VTC 20B - Pneumatic Counterbalance

Yesterday our VTC 20B had been running all day without an issue. A job was changed over and the machine was started, As it moved Z in rapid we got a Z axis excess error. After examining the problem in idle the Z axis is sat there at 80% load. It is never normally that high. You can move the axis around in hand mode or under 50% rapids, any thing more and the load goes to over 150% and alarms out.

Having spoken to Mazak they are certain that it is the Z axis electromagnetic brake on the servo motor that is playing up. Getting stuck on. This in my mind doesn't make sense, if the brake was stuck then wouldn't the load reading be low and only spike when the axis was moved? Has anyone had any experience with the pneumatic counterbalance on these machines? If this has leaked or lost it pressure could this cause the issues I am seeing. I have pulled the motor off the Z axis and this is going for testing tomorrow. The head is currently sat on a pile of wood on the table, if the counterbalance was pressurized correctly should I expect to be able to get some movement by trying to lift it by hand(I currently can't move it a mm).

Has anyone had any experience of pulling these out, rebuilding and pressurizing them?

Thanks

Will