Z axis sinking

We have a 1986 Morton Mill with fanuc servos and drives. (30M type). The Z axis is now sinking every time the power is turned off or the
E-Stop button is pressed. (Used to happen occasionally, now every time.) This happens even if the machine has been sitting at the same Z height for a long time. The machine has a tracing head with a dead weight counterbalance, a hydraulic counterbalance, and it would seem a Z brake on the servo motor. (At least the controller has a Z brake relay and the relay is powered and working.) I can still see the dead weight so I believe I can rule that out but I don't know whether the Z brake is never engaging (stuck open?) or whether the hydraulic CB leaks? Does it seem like the Z brake should be able to hold the head in position whether the hydraulic CB is there or not? I recognize any sort of valve that would hold the pressure on the CB w/o the pump running... Actually when the unit is sinking I can see MORE pressure on the CB than when the machine was running. It runs at about 400psi when running and goes up to about 550psi when it's sinking. Until it hits something, then the pressure bleeds off...
I really would like any input that people could provide on how it would be best to proceed in diagnosing this problem. Thanks.
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Not familiar with Morton mills but sounds like maybe a portion of the brake system is worn out or needs adjustment I'd look around for a hyd actuator and associated hardware that's similar to automotive disc brakes--On hyd pressure, the piston is pushed away from the disc allowing free movement but as soon as hyd is off then a set of belleville actuates the piston, stopping the thing from falling more than a few thousandths...often you can grind or shim someplace at disc or actuator cylinder get it back to adjustment.
ALWAYS USE A 4X4 LUMBER OR SOMESUCH SO AS TO SOLIDLY BLOCK THE HEAD IN THE UP POSITION BEFORE MESSING AROUND WITH BRAKES
--




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CK,
30M motors have an internal diode bridge that can be a weak point in the brake mechanism. If the bridge opens up, the brake drags and wears out. Someone needs to get a strap wrench on the ballscrew and determine if the brake is working. If the counterbalance gets in the way, you might need to remove the motor to make the test. AS mentioned above, BE CAREFUL!!
Fanuc sells replacement brakes but they are not cheap.
Warren Uptime Electronics, Inc.
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Warren wrote:

The brake isn't working, I can promise you that. We had the motor off when we were initially diagnosing the problem and I can spin the gear on the end of the servo by hand. From the sound of it I should need 6 people and a mule to spin the servo when it's off.
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The problem is the brake. I maintain a Chiron that eats brakes every 2-3 years. As far as the sitting long time. The machine is smart enough to say, hey, I've been waiting a long time. Put the brake on, kill the servo and quit wasting money (energy) holding dead weight.
Make sure the brake release voltage is proper so that you are not prematurely wearing out the brake.
Wes
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