Master cylinder rebuild - relief port?

[EEEliminator]

I'm rebuilding a master cylinder for a '73 RD350, brake fluid was all jelled inside the reservoir... it was bad. I got it fairly well cleaned out and I've been trying to clear this little tiny hole out that supposedly releases the pressure when you release the handle. I cannot get a sewing needle through this hole it is so small, I've tried blowing out with a rubber tipped blow gun at 130 psi and no luck.

I read you can use a guitar string, I tried this but I cannot get the string to clear the hole out. Some say you can cut the string on a 45 and spin it like a drill, not working for me. I also tried grabbing it close to the hole with some needle nose to keep it from bending and tried to push it through and tried soaking with carb cleaner.

At this point I've almost convinced myself that the hole does not go all the way through, except I can see it though the bore...

Does this hole really effect the functioning of the brakes? I know its there for a reason...

Here is what it looks like:

 

[El Feo]

Does this hole really effect the functioning of the brakes? I know its there for a reason...

Not particularly...but, it may affect subsequent acceleration.

It allows the brakes to disengage.

 

[anytwowilldo]

What do you see through the bore? Does it look like a corroded spot that could have fused closed? Better hardware stores will have numbered drill bits You should be able to find a very small one.

 

[EEEliminator]

What do you see through the bore? Does it look like a corroded spot that could have fused closed? Better hardware stores will have numbered drill bits You should be able to find a very small one.

The guitar string barely fit in the hole, not sure I could have gotten a bit that small. I wasn't able to get the guitar string to fit into the 3 jaw chuck on my drill. I ended up putting a bit in the drill and sandwiched the string between the bit and the chuck. This made the string offset from center, but with a little extra length I was able to guide the string into the hole. It ended up working and I got the hole cleared out!!!

Now I can clean it up paint it and reassemble it :banana

 

[Maddevill]

Try one of those jet wire cleaner tool sets. Either from Motion Pro or you can get them on flea bay. Used to clean pilot jets so one of the wires should be small enough for that port.

 

[ScottRNelson]

...but with a little extra length I was able to guide the string into the hole. It ended up working and I got the hole cleared out!!!

Perseverance pays off. :thumbup

I don't think we've helped you any on this one, but you've had good ideas for the rest of us if we get into a similar situation. Thanks for posting.