New Shimano m615 rear caliper fails and stops biting the rotor after 30 seconds of extreme downhill I never had this problem with my Hayes stroker trail. It only seems to start working after I stop for a minute or two. I am riding 30% grades so a lot of force for the short time period, then the brake softens up to the point where it loses 90% of the braking ability, and I can only use my front.
It's easy to determine that you're obviously losing your braking power due to heat generation. But hard to pin down exactly what's wrong without actually looking at your bike. There are a couple of things that could be going on, which could be a result of a mechanical issue, or an operational issue. 

Mechanical Issue

First of all, your m615 brakes are only middle of the line, they aren't intended for extreme downhill (check out the Saint line), but even still, 30 seconds isn't a very long time before failure. There are a number of questions I would ask you in the shop;


Has this always been a problem, or is it something new? 
How long have you been riding these brakes?
When was the last time you changed your pads?
When was the last time you had your system bled? and Did you use Shimano oil?


I would also inspect your bike, check what size rotors you're using and look for any obvious signs of oil leaks. I'd check the wear of your brake pads, and use a micrometer to check the wear on your rotors. I would look at the colour of your rotors, if they're blue then you've cooked them and they need to be replaced (Bluing steel makes it harder, harder steel means less friction–ergo–less stopping power). If nothing was visually obvious then I would bleed your brakes to check the condition of the oil, and make sure there isn't any air in the system.

Shimano Brakes use mineral oil, which is different than the DOT brake oil that would go in your Hayes brakes. Different oils have different boiling points, DOT oil is regulated (DOT 3, DOT 4, DOT 5.1-The higher numbers reaching higher boiling points) But Mineral oil is not regulated, which is why it's recommended to use the manufacturers oil. Some shops use bulk mineral oil made for other applications because it's cheaper to buy (Shimano oil is expensive), but it may not necessarily have a high enough boiling point. If your oil isn't pink, then it's either dead, or not from Shimano.  

Opperational Issue

This is the part where I grill people about how they ride. Improper riding habits are a mechanics bread and butter, I've replaced more parts from riders causing damage to them than from parts wearing out due to normal use. One of the bluest pair of rotors I've ever seen came off of a bike whose rider was probably a good 250lbs and rode about 10km back down a trail–that was ALL downhill–riding his back brake the whole way. You have two brakes for a reason, and the FRONT brake is the one with all the stopping power. When riding extreme DH, your back brake is for steering, your front is the one that stops you. You need to use them together when you need a lot of braking.

 

I gave my big friend with the blue rotors a long lesson in proper braking, then installed a new 8' rotor on his rear wheel and a big 9' rotor on his front. He never had brake problems again. It's important to make sure you have a braking system that can handle you and what you ride, if you're  bigger guy, then you're going to need bigger brakes, if you like to ride fast and steep, then again, you're going to need bigger brakes.



If bleeding your brakes or swapping out brake parts is a bit beyond you, then you need to take your bike to your local shop, they'll be able to tell you exactly what's going on, and may recommend you upgrade your brakes or your rotors. The alternative is to slow it down and ride gentler grades.