Brake is fine on minute and the next the lever pulls right up to the grip and doesn’t work at all, as I discovered descending rivington.

Does it just need a bleed or is it more serious ?
They are 2012/12 slx and not been used a great deal

Sounds like a bleed is needed. A few threads knocking about about this.

Master seal blew in my SLX rear at Christmas. Happened suddenly with so sign of oil. Is there some resistance to the
Lever going back out?

Got new compete brake and have the old caliper as a spare.

I returned my SLX rear. It blew at the master cylinder – again, no obvious sign of oil, so I spent ages bleeding it trying to get it back to how it was. You’d have a soggy lever until you pumped it a load, and after a few weeks the lever just never came back. They replaced immediately with a brand new brake.

Theres a few threads about similar problems with them, but they’re quite few and far between, I think the fact that the just replaced without even seeing my brake suggests there was a known problem. Mine too was a 2012 brake.

Edit: I also have some 2013 XT’s and have had no problems with them, despite plenty of Alps hammer, so I think they sorted the problems, don’t be put off.