Midmark 75 Evolution

When it "spills out" is the pump motor running when it happens? Do any of the pistons sink by themselves? Once it stops spilling out, do the pistons still go up and down?

No pistons sink by themselves and the motor is running. The oil cap is tight. After the pump replacement, it seemed to run fine as I was putting the chair through its paces up and down. Then, it started to flow out the top of the new pump motor. Very frustrating.

Ok then...couple more questions, what type of oil are you putting in it, don't say "hydraulic, be specific. And again, once the spilling out stops, do all the pistons still work?

The mIneral oil base that come with the replacement pump. The pistons were all working, but everything stopped.  I can hear the solenoids engaging, but nothing happening.  All the electrical connections are tight and receiving electricity when solenoids are engaged.  Again, everything worked for five minutes after the pump replacement.  I thought I had the problem solved, till the oil bubbled over and everything quit, again.

That is all good information, rules out some things I was thinking. Ok, if the oil level is correct, then it swells and overflows I can only assume that air is being introduced somewhere and forming bubbles. There is an anti-cavitation solenoid on the pump, maybe that would be something to check next. Obviously there is a flow problem, maybe a collapsed hose lining, something causing it to bubble up. Does the motor sound like it's straining when it did work?

It does sound strained.

This would be a good time for other members to chime in with thoughts or ideas!

Possible relief valves or cylinder check valves not holding, anit cavitate valve. Sounds like a valve or multiple valves/check valves could be failing allowing all the oil to be returned all at once instead of regulated as normally and the pum can't keep up with it. Each cylinder has an electric shuttle valve, these could be sticking open. Are all cylinders extended when this happens? What if you just moved one cylinder at a time and see what holds and what doesn't? And you're positive nothing i hooked up backwards?

sounds like your reservoir is overfilled. the cap is designed to "spill out" fluid if there's too much