Brake failure- BMW R1150GS (non Adv, non ABS)
 

After only just changing my rear brake pads (which worked great), I was coasting down a dirt road on a hot day in neutral when my rear brake pedal suddenly lost all pressure. The caliper was hot enough to fry an egg but after letting it cool down, some pressure retured. The brake works now but doesn´t feel quite as sharp as it was when I changed the pads.

Any thoughts?

It was heat. Over-use of the rear brake.

Yep, I´d sort of guessed that however the brake fluid level seemed to drop and return again later so I´m wondering whether I now might need to re-bleed the rear brake?

Some things come to mind :-
Maybe the caliper sticks a little with the new pads pushing the pistons in deeper. If so the pads could rub, and heat up the caliper. thereby heating the brake fluid so that it produced a little vapour.

How old is your fluid? If overdue for a change do it. It may have absorbed some water which will cause vapour to form.

It wont hurt to check by re-bleeding.

It could just be you were ridingthe rear brake whilst free wheeling.

Yes it was definitelly due to riding the rear brake while freewheeling however the new pads worked very well before this so I don´t think theres a problem with anything sticking.

When new, the pads were quite ´grabby´(if thats the right term) but after the failure, they needed quite a bit of pressure on the pedal to make the bike stop (almost like the rear drum brake on my old Kwak Z650!)

pads are glazed now? need to take off glaze.
gunk on piston pushed in causing sticking or too much fluid not allowing pad/piston retraction?

You can take sandpaper to the pads if you want to, scrubbing just a little bit.

The fluid and it's color is easily seen inside the rear brake fluid reservoir. Unless it looks like tobacco juice, I wouldn't worry about it.

I once had brakes fading on a R80GS from riding a pass downhill and having to much fun with all the hairpin bends and short straights where you could easily accelerate to 100km/h and have to get the speed down to 15, 20km/h for the next bend. This Kind of riding puts a lot of strain on the brakes and once the temperature reaches a specific level the brake will start fading. Your brakes will work the one moment and the next it will feel as if you have let go of the brake as there is just no more effect from the brake. This is caused by the metal of the disc changing character as it reached a certain temperature.(don't know the exact science behind this.)
This problem is much more common with trucks because of the heavy weight the brakes have to cope with, and is the reason these guys often creep down a mountain pass at only 20km/h.
This problem will go away by the time the temperature of the brakes have dropped to acceptable levels again.
The reason why your brake is not so effective anymore could be on account of the brake fluid that got to hot and caused some evaporation or you could have had damage to the disc. A problem with the disc is easy to pick up as you will feel a slight shudder through the pedal when braking. On cars and trucks you can fix this by skimming the disc but in the case of the bike you will have to replace the disc with a new one.
First replace the the brake fluid and deglaze the pads and then look at the disc as the possible problem as this can be pricey to replace.

It is never safe to coast downhill with any vehicle only relying on the brakes.
Always keeping the motor engage in the appropriate gear to help with braking also improve the handling of the vehicle especially when have to make an emergency stop.
This is even more so when traveling on gravel roads.

Top advice guys, thanks.

I think the disc is fine but I´ll take a look at the pads and sand them if they look glazed and will probably change the fluid too as the level looks a little lower than it was.

And no more coasting downhill with the engine off for me!