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For transmission pump output test, as TxJim said, is best/easiest with a flow tester (part of what John Deere calls a "hydraulic test unit".
This test is done with hydraulic fluid at operating temp, 120-140° F. Spec is 6gpm at or before 2500rpm. This equal 1.5gal in 15 seconds.
John Deere has you connect the test unit to the plug for the filter bypass (not all tractors have the ported filter cover)
BUT since you do not have this tester, next best is "controlled pump into a bucket"
FIRST I WILL SAY I'VE NEVER DONE THIS TEST!!!. If I were to do the test, here is how I would do it:
1) Install a hose with a 1/4 turn ball valve to the ported filter cover
2) Have a clean LARGE 5 gallon bucket, a helper, a way to time 15 seconds
3) Warm hydraulics
4) Have helper set rpms to 2100
5) Have helper signal GO and 15 seconds later STOP
6) Open ball valve at GO, close at STOP
7) Measure result, multiply be 4 for GPM.

If you just pull the hose off the filter cover, the fluid in the cover will drain out, you need to get the tractor started, up[ to rpm and shutdown in 15 seconds. I doubt the result will be accurate. I do think it will be messy.
 
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Before you try and test transmission pump output, I would like you to try something else.


Isolate the loader. To do this, connect the loader inlet hose to the front pump outlet line where they connect at C19281 ( part you used to replace the tee) . This will take the rockshaft and rear SCV out of the picture.
If all is well you have narrowed it down to the rockshaft or rear SCV.
If you still have the problem, it's narrowed down to the transmission pump (now test it), the front pump, or something related to the loader.

I am leaning towards the rockshaft being the problem. It could have a defective thermal relief valve or the rockshaft valves are not correctly adjusted, bypassing creating heat.

I think heat is what is causing everything to quit after 30 minutes.
 
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I agree with both of Zebrafive's replies EXCEPT I WAS thinking when testing trans pump GPM tech manual cautions not to close ball valve completely.
 

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I agree with both of Zebrafive's replies EXCEPT I WAS thinking when testing trans pump GPM tech manual cautions not to close ball valve completely.
TxJim,
I was not saying to do this with the JD hydraulic test unit (flowmeter).

Just a hose from the ported filter cover with a ball valve on the hose end. Closing a ball valve at the end of this hose would only send fluid to front pump, same as if the filter cover was not ported.

I am guessing you have used the JD hydraulic test unit, or have a manual for one. I have not used one or seen it's manual.

My JD 2030 TM only states use JD test unit (flowmeter) to test gpm of trans pump.
 
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