77 cb750k front brake

[77750]

Caliper piston won't retract after releasing brake lever. Only somewhat retractable leaves me with to much drag. New and shoes and reground disk. Loosed mountingbolts but still happens when applying brake. What makes piston retract?

 

[gunr]

I just remedied the same problem on my '77.
I pulled the caliper apart and popped the piston with compressed air, then plucked the seal from the groove with a pick.
Corrosion had formed under the seal. I ground a cheap allen wrench into a scraper and scraped the junk out. Cleaned the mess with brake cleaner and compressed air and reassembled.
Works fine now.
The replacement seal I ordered from 4into1 was to small and I wound up reusing the old seal.
The closed system of the master cylinder provides the suction to retract the piston.
I did also find the pin in the mounting bracket was not moving freely and had to clean and relube the pin. There are o-rings on the pin to keep out the gunk that may need replaced.

 

[77750]

I just remedied the same problem on my '77.
I pulled the caliper apart and popped the piston with compressed air, then plucked the seal from the groove with a pick.
Corrosion had formed under the seal. I ground a cheap allen wrench into a scraper and scraped the junk out. Cleaned the mess with brake cleaner and compressed air and reassembled.
Works fine now.
The replacement seal I ordered from 4into1 was to small and I wound up reusing the old seal.
The closed system of the master cylinder provides the suction to retract the piston.
I did also find the pin in the mounting bracket was not moving freely and had to clean and relube the pin. There are o-rings on the pin to keep out the gunk that may need replaced.

Thanks for the insight. I've got mine apartt now found a little rust and rubber needs to be replaced. We'll see.

 

[gunr]

So, learn from my mistake...
The correct seal for my '77 front brake caliper is #45215-533-004. It's 49mm OD and 43mm ID.
I ordered the wrong one that was 44mm OD and 39mm ID.
From 4into1the cost is only like four bucks so I'll keep it for the parts bin.
Luckily my original seal could be reused.