Couple of things to look at:

Your "reco" calipers (remove) them from the disc and remove the pads to check the following:

  • Are the slides moving freely/well lubed/not bent (actually quite common), and is the dust boot on the piston not stuck behind one of the pads?
  • Did you buy them from a reputable brake specific workshop/outlet? I have seen bodgy mechanics chuck a round section o-ring in as the piston seal - it doesn't work as the installed seal needs to be a square section to let the pads release properly from the disc.
  • Are the threaded fittings on the master cylinder the same size and/or could they have been swapped at any stage?
  • Is there any tool damage to the flexible brake line? They have a plastic sleeve in them and are not designed to be crushed off with vice grips, pliers etc to stop the flow of fluid.
  • Are the caliper attach bolts tight and are they the right bolts (not too long and hitting the inside of the disc)?
  • Are the pads actually sitting properly?
  • Check all of the metal lines for any form of crush damage (like the flexible lines) where fluid can get pushed under pressure in one direction easily but not back the other way when not under pressure.


There is other stuff to look at (such as the part number for the master cylinder) as a PBR drum/drum master cyliner does look the same as a disc/drum one (the drum fitting of any master cylinder will hold a small amount of line pressure on that part of the system to hold the linings close to the drum so that you keep a good brake pedal), however am not sure of the fitting sizes of the top of my head (20+ years since having anything to do with them).

That is a pretty good start, let us know how you go....

Dave.

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Disc brakes do run very "close". especially just after being done-up/repaired. This might be what you're seeing. Check the disc for warpage/run-out too.