I've got a 1980 Sportster I picked up about a month ago. Finally got it runnin' but the guy who had it last mixed DOT 3 and DOT 5 brake fluid in the front brake and it was all gummed up to hell. I replaced the master cylinder, and brake hoses, and cleaned the heck out of the calipers- We bled the brakes and everything seemed to be fine but upon trying to ride it for the first time the front brake is totally seized- Bike won't go forward at all- the front brake is always engaged. Any idea why this would happen? The calipers appear to be free from the gunk that was in there when I got it. Any insight would be highly appreciated

 

Does opening the bleeder release the brake? Could be a warped rotor being cocked in the caliper. Had that happen on the rear of my shovel.

Another suggestion, did you pull out the pistons out of the calipers? Could be gummed up in the bore or a bad seal.

Maybe the master cylinder piston not returning..

An internal check in the brake line. Old rubber can split inside and make a flap that is effectively a one way valve.


That's me thinking outloud, keep me posted on what you find.

 

As F&G says it could be either the master cylinder or the caliper that is stuck or seized.

Is the master cylinder new or a used part? You can get a good rebuild kit for it. You will need a good snap-ring pliers.

I use compressed air to pop the caliper piston out of the bore, lube the cleaned parts thoroughly with brake fluid, and carefully hand press the piston back in straight.