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Working on my '85 FJ600's brakes and it doesn't appear that the piston and/or caliper is moving at all. I started at the top and tried to bleed the brakes, but when I opened the bleed valve on the caliper I got nothing.
Where should I go from here? Should I rebuild the master cylinder? Should I look for a plugged or collapsed hose (and how do you do that)? Or is it something in the calipers (stuck piston)?
I'm 99% sure it's not a leak, there is no brake fluid anywhere it's not supposed to be. I haven't tried to bleed the rear brakes, but they are working. The calipers don't move at all on the front.
If bleeding the brakes gets me nothing, no air or fluid, what's the next step?

That does seem odd, but then again so do most problems. Did your brakes just stop working suddenly? over time? Is this a project bike that you've never ridden before? How much fluid is in the reservoir? When the cap is off and you squeeze it (be careful of anything shooting out and hitting your paint or your eye) do you see any fluid moving around? Is it full of gunk? Tell me more.

The front brakes were not working when I bought the bike a month ago. I didn't THINK it was a project bike, but it might turn out to be one!
The reservoir is full and clean. When I squeeze the lever the fluid level does not change and I see no bubbles.
It has been suggested that the pistons are stuck in the calipers.

if youre opening the bleed nipple and getting nothing ,the first thing to look at is the nipple.did the nipple have a rubber cover/if not,it likely blocked.new nipples are cheap.if you have to change the nipple youre going to lose all the fluid and start from square one,so you might as well check the seals,pistons,etc.xjrjohn

After some chin-scratching, my BF and I think we may be on to something.
When we pump the lever we get no bubbles in the master cylinder and no fluid leaves the cylinder. We stuck a needle in the bleeder valves and backed them out a few turns and no fluid leaves the caliper. This makes us think the pistons are stuck in the caliper.
How does our diagnosis sound?

I think your off. If nothing is coming out of the bleed nipple, then you have a blockage of some kind, or your master cylinder is bad. I don't know why you think it's the piston based on what you've told us this far. How firm is the brake lever when you pull it? If it's weak I'd guess your master cylinder was bad or maybe had the return clogged.

But even if there were a blockage or a bad master cylinder, wouldn't the fluid below the block/below the cylinder still come out when we backed out the bleed valves?
We're thinking that pistons stuck open would keep the fluid from coming out the bleeder valves.

Did you take the nipple off and have nothing come out?

OK, we pumped the brakes and opened the bolt at the banjo fitting on the master cylinder and got clear, fresh fluid out.
We opened the bajo bolt at the caliper at got nothing. Backed up to the brake hose union and tried both banjo fittings there and get nothing.
To me, that points to a plugged hose between the master cylinder and union, right?

Ding ding ding.

Don't worry about unclogging it. Get a steel braided brake line which gives you better performance and then top off with new brake fluid (can't hurt to flush a bit to get the old stuff out of the caliper/master