@Laitch wrote:Do you have the reservoir cover screwed on tightly when checking the brake pressure? Sometimes the mind wanders. 

More of an aimless shuffle really. 
When I tested at the cylinder with my thumb over the banjo outlet I couldn’t resist the pressure, so am assuming that reservoir to banjo is aok. Likewise testing at the caliper end of the brake line, not as conclusive as the olive was leaky when the swivel was unscrewed, but nonetheless, certainly a good flow coming out between the olive and the line.
Writing that makes me wonder if the olive should be leaky when unscrewed, but all the joints stay dry when everything is buttoned back up

If it wasn't for this-

'Follow on from replacing OEM master cylinder with a Chinese one.'

-I'd have said you have a classic case of worn m/c. Re-check all the hose joints because another potential problem is the leak that you can neither see nor feel, but provides pressure on initial test.

just be aware that if the two tubes within the master cylinder feeding the piston  are partially blocked ..(usual cause of such issues )  particularly the rear most tube within the master cylinder ..(called the equalizer tube  for want of a better explanation )...the piston inside the cylinder wont retract under normal conditions ...leaving the pump process wanting , and not being able to be bled


we have seen many cases of the tubes being blocked before ...and amazingly hard to see the issue when disassembled

there is a tiny hole deep in the master cylinder  that does this ......for reference see the front master cylinder and observe the two down holes ...if the front ...if the most forward one of the two become blocked ...action ceases ...as the feed hole wont allow the piston to return fully ///like wise the rear master ....but access to cleaning is restricted by the design of the fluid feed from the reservoir cap to the master cylinder  is a pain in the proverbial ...

a reason why many have searched and installed a cheaper than bmw replacement

good luck

Because the ChiCom master cylinder has the hose from the reservoir attached to the bottom of the cylinder, I had a bit of trouble getting all the air out.  I had to take the footpeg plate off and turn it so the hose was pointing up, and left it hanging overnight to get the last of the air bubbles out.  Sort of like what often need to be done with the front brakes.

The piston is designed for pushing the liquid towards the brake. The rubber lips are tight in the pushing direction but not in the pulling direction, like on a bicycle air pump.

When you bleed the circuit manually, you normally push the piston as far as it can. The spring internal to the master cylinder then brings it back, alone, to its rest position.
That return phase is delicate when bleeding the ks.
The problem is the vacuum that the spring assisted return motion generates. That vacuum is compensated partly by some fluid coming from the tank small hole, and partly by air coming from the front of the master cylinder, the rubber lips acting there as diodes (tight in one direction, let go in the other).
Same phenomenon if the bleeding is made by a too powerful suction.

Solution: bring back the pedal very slowly when you bleed the system. Otherwise, like dozens of other guys, you will pass liters of brake fluid through the not very accessible tank with your pedal remaining very soft.

Once the air bubles are gone at the calipers level and have not been replaced by bubbles entering the master cylinder, the piston return motion does not generate vacuum any more and the problem disappears.

k
Thanks everyone. Lots of useful stuff here. I have a recon. caliper on the way (even tho I have re pistoned and re sealed mine as part of this process).
On my third! six hour day trying to get this sorted. Today I have dismounted the m/c and hung it upside down so i can be sure it’s getting juice; and dismounted the caliper and set it on the ground so it is below the m/c. Also tried two different s/s brake lines. Not a sausage (English for, in this case, SNAFU).
As the s/s lines are new and mine, and the chinese m/c is new and feels like its working based on pressure received at the banjo, I have ordered the recon caliper because that’s the only original component left, but as some of you have said: I have a nagging doubt about the m/c because on the couple of occasions I’ve got some pressure build up it fades almost straight away. All the symptoms say I am losing pressure but I don’t fathom how I can have an air leak without fluid loss (and each element remains bone dry).
So: try again with the replacement caliper. Failing that deploy my spare chicom m/c. Failing that re install my leaky OEM m/c plus original hose.
I’m off on a 4k or so 3 weeker to the Highlands on Monday come hell or high water (back brakes are SO overrated - except when I want to turn the chair right - checking my Garmin route options as I type).

PS. also going to trim and re olive my s/s lines as I have had them off so often this week they probably would benefit from a refresh.

Hi, you need to think of the basics of the brake system. What it does is push a quantity of oil to the brake pistons. These will move (if free :-) ) as far as the volume that is pushed into that system. If no air, as a liquid can't be compressed, the volume stays a constant quantity. With new brake pads less volume displacement is required to have the brake pads 'grip' the disk. Worn pads will require more volume, as the pistons need to move more towards the disk. With all what you have done, and assuming your disk is not below its minimum thickness and the pads relatively good, I wonder if your Chinese m/c can generate enough volume. It's not only the diameter of the plunger, but also its stroke that determines how much volume gets displaced. Not enough is no or little brakes. This is anticipating that, as Charlie mentioned, both channels (the little holes in the bore) are fully open and providing liquid.

Thanks everyone. I let the fact that the mc was new blind me to the obvious.
Swapped in my spare this a.m. and even without bleeding its reasonably effective. Bit of tweaking to do on the push rod length and detente but there is light at the end of the tunnel.
When I get back from my trip I’ll dismantle the broken one, and if I find the issue will post pics.
Thanks again. 

If you're lucky Stu, the o-rings will be under-spec rather than the cylinder being overbored or the piston under-sized. However, the smart money's on one of the non-rubber bits being the wrong size.

BTW: been caught in a similar situation with a Guzzi rear master cylinder. I'd overhauled it twice before with no problems (high mileage bike) so had no reason to think it wouldn't 'go round' again. Symptoms exactly as you describe. Talked to my local Guzzi parts dealer who was brilliant (anyone who would brew me a cup of coffee on a Saturday morning as soon as I walked into the shop had to be alright!) and he told me that they rarely take a third overhaul as the leak this time round was most likely caused by the bore being worn out. Easy way to check? Hold it up to the light and if it was nice and shiny inside, it was screwed because the anodising was worn through.

I could see my face in the reflection...