Front disc brake adjustment

[billdn]

Where the old girl had been standing over winter, the disc brake had seized on. So drained fluid,  stripped,   cleaned calipers, fitted new pads c/w new washer behind floating pad, new fluid and bled satisfactorily.  However I can't adjust the gap between fixed pad and disc   no matter how much I twiddle it. With calipers off the brake arm moves freely and is not seized.
I've noticed the floating pad projects 5mm or so out of it's caliper, should it go fully home after using the brake? I renewed o ring in calipers and there appears to be no fluid bypassing the pad. I cleaned up the piston which was gunged wicked   , it looked OK? Can't work out what's going on 😤

[Oddjob]

No it doesn't go fully home, it retracts just enough to stop the brake binding, did you clean the groove where the seal sits inside the caliper, if not that's most likely the problem. The idea is that the seal distorts as the pad is forced outward by the fluid pressure, this distortion is what allows the pad to retract. The floating pad will only move back around 1-2mm in total, just enough to allow the pads to release from the disc surface. If the groove is full of crap (and most are) the seal won't distort properly and thus the pad won't retract properly.

The fixed pad should be set using the adjusting spring and screw, there is a set gap but I tend to get the pad to just touch and back it off a little, always seems to work just fine afterwards.

Do the setting of the fixed pad before you pull the brake lever the first time, or fully retract the piston into the caliper and then set the gap.

[McCabe-Thiele (Ted)]

You mention the brake was seized after standing over the winter is there a possibility that the system was still pressurised after applying the brakes?

There is a fine hole in the master cylinder body that allows the fluid to return to the master cylinder. Simon had an issue when he used a previously good master cylinder that he had fitted new seal in - it did not release the pressure when the brake lever was released. Easy enough to check by opening the bleed nipple to see if it de-pressurises the braking system.

[billdn]

Thanks Oddjob and Ted for advice. Been a while as other life stuff took over. But now resolved,  would have helped if I remembered the difference between "fixed" and "floating" pad.
Still won't forget that error in a hurry. Or "Doh" as Honer would say. 🙄