Denford Software & Machines

I'm learning to program with VR Milling V5 and driving my VMC 1300. I've got to the point where I want to make a manual toolchange during a program.
I'm mostly using "G and M Programming for CNC Milling Machines" as my reference.
When I complete the moves with first tool, I programmed the toolchange with
M05
G91 G28 X0 Y0 Z50
M0 T06
similar to page 9 example
When the VMC gets to the G28 line, the machine starts to go up in Z, but makes a really bad noise and stops moving without going home.
I can just do the M0 T06 line without the preceding G28 line, but that makes a non-vectored return home which could crash
I tried variants of the G28 line like G91 G28 X0 Y0 Z0 without any joy in homing.
Of course I can G91 G00 X0 Y0 Z50 then M0 T06, but I'd like to know what I'm missing on G28

I'm now working on the theory that the noise I hear is due to the Z Axis brake still being on when the machine tries to home. I am experiencing this problem when I try to do a toolchange without a prior G28 too. I can see the Z Axis brake state on the Mint i/o debug screen as Output 15, so I'm going to step through some programs and watch the Z Axis state.

Having spent some time over the holiday, I can confirm the problem is with the Z axis brake, so not a software issue. I suspect the Z brake solenoid is starved of power when the 3 axis drives all start up on rapid, and the solenoid lets go and the brake springs on. I'll start a new thread in the VMC 1300 place.