82seca 550 soft brakes

I have a seca 550 thAt I have rebuilt the master cylinder on and put new break fluid it and bled the brake line. If I tie the front brake level close to the throttle and keep front brake on over night I have a firm front brake the next day but if at any time I don't tie up the brake level over night. nmeaning I leave the front brake in the resting position overnight I get a soft break in the morning. Does anyone know why this is and how to fix it. Thanks for your help

 

Either you do not have a fully-sealed system, or you didn't get all of the air out and are slowly bleeding what's left.

Did you use new crush washers, or at least anneal the old ones?

Did you rebuild the caliper?

Are you using the 30+ year-old brake lines? (don't do that)

 

I think my problem is my old break lines but I took it by a bike shop that specializes in 70 and 80 bikes and they did not think it was my break lines they thought I still had air in the lines. But whe I bleed them there are no bubbles so I though the air was out of the line. I looked at the caliber and the piston moves freely in the bore so I think my caliber is okay. Thanks for your advice

 

Try k-moe's suggestion to use a hand vibrator or even orbital sander (no sand paper) and work your way up from the calipers and brake lines. Be sure to tie the brake handle. After a compete rebuild of calipers, m/c and new brake lines I reverse bled my brakes using a syringe and tubing and pumping fluid through the bleed valve and up the lines into the m/c. After doing that I lightly tapped the brake handle a few times and got a few bubbles. Once the bubbles stopped I had firm brakes. You might have some air still trapped in the m/c, so remove the m/c cover and lightly tap the brake handle a few times.