Rebuilding GL1200 brake calipers

most likely you have a gunk buildup in the seal grooves and will require popping out the caliper pistons and cleaning out the grooves with an angled dental pic being careful not to scrape the bore, cleaning the pistons if they are not pitted(replacement time) and putting in new seals.
And/Or the return port in the master cylinder is plugged and not allowing the pressure to release, this can be tested when the condition shows up, stop and crack open the banjo bolt at the caliper and if the fluid spits out you know it's in the return port and needs cleaning out with a wire from a wire brush or the smallest welding tip cleaning tool works for me, spray cleaning alone will not clear the clog.

These conditions happen when the brake fluid, DOT 4, is not flushed out yearly with a fresh can of DOT 4, 2 years at the most as moisture enters the brake system and causes a chemical reaction in the brake fluid to an acidic compound which eats away at the rubber seals and brake lines.

First look in your master cylinder . See if the fluid is gunky,very dark and or has a slime buildup in the bottom . It could be the master has a plugged return port . this is a very tiny hole in the casting ( think size of a single bristle of a wire brush). If your fluid looks like this , you might first take apart the master and clean it before getting into the calipers . You will have to disassemble the master to clean it .
pull the calipers off the brakets . remove the pads . If you have compressed air ,put a piece of wood in the caliper where the rotor would fit ( i think 1/2" plywood). blow some air lightly into the hose port. this should expand the pistons out . The pupose of the wood is to keep one piston from popping out and leaving one stuck in place . Once you have pistons out remove the rubber seals . Normally in the seal groove you will find a buildup of corrosion . this exerts pressure on the seal and causes the piston to not return /relax . If you clean these grooves , clean it all up with new clean fluid and reassemble .
Both of the above can sometimes be put back together with the old rubber parts . Once cleaned and all works good . I prefer to have new seals myself . So you might want to source the master cylinder rebuild kit and also caliper seals . If you order caliper seals/dust boots remember they are different size pistons on right and left side .

It has been my expierience that if u have corrosion around the seals in your caliper as newday said will cause the sticking of the pistons ... More than likely if you have it in one caliper it's throughout the entire system ... I have just finished going thru the entire system on my 83 interstate an in each master cylinder both have tiny pin holes make sure u clean those thoroughly if they get any trash in them at all you will not be able to bleed the brakes ... This can't be stressed enough as I can tell you from expierience .. I pulled mine off 3 times before I got it right ... Also if you have the brake pressure control valve you may as well pull it an clean it as well then blow all the lines out ... Hope this helps it is a very labor intensive project but a very good learning expierience ...

I had to rebuild all 3 of mine. I had the opposite problem, mine were leaking. But when I took them apart, I found a lot of gunk buildup, and some minor corrosion. After a thorough cleaning and polishing, I replaced the seals, and they worked fine. The calipers are aluminum, and if you find severe pitting on the sides of the bore, you will need to replace them, or they will leak and not hold pressure.


Since you said they were sticking, you should know that a plugged return hole in the master cylinder will also cause that, by not releasing fluid pressure on the pistons. It could be just that, and not the calipers at all. But with old brakes, I like to take everything apart and check it out, and while you have it apart, might as well rebuild it. If the engine quits, you can just pull off the road. If the brakes fail......