4l60e issues.
https://ls1tech.com/forums/automatic...en-record.html
So, its just not being rebuilt properly for performance use.
You mention 110 psi in 1000 RPM. More useful would be the pressure in Reverse and/or what pressure you have when you blip the throttle.
When properly built and tuned, a 4L60E can handle a lot of power. However they are also delicate - a small leak caused by careless assembly or even a bad engine sensor and can quickly ruin them. (Probably true for all trans, but perhaps more so for the 4L60E.)
While a Level 7 trans is very expensive, a Level 2 from e.g. RPM or FLT should suit you just fine. Cost is about $1500 plus shipping.
Strange to lose both 1st and Reverse as they don't have that much in common. When the shifter is in D1, the low/reverse clutch is engaged in 1st gear, but not in D4/OD; it is also engaged in Reverse.
Lost reverse is often due to a bad teflon seal on the stator shaft, but that won't affect 1st gear. Same with a broken sun shell.
However, I don't have much real-world experience with broken transmissions. Little reason to try to figure it out anyway as the trans needs to be removed and rebuilt; and the problem should then be obvious.
It might be a problem with the low/reverse roller clutch which should be replaced during a rebuild.






