small plastic cap inside front break reservoir

[angrypants]

sorry if this has been asked before i've searched here and googled but found virtually nothing on it.
this is on a 1980 gsx250. the reservoir is rectangular metal with the little window

what ive been puzzled about is what looks like a little clear plastic dome, maybe 15mm in diameter and 5-6mm high. at first i thought it was part of the window.... it loosely fits it....but now i think it sits over the two holes at the bottom of the reservoir, its almost snug there, and the dome has 3 short line near its edge that seem to match with markings in the countersunk hole that has the tiny return hole and large hole down into the cylinder part . The plastic dome has a few small holes in it too .

i didn't pull the master apart, i'm helping the young bloke service it, he says that's where he found it, but he is like a bull at a gate etc etc and in such a hurry prone to forgetting details ;-)

is that where the thing is meant to go?
What is its purpose?
it appear to fit / just push in raise part of dome up, should it be held in by something, have we lost an o ring or similar?
is it necessary?


sorry i should have taken some pics last night

thanks in advance

grant

[gsrick]

I'm not familiar that master cylinder, but many have some sort of plate that lies over the inlet and bypass hole. When bleeding the brakes pressure can force the fluid up and out of the reservoir and land on the tank or fairings and ruining the finish. The plate or bubble keeps it from doing that.

[Steve]

I will agree with Rick on that.

.

[angrypants]

Thanks guys