs off into a large baker's tin that I keep the caliper in. When the piston comes free, the fluid simply finds its way out. Having done dozens like this, I stand by it without reservation. My rational is that, although compressed air and grease guns can do the job, why not use what was originally designed to move the pistons? Certainly not grease, nor even compresed air. Having said that, I realize that everyone has his/her preferences, so, whatever works.
Ken Daugherty: Remove the fitting and install a common grease fitting and pump away. A (hand pump) grease gun will exert even — and controllable — pressure. One cylinder may come out ahead of the other. If one starts moving put a clamp on it until the second piston moves. This will work on calipers and on wheel cylinders.
Jim Breazeale: I use compressed air (yes I still have all my fingers) and remove the piston on the outside caliper half (the one with only 2 inlets, one for the crossover tube and the other for the bleeder screw). After the piston is removed on that side, I remove the bleeder screw and put it in the threaded hole on the other side of the caliper vacated by the crossover tube. That leaves just one opening for my compressed air nozzle. I then blow out the other piston, remove both bleeder screws from the caliper and then commence the rebuilding process.