Having trouble with Master Cylinder - how to test.

[LTGTR]

I recently fitted a 14mm front Master Cylinder I purchased from Mikes.
Its for the twin discs on my 77D.
It worked okay for a couple of weeks (bled real easy) even though to my surprise it didnt feel much better than the original (still wooden).
The other day the lever sunk all the way to the handle bars. Trying to bleed it was impossible and it wouldnt even let any fluid gravity feed to the nipples.
I checked the compensation port wasnt blocked and when I tried to bench test it, with my finger over the outlet hole, there is no pressure to blow my finger off or squirt fluid out.
Am I testing it correctly ? If not, how do I prove the unit is deffective before going back to Mikes.
The twin discs have been a pain in the arse for all the reasons explained on this forum, that I am well aware of - I only kept them for Australian delivered originality which is looking like a fuckwit thing to do.
Another question - if I prove this MC is rooted and its going to cost me, I will take the opportunity to revert to single disc - will a 13mm MC be okay or do I have to have 1/2 inch (12.5)mm.
Thanks for any help - Ray.

 

[Max Midnight]

If it won't pump fluid then that would seem to suggest your m/c is Donald Ducked?

As there is only a difference of some four percent difference between 13 and 12.5mm m/c pistons area I doubt that you will notice much difference although the smaller one will increase the lever ratio a bit.

If the lever feels wooden then that would suggest that you need to increase the lever ratio.
To establish what would be the correct size you would need to work out the total area of the pistons in the callipers and divide it by the area of the m/c piston. Aim for around 24 to 27:1, depending on what sort of feel you like.

Have a look here

 

[arcticXS]

Another question - if I prove this MC is rooted and its going to cost me, I will take the opportunity to revert to single disc - will a 13mm MC be okay or do I have to have 1/2 inch (12.5)mm.
Thanks for any help - Ray.

I have a Euro 77, that also came with 2 discs. Unfortunately, the discs are smaller than the US market single disc, if I understand this correctly, 267 mm vs 298 mm. And the 77 and newer caliper is a rather cheap and poor setup, even compared to the 72-76 caliper, one 38 mm piston vs two 48 mm pistons. And it will need a 9-10 mm MC to give a reasonable ratio with a single disc setup.
My advise is to do what I did, find a nice, solid cardboard box, clean your stock discs, calipers, fasteners, brake hoses and master cylinder, then put everything in that box, and put it on the shelf. Then get a single 298-330 mm Yamaha or Ducati disc, a Brembo 4 pot caliper, and a Brembo 13 mm master cylinder. Mock it all up, make the required disc spacer and caliper adapter bracket. It's not that hard.
Alternatively, fit a front end and brakes from a 76, 75 or even a. TX750. They all have the better calipers.