Over the years, I've lost the rear brake completely on a number of occasions on hard and fast down hill descents.

Can i use dot 5.1 or need to stick with dot 4.

It's usually towards the last 1/3 or 1/4 of a reasonably long mountain descent that they completely disappear and the pedal can be pushed to the floor with nothing there,although it does return up.

This even happened when the bike was new.

Keen to hear any thoughts, mods, tips or maybe i just need to use the front brakes more instead

 

Definitely learn to use the front brake efficiently. As weight shifts forward a majority of your braking capability is from the front brake. If you have to brake so hard as to experience brake fade on a downhill use the engine braking, downshift and allow the engine to slow and maintain reasonable speeds. I damaged a front rotor on our car on a long roughly 2 mile downhill on the route to my job. I made the mistake of coasting and using the brakes only. My job location was a few hundred yards from the bottom of the hill, so the brakes didn't get the chance to cool while moving with some air flow. Had to replace rotors. From then on I'd use the gear box to maintain reasonable speed down the hill. It was steep and long enough that I could reach well over 60 mph (100 kph) if I could let it go, but curves and speed limits held me under around 50 mph.

I'd say you need to make sure the rear caliper is clean around the piston so the caliper works well, then put in new brake fluid, DOT4 if that is what is the rating for fluids there. Then learn to use both brakes to control speeds and have the bike in the right gear to help aid in keeping the proper speed for the road and any speed limits.

 

Downhills: first gear (engine braking) + gente front brake (careful with locking). I typically dont use rear brake on hard downhills:
1. less efficient as most most of the weight/braking power is on front
2. on 4-stroke engine braking is good! and engine braking doesn't brake traction as much as rear brake