what is the best method to purge braking system?

I have to do a brake job on old Betsy and need to totally change the brake fuid at the same time.
Is it best to totally drain the system, do repairs and then re-fill
eveything?
Or just undo what I have to and block the flow and then fill and bleed until it is all done?
Or does it make any difference?
If it will not affect or cause issues, number 1 would be best I think as I have to replace brake caliper seals in all 4 wheels. And I do want to totally change the fluid as it has been years since it was done - like 20 or so....
1976 smogger if it makes any difference....
TIA...
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I notice vette brakes has a complete kit with s/s lined calipers, master, lines and hoses. I may go for that and be done with it. Has anyone every dealt with Vette brakes? Their prices seem pretty good....
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I have used VB before to replace my stock calipers on my 69 with o-ringed units. The brakes kept sucking in air and always needed to be bled. No problems with VB.
Vito
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