Front brake locks up when bike gets hot usually within first 10 miles stop and bleed it and it is ok for a few hours.

faulty brake master cylinder
replace with a new unit and bleed the brakes

YOU NEED A TECH 1 SCANNER TO MOTOR REHOME FUNCTION PISTONS IN THE ABS BRAKE MODULATOR SHOULD BE RETURN TO THE UPPER MOST POSITION.BEFORE YOU CAN BLEED BRAKES.IF YOU DONT HAVE TECH 1 SCANNER SAFELY RAISE SUPPORT FRONT OF VECHICLE USING JACK STANDS. WITH FRONTS WHEELS OFF GROUND EMERGENCY BRAKE APPLIED WITH PUT VECHICLE IN DRIVE LET WHEELS SPIN AT 5 MILES PER HOUR THEN APPLY BRAKE PUT VECHICLE BACK IN PARK.TURN OFF CAR. THEN WAIT FEW MINUTES THEN START ENGINE AGAIN IF THE AMBER ABS LIGHT STILL FLASHES AND DONT TURN OFF HAVE VECHICLE TOWED IN LET DEALERSHIP PRESSURE BLEED AND USE THE TECH 1 SCANNER TO BLEED AND FIX YOU BRAKES FOR YOU.

Master cyclender working ok?

Try putting the bike on the center stand and get the front wheel off the ground with a floor jack. Spin the front wheel by hand and press on the rear brake pedal. The front wheel should stop. Likewise with the bike on the center stand and the rear wheel off the ground you should be able to lock the rear wheel with the foot pedal. On many Goldwings the rear pedal also operates one front disc. This is for people that refuse to use there front brakes to stop or slow there bike (80% to 90% of your stopping power is the front brake). Try bleeding the entire system front and back, you may just have air in the system. If the rear brake still does not work you may have a bad delay valve in the system. If you need to take the bike in to a shop, try to find a mechanic that is familiar with the system so they do not replace parts unnecessarily. That way you minimize your shop time and maximize your riding time.

dont get into the habit of using your rear brake anyway it'll get you in trouble...rear brake usage should only be left to the professional road racers. you should be glad that your rear brakes arent working..i have a friend that even disabled his because him and i both almost lost it when a car pulled in front of us and the bike was getting side way and wanted to dump me off..high side...you need to get into the habit of learning to control your speed even if your flying down the freeway doing a 100 with just the front brake..also replace the pads often especially when they start squeeking and always remember that new brake pads alway need to be broke in easy... no hard quick stops within the first few miles or you'll put a glaze on the pads and they'll never work right!...but if you want a rear brake, it sounds like your rear caliper is shot ..it needs a rebiuld!
can you rate my expertise! thank you.

sounds like your brakes need bleeding and the abs sensor adjusting

Possibly the Proportioning valve. its where the lines go after the master. it has a bleeder screw of some sort on it. The pins inside wear out and stick causing uneven pressure. Replace or rebuild it if that be the case. Also chect the master for fluid leaks at the end of the piston where it connects to the booster push rod. If its leaking rebuild or replace it. 

from my experience its sounds like ur front rotors may need resurfaceing...if ur doing that...may as well replace the brakes on the front as well...sometimes glazed front brake pads may make a sound but never heard of it as a grind, usually its a squeal...have ur dealer check ur rotors for hot spots in which u would need them replaced if they are overheated..also check the front pads for cracked brake pad material

I have a couple ideas for you. first make sure you bled out all 4 wheels making sure to get the air out. also check your abs module and see if it has a bleeder screw on it and bleed it. The second thing is after you drive it, does it seem like the brakes don't release after braking? is it ok when it's rolling? you may have a collapsed brake hose from your body to the calipers, allowing pressure through, but not back. This also makes sense after a few miles, when your brakes are just getting warmed up. after it starts locking up, jack it up and see if the wheel will spin. then see if the caliper piston will go back in. if it doesn't want to go in as easily as it should, or not at all, open the bleeder and see if it goes back. if it does, there's your problem, the hoses.
good luck!

They're turning hot because you clamped off the rear lines. Now the fronts are being overworked - they're heating up because they're doing 100% of the stopping instead of the usual 70% or so. Replace the line and bleed the system before you destroy your entire braking system.