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Hello, Welcome to Just Answer
So far it sounds like the compressor suction valves are going bad,
You can do this test, close the liquid line service valve at the reciever, and hold the systems pressure switch closed or force the compressor contactor in manually so the compressor will keep running, Then tell me how low the suction pressure goes,
If the compressor is good, it will pull into a vacuum
If not, then the valves are bad,
We can go from there with coaching on how to fix it after you get that test done,
Hello and welcome to just answer, I am servtech50 and will try to assist you today.
My first question to you is, was the evaporator and compressor sized properly for the load or area of the box? If this walk-in has been working properly for a long time then that is not an issue. (I am not sure if you are building a walk-in or repairing it). If this was a working system then I would check the TXV. 25psi suction pressure is equal to 21 to 22 deg. in relationship to a PT chart. (pressure = temp.) and if your suction line temp. is 42 deg. at the sensing bulb of the TXV then you have 21 to 20 deg. superheat. which means the evap. is starving. You should have a 10 to 12 deg superheat at sensing bulb. Sub-cool should be anywhere from 18 to 30 deg. depending on your ambient, if you have condenser fan control or a head pressure control (head master) for cold weather operation. So if you are building a cooler first make sure your evap and compressor are sized properly for the load. (for medium temp and your load your compressor and evap. should be sized for a 25deg evap temp.) Make sure you are using a TXV for 134a and feeds at the proper rate to achieve a 25deg. evap temp. and a 10 to 12 deg. superheat. other words if you are using a 1/2 horse compressor (6000 btu) then your TXV must be a 1/2 ton expansion valve not a 1/4 ton or a 3/4 ton expansion valve (one would starve the evap and the other would flood the evap. in either case you would not be able to adjust properly). Do not worry about the sub-cool at this point. your 11 deg. sub cool temp. is due to your condenser coil being flooded due to the starving of the evap. And after reading all of this you could also be just low on refrigerant. Does your system have a receiver and a sight glass? There should not be any bubbles in sight glass with 134a. But according to your head pressure of 120 psi the system should be full, unless you do have a headmaster control then it could be holding back the refrigerant.