Lost rear brake pressure today
 

For the first time in 5-days I went to ride my bike into work. I noticed at the first stop sign the rear brake pedal bottomed out to the frame. I tried to pump it, nothing. So I went around the block and headed home. I looked at everything and it looked intact. However, I could push the pedal down by hand almost all the way.

I didn't have time to look at it in great detail as I had to hop in the truck now and go to work. So when I get home what should I be looking at? I have never opened the brake fluid reservoirs and assume they are still within spec.

Start with the simple stuff.
A leak would be obvious so I assume that's not the issue.
If the master cylinder is low enough, it can't pressurize fluid to the slave (brake) cylinder, so check fluid level. This too should be obvious because the reservoir is located above the master cylinder, and do you see fluid level in the reservoir? Add fluid if its low. Check the pads. Are they still there with plenty of pad remaining?
After those simple observations, I'd bleed the brakes and see if that helps. If not, I'm just guessing one of the cylinders is bad.

^+1
Check fluid level, look for leaks and make sure that pads are in good shape. If all that checks attempt to bleed hydraulics and watch for air bubbles at the bleeder. If that still doesn't give you a firm pedal then the problem is usually the master cylinder. You may be able to fix it by taking the piston and seals out of the housing and cleaning them. Look for any signs of tearing of the lip of the rubber cup seal and if it looks good put it back together and see if it will pump up. If it's still a no go then replace the rubber seals in the m/c- that should get you going again.

Ok, so I'm back home for the day and looking at the bike. I checked the brake fluid and I'm right in the middle. The fluid appeared clean, although it looked like a little sediment at the bottom of the reservoir. No sign of a leak anywhere. Pads are good.

I'm not sure what this part is, but it is loose.

https://farm9.staticflickr.com/8744/...9c2e7fa7_c.jpg

This is a video of the rear brake as I pump it. I gradually tightens up as I pump it but the pedal still goes to the frame.


This is another video of the pedal as I pump it.


The bike only has 6,800 miles on it and I've never had any brake problems. Rode it home from work on Wednesday and all was good. I don't know how it could go to this just sitting for 5 days.

I don't have any brake fluid here right now, so would prefer to not bleed them until I can get some (tomorrow).

That's your switch for the brake light and should have nothing to do with your issue. Don't try bleeding until you have fluid. Do you have one of those mityvac type units to suck fluid from the slave cylinder instead of trying to push it with the master cylinder (brake lever)?

I don't have any brake tools. Did a quick search on Harbor Freight and can pick one of these up tomorrow.

Search results for: 'bleed brakes'

Add fluid and try it first. When my fluid level isn't full the brake often does that to me too.
The question is where has the fluid gone?
Might be a good time to upgrade brake lines?
As to bleeding tools...my homemade bleeding tool. I drilled a hole in a bottle top, stuck a hose through it, all the way to the bottom of the bottle, and then put some fluid in the bottom and put the top back on. Attach it to the bleed screw and you're off and bleeding.

As for topping it off, the owner's manual says to not mix brake fluids, even of the same DOT. Is there any real concern to topping it off?

Also, anyone know what DOT would have come with the bike? Manual says either DOT3 or DOT4.

True, but I do top it off.
It's worth it to see if you can return pressure to the brake.
Then bleed it out with new fluid, if it holds.
If not, then it's time to take some stuff off the bike and take a closer look.
Start simple.

Best $30 I ever spent - keeps your sanity.

Just don't draw too much vacuum it can drain the reservoir and line fast and possibly suck air past seals that are meant to seal against pressure, not vacuum. Light suction does the trick and fast too. Took me about 3 minutes to bleed the back brake on my Zephyr 550 from a dry rebuild.