Front master cylinder bleed port ?

[Grewth]

There are two small ports on my F1 front master cylinder that allow the brake fluid to enter the piston bore.
The little tiny one appears to be blocked.
I've tried poking it with a sewing needle etc, but so far no luck.
Thinking about drilling it now, but unsure of what size to drill.
Anyone know  ?
I'm thinking too big a hole is going to have a bad effect on the master cylinder.

[K2-K6]

The bore size effect controls different aspect, the one nearest to line output has to be fully closed to build line pressure. With an increase above std, that would delay closing and increase lever travel before caliper "activation" but once past that, the master seal would operate the same as system to line is fully isolated from reservoir.

You can get 0.3mm pcb hand drill bits that should be under that size, but check initially to see if it's OK.

Have you got the mastercylinder apart, or trying to clear in situ ?

[K2-K6]

There are two small ports on my F1 front master cylinder that allow the brake fluid to enter the piston bore.
The little tiny one appears to be blocked.
I've tried poking it with a sewing needle etc, but so far no luck.
Thinking about drilling it now, but unsure of what size to drill.
Anyone know  ?
I'm thinking too big a hole is going to have a bad effect on the master cylinder.

A dressmakers Pin is usually thinner than a needle and with less taper from tip into shank, they often pass through smaller holes but helpful with pliers to hold them ordinarily such that you can gently wiggle them.

[McCabe-Thiele (Ted)]

Have you tried a wire from a wire brush?

[Bryanj]

Single stand of multi strand, small electic cable

[Oddjob]

How about a really small interdental brush? I’ve used them very successfully on jets.

[Grewth]

Many thanks for all the advice.
I'm thinking about the wire from a wire brush to start with.
The cylinder is totally stripped down,  I was lucky enough to find a pair of circlip pliers that were long enough and thin enough to work.

[Bryanj]

Put a small torch into the reservoir and see if light comes through to holes

[ozzybud]

Get a set of guitar strings..  Great tiny strong wire.