Sorry, meant front hydraulic brake. I haven't messed too much with disc brakes before and I was hoping that you could help me troubleshoot this, as the service manual doesn't help too much.

I have no front brakes. No resistance at all in the front brake lever other than the return spring. I can pull the lever back all the way to the handlebar no problem. When I pull on the lever there is no bubble or evidence of pressure in the master cylinder. I have loosened the bleeder valves and fluid runs out. No pressure appears evident at bleeder valves when "pumping" the lever.

Background: I believe all of this started a few months ago. (this is my friend's atv) The bike had been sitting for several months between rides. When loading up for a trip, the front right wheel was locked up. The wheel and hub were removed at that time and the wheel hub was sanded down to remove rust, which freed the wheel.

So, is this a piston problem in the master cylinder (since there is no pressure) or is it a wheel cylinder problem?

 

Assuming the caliper piston got pushed back in when you disassembled it, it can take a bit of pumping to extend it back out to where the pads contact the rotor.

Another possibility, since you said it was rusty, is that the brake piston and caliper could have been rusty, and when you push the piston back in to the cylinder it will then leak.

I'd start by generally inspecting things, and then try vacuum bleeding.

 

I will take a closer look at it tonight. However, I do not believe that there is any movement in the wheel cylinder pistons or no visible leak. Remember there is absolutely no pressure when retracting the hand lever.

Is there an easy test to rule out the master cylinder piston/cups?

Also, can you take out/examine the wheel cylinder pistons without having to remove the wheel cylinder from the brake panel?

 

Sounds like a master cylinder problem to me. If the MC were working, you'd get back pressure because there's 2 front brakes. I'm assuming you have a full resevoir. The MC is not hard to take apart. There's some seals and springs in there. I'm guessing there's dirt or something in it. Something unrelated to when you sanded the caliper, but somthing that may have caused that caliper to stick in the first place. Taking the MC apart and checking it is easier than taking a caliper off and then apart.

 

Yeah, you could have air in the MC's cylinder, and surface tension sometimes keeps it from filling (vacuum bleeder solves), or there could be a bit of debris in the oraface between the fluid and cylinder.

Also, if the lever doesn't return fully (wrong lever, hitting hand guard) it won't open the port between the resivour and the cylinder.

 

That is kind of what i was thinking, but i didn't know how the lever suppose to be acting.

That is encouraging that it could be just dirt rather than needing a MC rebuild. I will take a look tonight. Thanks for the help!

 

Ok, it may have turned out to be a comination of problems.

I removed the Master Cylinder. removed the oil seal and and poked in the holes, trying to clean out what I could. I attempted to take out the piston, but had a hard time removing the snap-ring with out snap-ring pliers.

So I was able to create some pressure and had the MC bubbling with the pumping of the lever. After bleading brakes, the wheel cylinder began expanding, however, only on one side. (with the wheel and hub removed)

After it expanding, probably further than it should, it began leaking. So:

1) how do I retract the wheel piston (do I have to drain fuel line)
2) could the leaking be only because it expanded too far
3) how do I get the other side of the cylinder unstuck? (use a c clamp?)

 

If you pumped the master cylinder with the calipers not on the rotors, the normal thing that would happen is one of the caliper pistons (whichever one has less friction) would pump its piston out of the bore.

The caliper will have a rubber boot that seals dirt out, and farther in the bore the caliper seal. You will want to disassemble the caliper that came apart and clean everything (absolutely spotless) in clean brake fluid, then reassemble.

You will then need to bleed the system. I'd highly recommend a vacuum bleeder, like a mity vac. Use a bit of finesse when opening the bleeder valve, in that you can vacuum a motorcycle system dry quickly if you just open the bleeder fully while pumping away.