Motorcycle Questions? Ask a Mechanic.
It sounds like debris has built up in your master cylinder reservoir blocking the small fluid passages located in the bottom.
This causes the piston to pull fluid back toward it from the line rather than allowing the fluid from the reservoir to make its way down into the master cylinder bore.
It is common for this to happen when a brake system is disturbed after it has been in operation for a long time. Corrosion naturally builds up on the cast aluminum and inside the steel lines. It typically can be seen as metallic streaks in the fluid. Sometimes it can be fixed easily by just flushing the reservoir and sometimes is takes a full system flush and master cylinder rebuild. It can be tough at times.
At the very least I'd say you will need to remove and disassemble the master cylinder to inspect the bore and seals and clean those passages.
I hope this helps. Thanks, P.J.
You're welcome! Take care. P.J.