There are two reasons i know of . first the oring in the piston is either cut or twisted. The other is that the o ring on the piston is to small. The only cure is to replace it either way.
Seal in brake caliper is leaking and needs replacement Flexible brake line to the caliper is leaking and needs replacement
just replace the whole caliper.
Brass washers replaced when caliper replaced?Fitting tightened completely?
Rebuild the cylinders on that caliper.
If the caliper is sticking or leaking it is possible to buy a rebuild kit and replace all parts that are worn inside. If the caliper has a cracked or broken housing then the unit can not be repaired and must be replaced.
If it is leaking, cracked, or the piston is seized it needs replaced.
That depends on where it's leaking out; if it's at the master cylinder, you have a bad master cylinder, if it's at the wheel cylinder you have a bad wheel cylinder if it's at a caliper... well, you get the idea. If you know how to repair a master cylinder, wheel cylinder or caliper, you can often get by with a little savings but it's usually best to just trot down to your local auto parts retailer and get a new or rebuilt one.
NO, and in fact what you are proposing is extremely dangerous !The only fix for this is a new part fitted by a competent professional.
how do you get the brake caliper push in to put new brake on
Probably brake fluid, your caliper seal may have gone bad
what is the best way to bleed brakes when installing new caliper on front left wheel
Failed caliper, wheel cylinder, hose, line, Vehicle is unsafe to drive.