Sticking brake

Left front caliper is sticking while on a ride. Popping the bleeder releases it. Causes?

Thanks

 

#1 cause is water in the brake fluid.
#2 is not rebuilding the brake system on a 32 year old motorcycle.

#2 takes care of #1.

If #2 has been done then start changing your brake fluid every 2 years. Brake fluid absorbs atmospheric moisture (on purpose(with the exception of silicone based fluids(which are crap))). As the brakes heat up the water in the system boils, increasing pressure in the lines and causing the caliper piston to apply the brakes.

Note: #2 absoutely requires that the pad retention pins, and caliper block pins be shiny-smooth or the pads can end up not retracting, which causes the brake to drag, and eventually lock as the brake fluid expands (or on rare occasion catches fire (leading you to pour your big-gulp onto the left rear wheel of your wife's Miata, and drive very slowly on the way back home. Not that I've done that).

 

Thank you for the help. Calipers have been rebuilt (not quite a year ago), but I recently swapped handlebars, and consequently the top brake line. I'm running DOT5 which has up until now performed flawlessly. Should I drain and flush the entire brake system? It does appear some water is collecting in the top of the reservoir.

 

http://en.wikipedia.org/wiki/DOT_5

dot 5.1---ok
dot 5----no no

 

Searching the forums for flushing instructions, can anyone give me the down and dirty? When I rebuilt the brakes last year, I just replaced everything else, so I didn't need to flush. I have stainless brake lines, and have no reservations about going back to DOT3.

 

I haven't ever needed to flush DOT5, but I would probably remove the brake lines, and use isopropyl alcohol to flush out the DOT5 from everything. I'd then put the brake lines back on and flush the alcohol out with DOT3 or DOT4 before bleeding the remaining air out. Just flushing with a glycol based fluid could cause bits of solid "goo" to form if any of the DOT5 is left behind.

 

Do you flush it from the master cylinder? Just isopropyl alcohol?

 

I suppose that you can flush it just by running it through the master, but I'd be leery of damaging the piston seals since alcohol doesn't lubricate much (except my personality). Personally I would tear the system down to the main components (leaving the master and the calipers intact), and flush them separately with a spray bottle filled with 90% isopropyl, followed by shop air (or canned air if you don't have a compressor).