I was scratching my head as to exactly how more fluid gets from reservoir into circuit, when you release the lever, preventing the caliper pistons from getting "sucked back in" (actually pushed back in by atmospheric pressure). It's simply that the first seal on the master cylinder piston allows fluid through when it goes backwards. Good explanation here: https://www.helperformance.com/brake-doctor-master-cylinders If the new seal is wrong in some way, or the little holes behind it blocked or obscured, that could be the problem.
Recidivist I think we are both in agreement on this. Mc seals/bore or blockage in orifices. Any known working Master and reservoir connected to the line or lines would be the easiest way of checking. At a loss now for any other causes without an actual leak.
Don’t worry I will update soon! Busy old time between work and our 11 month old at the moment but hopefully get some garage time again soon.
Got an hour or so at it tonight, I think (tentatively) that I might have found the problem! Pulled the calipers to bits and it took serious pressure to blow the pistons out, compared the seals to some fresh ones and they’re significantly bigger. Have cleaned everything up again and will try the new seals over the weekend. Really hope this is it!
My post 16. Hope you sort this. Pistons not free moving, wrong seals. Great learning curve his one! Good luck, let us know
Boom! I have brakes. Still not perfect, there's probably still a bit of air in there as there's a fair bit of lever travel but ultimately everything appears to be working correctly. Thanks for all the pointers as always! The only bugger now is the bike has sat for so long it won't start
