I bought it off eBay, there were loads on there. I fitted it today on the caliper and it's helped but the braking force is weak. I think the continued presence of air was from a leak in the master cylinder. The rubber around the inlet from the reservoir was perished, I carefully cut some rubber away and sealed it with JB weld. This seems to have improved things, I think I'm now in a position where I just need to bleed better. I've left it overnight with the pedal clamped in the on position. If that doesn't work I will try new pads as they are only 2mm thick. It that fails I'll remove the system and hang it up.

Yup the airbox is in the way on my setup. But removing the airbox is about five minutes. You can spend a hour or more trying to bleed the rear brakes if you don’t crack the banjo bolt. Sometimes it’s just easier to bite the bullet and take some things apart to get proper access. The upside is once it’s done you don’t have to do it again until something gets broken.

That's a a good point, I'll order another banjo bleed nipple. There probably is enough give in the airbox to squeeze the slightly bigger banjo bolt with bleed nipple in there. I'm still surprised that it's such a pain in the a*** to do. 

9 hours ago, dan williams said:

Yup the airbox is in the way on my setup. But removing the airbox is about five minutes. You can spend a hour or more trying to bleed the rear brakes if you don’t crack the banjo bolt. Sometimes it’s just easier to bite the bullet and take some things apart to get proper access. The upside is once it’s done you don’t have to do it again until something gets broken.

We may be misunderstanding each other. Agreed, taking the airbox off is easy. I had to take off a lot more than that to remove the entire brake system in one piece. But if you fit a banjo with a bleed nipple, it has to stay there when the airbox goes back on, and that is where i saw the problem.

9 hours ago, trapezeartist said:

We may be misunderstanding each other. Agreed, taking the airbox off is easy. I had to take off a lot more than that to remove the entire brake system in one piece. But if you fit a banjo with a bleed nipple, it has to stay there when the airbox goes back on, and that is where i saw the problem.

Maybe because mine doesn’t have the bleed nipple, just a button head screw. Point taken.

2 hours ago, dan williams said:

Maybe because mine doesn’t have the bleed nipple, just a button head screw. Point taken.

Button head screw: that’s a good idea ?. With a copper washer, of course.

What you need is this https://www.amazon.co.uk/dp/B084QHHG3C?ref=ppx_pop_mob_ap_share

These large syringes let you get some nice pressure to force the air out. 

Get the front wheel above the rear wheel so the rear caliper is lower than the master cylinder. 

Use a long run of clear pipe on the syringe to the caliper bleed nipple, remove the reservoir under the plastic and allow the black pipe to reach out of the bike into a drip tray/container. 

Open the nipple and force some brake fluid through under a lot of pressure, close the nipple and pump the brake pedal really hard a few times to break up any air bubbles in the system, bleed once again and try the pedal, it should be spot on now. 

Refit the reservoir and top up with fluid, tap the reservoir to make the trapped air in the black pipe rise into the reservoir. 

Thanks Bassound, that sounds like an excellent plan. I'll give it a go. Thanks.