vanbiker
CB750 Member
It's frozen, I need to free it up so I can rebuild if possible. It's been soaking in vinegar, no luck so far. Suggestions?
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amc49
CB750 Guru
Compressed air and a shop towel over piston, it can kill you if it pops loose all at once. Past that not working make up a grease fitting by tapping a 1/4"-28 hole in a spare banjo bolt and then use a grease gun and a grease zerk screwed into your new hole, sealed correctly the 2000-2500 psi will push piston right out.
If by chance a double piston caliper you have to limit one piston coming out the easiest to make it push the other out, swapping from piston to piston to work them both out.
If by chance a double piston caliper you have to limit one piston coming out the easiest to make it push the other out, swapping from piston to piston to work them both out.
Dave
CB750 Junkie
or ... Hook it back up to the master cylinder and pressure bleed all the air out then use the brake lever to push it out.
Let it hang in some kind of container to contain all the brake fluid that will pour, or possibly spray, out if it works.
If it's stuck mid stroke, you can clamp it in a vice or c-clamp (using the proper jaws or pieces of wood on either side to prevent damage to the surfaces) and close it a little bit to at least unfreeze it. Close it as little as possible as the farther it's sticking out, the more pressure you can build behind it to push it out.
Let it hang in some kind of container to contain all the brake fluid that will pour, or possibly spray, out if it works.
If it's stuck mid stroke, you can clamp it in a vice or c-clamp (using the proper jaws or pieces of wood on either side to prevent damage to the surfaces) and close it a little bit to at least unfreeze it. Close it as little as possible as the farther it's sticking out, the more pressure you can build behind it to push it out.
amc49
CB750 Guru
Actually, the pressure is the same with piston almost out or all the way in, pressure figures off the OD size of the piston itself, VOLUME is what changes, pressure stays the same. Still, a good idea, once broken loose EITHER way they always get easier.
Instead of ruining what may be a rare hard-to-find banjo bolt you can match the bolt to a common metric bolt and drill that to make a custom grease fitting too, what I did last time. Make it a lower property class number (8.8 vs. 10.9), it makes it easier to drill.
Instead of ruining what may be a rare hard-to-find banjo bolt you can match the bolt to a common metric bolt and drill that to make a custom grease fitting too, what I did last time. Make it a lower property class number (8.8 vs. 10.9), it makes it easier to drill.
If this is for a sohc bike I would just look for a good used one that isn't stuck. They are easy to come by. Just put a seal kit in the different caliper and get to riding.
amc49
CB750 Guru
Are the SOHC ones the same design as the 500/550? In that they have no dust seal to stop that and water gets right into them. The only corrosion prevention being that you coat the piston outboard of the actual pressure seal with grease that eventually washes out. A real stone age situation there.
yes the 500 caliper is similar, looks identical it is just a bit smaller then the 750 caliper
CivilPenny
CB750 Member
I have the same problem with my piston. The son of a bitch is rusted in/seized. I've tried grease even and it won't come out. The grease just came out right around the zerk. I may try using a propane torch and putting some heat into the sucker for hopeful expansion. I'll let you know how it goes
amc49
CB750 Guru
If the grease came out easy enough then the zerk is not sealed well enough. I use 1/4"-28 as the tap there but the hole needs to drill tight (small ID that matches thread inner diameter tight), if you take too much wall out the grease leaks around the threads because they do not fill the airspace well enough. Done correct the part should crack or break with little leaking.
Put heat into the outside aluminum casting and an ice cube inside the piston.
Put heat into the outside aluminum casting and an ice cube inside the piston.
AlabamaCB750
Competent CB750 owner
I use the grease gun method using a bleeder bolt from the caliper. Most grease guns will fit it well. Put the caliper inside a couple of old socks so when it does pop out, the piston and grease won't fly everywhere. Make sure and thoroughly clean out the caliper afterwards. A Dremel tool with a wire wheel works well to smooth out the piston hole and remove corrosion. Use a new piston when you rebuild it, they are cheap. Rebuild kits are cheap and easy to find too.
AlabamaCB750
Competent CB750 owner
By the way, screw the bleeder bolt into the hole where your brake line normally attaches, and seal the hole where the bleeder bolt normally is with an appropriate bolt. Or, another bleeder bolt will work if you have one, just be sure to close it before you apply pressure.
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