Hi, I’m having real problems bleeding the rear and front left brake circuit after changing the lines. I’ve tried doing the front first, the rear first  and also tried using a gunson pressure bleeder that uses a tyre at 20psi - still no luck.
I’ve now blanked off the front disk completely by using a brake union with loads of copper washers at the rear v junction so and am just trying the rear but still no luck. I think the problem is bleeding the air out of the actual rear master cylinder. I get some fluid at the bleed nipple but mainly air. The system was working and I’ve changed the mcyl over to a different one.
Any tips on how to bleed a system from scratch?
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Take the rear caliper off and raise it as high as possible.Then bleed system again - this normally works for me.
I reverse bled mine with a syringe. Â I do it on most of my bikes and seems to work well.
had similar problems with a friends
best to double check there is no debris in any of the pipes by carefully blowing through with an air line, also check the master cylinder  carefully.  Block the outlet with a suitable bolt and check it pressurises.Â
Nip to your local friendly vets and get a large syringe and find some clear pipe ( windscreen washer pipe ideal),  open the front brake nipple and push syringe full’s of fluid SLOWLY through the system with some one watching the tank .  do same for the rear.  You often find  the brake pedal down to something heavy over night help
if you have a remote fluid tank like the v50 check the pipe down to the master cylinder is not blocked or kinked…Â
I have just had this exact problem with my v50 mk3, after much messing about, it turned out the the adjustment on the brake pedal was to blame, it was depressing the piston in the master cylinder slightly too much when it was at rest, causing the master cylinder piston to slightly block the hole that feeds the cylinder with brake fluid delivered from the resiviour, (the master cylinder piston has a undercut section on it to allow brake fluid to flow into the cylinder). All I did was adjust the brake pedal so the piston in the master cylinder came out a bit further and it bled easily.