No harm done.

Opened the bleed nipple, and a puff of steam came out.

I can only think that the fluid absorbs water when the bike is laid up over winter, then it boils and pressurises the brake.

It always happens in traffic...engine heat on the master cylinder must be doing it.

Ordered some Motul Dot 5.1 in hope of a cure.

Anyone else had this?

Luckily, I was only in Bodmin, and a small crowd of helpers soon gathered, blokes arrived with toolboxes etc.

The Cornish are 

a) Very kind

b) Love fixing things.

See this story

https://adventurebikerider.com/forum/viewtopic.php?f=28&t=51984" rel="nofollow - https://adventurebikerider.com/forum/viewtopic.php?f=28&t=51984

Needs to be bled 'just right' and set up 'just right'...I've now got mine set up with a good bit of slack in the pedal, so you can tell straight away if it's on the point of locking itself up.

I'll report back on what difference, if any, Motul DoT5.1 fluid makes.

I've also seen a sudden fall in the fluid in my clutch MC, so there's an Oberon slave cylinder coming too. Hopefully the new fluid will help here too, and that has to work in a very hot environment.

Ironically, the front brakes have the easiest time...cold air all over them.

Now waiting for the fluid

Most who get rear lock ups have put it down to incorrectly adjusted input rods. 

I have an RSV4 rear master fitted to AP rearsets and the brake works really well. Certainly capable of locking up the rear wheel.

New fluid in, brake bled, all well.

Then today riding slowly up a steep hill in traffic with a hot engine, it locked up.

Undid the bleed nipple half a turn, and a spectacular ejaculation of boiling brake fluid resulted

The caliper is now leaking when the brake is applied...am going to order a £16 heat shield, strip the caliper and rebuild with new seals.

Good job I've got some brake fluid left over.

On a brighter note, the Oberon clutch slave works really well!

I had to cut it down to get it to fit, and lined it with heat-reflective insulation on the engine side.

Brake freeplay checked and all OK (I run slack back brakes on all my bikes anyway). Fluid topped up (but not over-filled) to replace what was lost. Linkage moving freely.

Will let you know how it goes. If it still gets hot, I will try insulating the line, which runs horribly close to the engine. If that doesn't work, then it's time to get some aftermarket footrests that relocate the master cylinder away from the engine and into the airstream...there is a duct in the bellypan which is obviously designed to cool the MC, but it's not going to do anything at 10 mph!

None of the above was any help.

In the end...purchased another new master-cylinder and fitted to the AP rearsets.

Ditched the Brembo caliper and replaced it with on from Hel (which is just up the road in Devon).

Sat and thought about things for ages while bleeding the new caliper in.

Does the reservoir get hot?

Well less so than when it was under the fairing, but there's still a lot of heat around that area.

Does that create pressure in the reservoir?

Yes because it's not vented.

Looked carefully at the cap, and there's two little dimples on the inside.

Hmmm, drilled them through with a very small Dremel drill.

Have stressed-tested bike by riding slowly in high temperatures, and all was ok. Went for a longer ride, and the brake works perfectly and no longer gets hot.

Was it either of the expensive components that I replaced, or just the two little holes I drilled.

I've no idea, but anyone experiencing this problem would do well to drill the holes first!

The reason that brake systems aren’t generally vented is that the fluid is hydroscopic. Not an immediate problem but your fluid life will be reduced and you may introduce some corrosion if you don’t keep on top of fluid changes. 

I checked the condition of both carefully.

Heartily recommend switching to the HEL caliper.

It's not at all grabby in normal use...came with EBC organic resin pads, not sintered metal.