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Rear master cylinder site glass


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Hi all,

I've been having heaps of fun with the YZ250WR I recently picked up, and enjoying the shed time getting it into shape, however, had a funny/weird thing happen today when I was putting some new rear pads in.

I purchased new rotors for both front and rear, and while I was replacing them I've also replaced the pads (obviously) and brake pins with hex heads. As I was putting the pads in this morning I needed to move the piston in a little to make room for the new pads. Using a screw driver I levered the piston in a little and at this point the site glass window in the rear master cylinder popped out, spewing brake fluid all over the shed floor.

I've pressed the window back in, installed the brake pads, filled the cylinder with fluid, bleed the line and the brake seems to work OK and doesn't seem to be leaking.

Do you think this is going to last? I assume there is plenty of pressure under normal conditions, so i'm not terribly confident of it lasting, however, looking at the make up of the thing, it's definitely how its been made i.e. the only way to get the window in there is to press it in.

Should I put some kind of epoxy on the window to keep it in?

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I think you are ok. That part of the mc shouldnot have pressure in it. The fluid should have shot out of the vent in the cap when you pushed your pads in. I believe the mc has a dirty vent or an aftermarket cap. The aftermarket caps often are not vented properly.

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Thanks. I didn't know there was a vent in the cap. I'll pull it apart again tomorrow and see if I can find a blocked vent.

Spent the rest of the arvo trying to sort out the electrics. You should have seen the wiring on thing! Everything was just wrapped up in the air box, plugs not covered our anything. A trip through any water and this thing would surely have stopped.

I should have got one of these bikes new in 2004....I'm loving it!

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One more question....has anyone else had this happen to them?

Today it seemed to work OK and didn't leak. Just wondering if this has happened to anyone else and whether the problem kept occurring or it posed not further issue in the future.

Cheers.

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The cap should be loosened when pushing the piston back to install new pads, especially if fluid has been added over the course of the pads wearing. The cap vent is above the the cap sealing boot, so if if there isn't enough volume left in the M/C for the fluid to being returned to the resevior, pressure will develop when the boot is push up against the cap (sealing off the vent). The M/C is vented like this to keep moisture out of the fluid.

When replacing linings, it is a good time to replace the fluid too.

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