I have a 1940 1-1/2 ton panel fire engine with the heavy duty non-synchro truck transmission with a PTO to drive the fire pump. It started to grind gears a lot this year while shifting from 2nd into 3rd and 3rd into 4th. Yes, I am properly double clutching. The grinding occurs while trying to move the shifter into the higher gear with the clutch fully depressed.

THINGS I HAVE DONE OR KNOW:
1. Adjusted the clutch from the 2" free play to about 1". Afterward the grinding remained while shifting from 2nd into 3rd, but disappeared when shifting from 3rd into 4th.
2. Did not find any obvious mechanical issues in the external clutch system.
3. Transmission/PTO leaks gear oil. The floor drip pan has collected a fair amount of liquid. It appears to leak even while parked. I do not think it is just dripping off the transmission/PTO.
4. I have not found a good way to access the fill plug as the PTO shifter mechanism is right in front of it.
5. I took the top off the transmission.
6. Gear oil level is 1/2" to 1" low. It is difficult to determine more accurately than that.

HISTORY
I do not drive the panel very much. A mechanic installed good internals from a spare transmission in 2014. The reason was that the transmission jumped out of 3rd gear when I bought the fire engine in 2014. He used all new bearings, seals, gaskets, and clutch components. I have driven about 700 miles and operated the fire pump about 25 hours since then.

QUESTIONS
Are there any defects visible in the pictures?
Does the clutch need further adjustment? I just looked in the "Ford 1940 Truck Reference Book" which gives a free play spec of 1-1/2" to 1-3/4".
Is it possible that the low level of gear oil could contribute to the grinding? Should I drain the transmission and fill with a product such as StaLube 85W90 GL-4? The Ford book specifies S.A.E. viscosity 90 for winter and S.A.E. viscosity 140 for summer. StaLube makes the 85W90 and also 140 in GL-4.
All thoughts are very welcome. TIA

Don't know if they help, but here are a couple of pictures.



This is the fire engine pumping from the Genesee River in Rochester, NY from 2022:

 

i would think its the clutch . try removing most of the slack .. it will turn the clutch bearing if no slack ,but just for a test . you say it was better when you passed from 2 inch too 1 , so maybe theres slope in the pivots. try removing more all you want is for the clutch bearing not to spin .by the way you got quit a cool panel !!!

 

i just this year had a like wise clutch issue in a 33 woody , in these old fords the to bearing cross shaft is known to wear, along with both bushings that support it in the trans housing, the to, forks use a single tapered pin into the shaft that cracks. with some aid from van pelts parts ,we found it was best to replace the shaft, bushings. pins, anything that can wear with new,