Walk in freezer pressure at condensing unit with zero degree box"

The compressor will normally be 100 to 110 degrees. I would suspect the thermostat or mouse damage in the freezer area. Also, if you keep it overstocked, to the point of poor air circulation, you could just have ice blockage. Put the frozen food in a styrofoam cooler or a good heavy corrugated box and give the unit a good full defrosting.

The condenser is the 'radiator' located under the unit near the compressor. If it's clogged it will cause this. Make sure, also, that the fan at the condenser is operating.

If you've not yet cleaned the condenser, do so with a vacuum both sucking, and then blowing, to clean it well. BE READY FOR A DUSTY MESS!!

why yes there is. Generally speaking, the head pressure should be around the "ambient +30" rule.
That is, measure the entering air temp to the condenser, say 75 degrees. Now add 30 to that and get 105 degrees. Look at a P/T chart and see what the pressure is for that temp which is about 253 for R-404-A.
The evap on a freezer leads the load so, as a general rule, the evap temp is going to be around minus 10 degrees to get a 0 degree box. Coolers are different. So, the pressure for a minus 10 evap is around 25 or so. But the unit has to be close to operating temp. If the box is warm, naturally the pressure is higher. And what influences the pressure is whether or not it's a TXV or a cap tube system.
If a TXV and no receiver, charge by subcooling. If a receiver, fill by sight glass. If Cap tube, charge by superheat.
Hope that answers your question.

When they replaced the evaporator coil they had to re-charge the unit. Sounds like there may be a small leak.

Head pressure should be around 250#.
Be sure condenser is clean. Use a good foaming cleaner.

refrigeration systems are sealed. The only time you add refrigerant is when there is a leak. Maintenance wise, need to keep condenser and evap coils as clean as possible. Make sure all electrical connections are good and tight. Make sure that the compressor and/or condensing unit has good air circulation around it.
Hope this helps ya out.

I am assuming you have some refrigeration smarts. As a general rule the evap in a freezer is 10 degrees low than the box temp. Take the pressure of the low side then look on a P/T chart for that refrigerant and determine if it is where it needs to be. Usually somewhere around -10 to-15 degrees for a zero or colder box.

You have to defrost the coil by turnning of the unit and to make it faster leave the inside fan/s running.But if it's building up ice and the fan/s inside works ,is most probably that the unit is low in refrigerant(freon) , or the defrost system is not working.

The pressures depend, somewhat, on the expansion device,i.e. a TXV or Cap Tube. Take a pressure reading, then convert it to a temperature. The evap temp should be right at 10 colder than the box temp. Remember, heat flows to cold so the evap has to "Lead the Load" in order to cool. In many cases, the condensing pressure is usually ambient temp, plus 30, then convert to a pressure. These are general accepted practices that have been used for many years.

Hope this helps

Check out "victory-refrig.com" for manuals, diagrams, etc.
Cap tube systems are "critical charge systems". Can be difficult to get right. You did not mention pressures that you had. The split on the evap vs.box temp is almost always 10 degrees for freezers. If you want a -10 box, your evap needs to be -20. As a general rule of thumb, you want the high side to be "ambient +30". This seems to work most every time. Your problem might be operations related. Them leaving the door open alot and so on. It could also be a problem with the evap fan or the control to it. It could also be that since there had been a problem with the unit, they are now constantly checking it. Could be checking during a defrost cycle. When your box is at 0 degrees, your pressure should be about a -10 and no less that a -15 degree for the evap. You may have to babysit this thing and see how far it will pull down. You may have a weak compressor. Also, I have had many Hobart freezers fail due to dirty condensers. This causes the oil in cap tube systems to gel and clog up the cap tubes. The only fix is replacing the cap tube. There are alot of variables here so be patient, it's not going to be a quick fix. One last thing, since R404A can fractionate, you really need to pull the charge out and weigh in the correct charge with virgin refrigerant. This will eliminate 2 things. 1, it will ensure the correct amount of refrigerant and, 2 no chance of having a poor mixture of refrigerant in the system causing inconsistant readings and performance.
Hope this helps. And remember, cap tube systems are performance measured by the amount of superheat just prior to the unit making temp and cutting off.