master cylinder bleed hole size GT550 GT750

[bigsteve550]

Hi

Help I am restoring a master cylinder for my GT550 a model.

The question is there are 2 holes in the master cylinder base were the plastic brake fuild cap goes, one large one and one counter sunk one with a VERY small hole in it. Which is a air bleed hole I think

I think mine may be blocked.

Does anybody know if its just a plane old small bore hole that goes straight down in to the piston bore hole without any restriction or valve in it.

also does anybody know the size of it (Small is not the answer I am looking for) as mine will not let even one strand of a wire brush bristle go down it.

Thanks in advance

[Alan H]

It does go straight through, but I don't know the size. I must have some thinner wire than you as I definitely 'poked' mine through when I did the master cylinder. Copper wire is best as it won't damage the drillings. A single strand out of something like 1mm multistrand cable should do the trick.

[Suzukidave]

I always use 1 bristle pulled out of a handy wire brush as they fit perfect .. be careful not to punch a hole through the rubber plunger cup inside .

[bigsteve550]

Thanks guys

[Allan k]

I used a long thin needle (for clothing..) Everything dissambled ofcourse. It was on my GT125 but same unit as the 550.

[Coyote]

That hole definitely needs to be open or your brake will lock up. Not pretty.

[Allan k]

That hole definitely needs to be open or your brake will lock up. Not pretty.

And you won't be able to get air out either..(I found out the hard way )

[bigsteve550]

Progress thanks to the info on this thread I went to ebay and brought a mirco drill set.

from 0.3mm to 1.5mm which arrived today.

The only problem was they are so small they will not fit my drill chucks, so i used a round modeling knife handle to hold the drill bit which was then put in the the chuck. I had the drill in the vice and offered the master cylinder up to the drill bit, anyway after about 30 seconds on drilling the bit broke through in to the main chamber, I then went up to to the 0.4 mm size which also went through after a few seconds. I dont think i will risk the 0.5 mm as the hole looks clear now.

So will reassble and try to bleed it again

[Suzukidave]

Wow !!! to need a drill bit to clear the bleed hole .. that must have been one crudded up master cylinder

[Suzsmokeyallan]

I agree Dave, usually on such old cylinders I use a drill turned by hand to simply clean out the crud and reveal the original taper of the hole in the casting, then a needle to carefully push the crud out of the hole itself.

[bigsteve550]

The front brake is all bled now. thanks for the tips on bleeding the brakes did it with the reservior off the master cylinder, took about 30 minutes to get all the air out.

The front brake had been sitting with old brake fluid in it from 1980 so yes it was a long time blocking up the hole. the master cylinder just had a dirty brown gel type stuff in it when I stripped it.

It was a gey white power that came out of the hole, already tried clearing it with wire and a air line with a 100 psi.

another job with a tick against it.