As it is today, radios and electronic instruments for the most part were LRUs. If an aircraft had a bad radio to was removed and replaced either by the normal crew or with assistance from techs who worked in a radio shop. On a "line maintenance" unit (squadron) the crewchief and his mechanics accomplished maintenance and did repairs but were limited on how in depth they would go. As you mentioned like instruments, more complex components (like engines) were usually rebuilt at an intermediate or depo level maintenance unit.
The intent is for the mechanic to get an understanding on how the item worked. As far as the chemicals -
When the Crew Chief got the Form 4 filled out and noted instrument or radio or hydraulics or electrical issue, he parceled out the problem to the specialists. For example he might spot some oil/hydraulic streaks on prop spinner, requiring a look see at pitch controls, etc.
In addition the Electronics tech installed or changed the radio frequencies every day to conform to the frag order for 'C" Channel (fighter to bomber communication - tagged to the same radio frequency for that specific Bomb Division they were tasked to escort), as well as adjust volume.
At the end of every mission the routine drills were to conduct a pre-flight inspection to see if any issues were noted beyond the Form 4, the a/c would be refueled and the wing tanks installed and fueled (subject to change when tomorrow's frag order was deciphered - after May it was 'usual' to install the 110's. Oxygen bottle was replaced and flow tested, pee lines flushed, seat fasteners/bolts inspected.
During D-Day campaign the racks were left free as the flight line really didn't know if the mission called for bombs, combination bomb/75 gallon wing tank, or 110 pair for long range escort). Oil, hydraulic fluids and ammo replaced, mag checks conducted, air pressure checks for tires, inspect glass elbow connections from external tank to wing fuel line, etc.
The individual Flight and Squadron Line Chiefs would conduct a post mission inspection and talk to each of the crews.. sometimes the dialogue would flow to whether an Engine change should be scheduled for a high time engine or whether a radiator/oil cooler flush.. sooner or later a particular aging beauty would be down graded to War Weary and removed from combat ops.
If necessary plug replacements and run up were conducted before leaving the line.
An engine/wing/repair/patch job went to Service Group Hanger. Installing a Malcolm hood originally was installed at a Wing Service Group but later deployed to the Fighter Group Service Group at the base. The Engineering officer for each squadron worked with the Flight crews to dispose of Form 4 issues and post status of all squadron a/c on the bulletinboard and send report to Squadron and Group Ops for next day planning purposes. Ditto any issues with pilot status.
The Engineering Officer co-ordinated closely with the Squadron Line Chief to review ammunition feed/stoppage issues, scheduling bore sighting for recently acquired Fighters, RTD ships after crash landing and/or wing replacements .
On the actual morning of the mission, the Crew Chief and Asst CC and Armorer would get the mission profile from the Line Chief a couple of hours before Start Engine time and head for their fighter. They would note whether the mission called for a change in external load. If mission was a fighter bomber sortie the ordnance operation would bring bombs on trolley's and the fuel trucks would upload the previously loaded fuel in the 110/75 gallon wing tanks and the tanks would be off loaded to replace with the bombs - ditto if swapping 110 for 75's. Conduct pre-flight inspection and engine/systems check out to look at mags, oxygen flow, instruments. They would touch up any spots or dew or snow or ice in the windshield/canopy.
When pilot arrived the CC would go over the routine, help the pilot put on his chute and check his straps. At SE time the CC would usually hop on the left wing and help the pilot (extra eyes) taxi out of revetment and onto the taxi way until they got to the active - then hop off and get clear of runway until all ships were off. He would join the rest of the crew and head for chow line for breakfast - then sweat the mission out until his pilot returned (with HIS ship).
There is more but I have run out of memory cells for the moment..