I'm not an expert on the subject but the valve they are talking about is the proportioning valve which limit the amount of fluid sent to the rear of the vehicle.
The confusing part is why only one wheel is affected. Generally, the prop. valve does not differentiate between the two rear wheels. It'll send the same pressure to both.
One possible issue could be that there are some foreign materials in the line going to the left side brake.
When you say the rear fluid was changed, did you actually bleed the system? Your rear left brake cylinder is the one that is farthest from the master cylinder so that is the wheel one should start with when bleeding the system then followed by the right rear, left front, right front.
Your mechanic is right in saying that they will have to analyze the entire system as it could be any number of things that can cause this.
Besides the above, your wheel cylinders may need to be checked for proper operation, lines need to be inspected, master cylinder need to be inspected etc...
I think it is well worth the money to have the whole system checked by a qualified technician to make sure that you identify the root cause of this issue. Definitely don't drive the car until you get this rectified. Imbalanced brakes can cause all sort of trouble up to and including total loss of control during hard braking.