Hy coal, It could be that the compensating port in the master cylinder is plugged with something, or the master cylinder is not returning to its full at rest position. When you have the cap off of the master cylinder you should be able to see two ports a larger port to the rear and a much smaller port forward of that, that one is the compensating port, check that port with a bit of fishing line to make sure it is open, hope that helps.
