2014 gasgas rear brake master cylinder

[kasper]

Hello
I have a 2014 GG Replica, it has an "older" rear brake master cylinder (part number BT660002811) as it wasn't available quickly enough, I bought the newer version (part number BT66000GG-CSV-1). The banjo on the older is 8mm and the more recent is 10mm, I made the new house with proper banjos. However, I am not able to get it working correctly. 
The brake is really weak, almost non-existent. There is a chance, that I still have air in the system, but pumping the brake does not make it any better. 
Checked/done so far:
1) No leaks;
2) New hose, proper armoured hydraulic hose, professionally made
3) Probably no air, I injected the oil from the calliper side, with a large syringe. Slowly pushed thru a few hundred millilitres;

 

[arnoux]

Just my take on bleeding Trials brakes (and we do a lot of them as I have a bike business) is that you are way better "pulling" the fluid through at the rear bleeder screw, than pushing it through like you are.

If you have air behind the caliper pucks pushing fluid in does not clear that. The fluid takes the path of least resistance and goes from the bleeder straight to the reservoir. Much better having the reservoir cap off, fluid level full, and draw the fluid through the system, this then draws out any air bubbles deep in the calliper. I have always found this a much better way to bleed rear brakes.

Couple of notes: always make sure you have a little freeplay on the actuating rod, so the piston comes all the way down. That makes a clear path from piston bore to reservoir.

Try and use a vacuum bleeder for the best results, but a syringe works also, just not as efficiently. And don't worry about seeing bubbles in your tube from the bleeder. When doing this method you will always get some air draw from around the bleeder. Just pull a full reservoir of fluid through and that should be sufficient.

[rotors7]

Rightly or wrongly I always use ptfe tape around bike or car bleeder nipples to try to stop the 'false' bubbles.

It usually does the trick for me when vacuum bleeding. 

[kasper]

Yesterday, after posting it here I read more about the bleeding of the rear brakes. I understand that the process is tedious. I will give it another try, unfortunately, I don’t have the vacuum bleeder, but I’ll give it a try with syringes once more. @arnoux made a valid point that the back of the pistons might still have air bubbles trapped in them. 

Regarding my original question… The master cylinder should be interchangeable as the rear callipers are the same on both master cylinders. So there should be no difference in the length of the stroke or the volume of the brake fluid. 
With the older master cylinder, there is a chance to open the top lid without unbolting the master cylinder itself from the frame. With the new one, there is no chance to do it. Also, for future users/others, the brake hose banjo on the cylinder side is 10mm instead of 8mm. It is no 1-2-1 swap. 

And thank you both @rotors7 @arnoux

[kasper]

It was air, as I don't have a vacuum pump, here are the steps I took, which might be useful for someone else as well:

1) Check the calliper on the bike and ensure it is adequately filled. In my case, it was already almost full.
2) Remove the bleeding screw.
3) Take off the hose banjo bolt. It's a good idea to have two new copper washers (10mm) ready.
4) Find a thin and soft wire and use it to stir and disturb the screw holes as much as possible. You can also tap the calliper gently. This will help the air escape from the piston and other cavities. Make sure that the oil level is clearly visible from the banjo bolt hole the whole time.
5) Put everything back together.
6) Next, use a syringe to slowly backfill the entire system from the bleeding screw. The goal is to push the air into the master cylinder, so take your time and do it as slowly as possible.