Interesting this as you did say you had replaced callipers and master cylinder.
I am going to make the slightly rash assumption that all the new parts were "good". and fitted properly, as
@lynall mentioned this may need checking.
If you replaced the callipers, the guide pins should have been replaced at the same time. If not replace these or take them out and clean them up. I had one more or less rust into place, but once taken out and all the rust cleaned off it worked fine. Doubtful guide pins on opposite corners of the vehicle would go at the same time, but not impossible.
It is odd that 2 callipers are doing this. The chances of two flexi hoses delaminating at the same time same very low to me.
EDIT, I'm an idiot did not notice till now that you have replaced 4 brake lines. Did you mean flexis or the solid lines? If the solids than you can ignore the latter part of this post.
I think the fault must lie in the pipes connecting the master cylinder to the two callipers, forming a one way valve.
I don't know because I have never had to investigate this, but they may connect into one somewhere? Saabs used to separate the dual braking system that way, but most manufacturers do it so one line is front brakes and the other is the rears.
Anyway, I'd follow all the brake lines back to the master cylinder, disconnect them from the flexis )and see if there is a blockage of some sort. A bit of a seal or something may have worked its way down from the old master cylinder, into the pipe(s) and be causing the problem.
If you can pump air or fluid in both directions, collecting the old fluid in a jar or something, you may see some offending material come out.
If it is this, it should be a relatively cheap fix. no longer any point in chucking money, by replacing parts, at the problem.
I suppose it is faintly possible that a pipe may have rusted internally and a shard of rust is causing the same problem.
Best of luck with it.
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