Originally Posted by FranksCustomTrans

Yes the wrong gasket can cause considerable issues, Been there seen and even done that long ago, Now on the line presure, Yes the pump could be tired but unless it gouged or visably worn it seems unlikely,
Have you checked the TV LIMIT valve spring? Broken, wrong spring,missassembled or sticky valve could cause your presure and shift timing issues . I would look at that first myself.
If you do change the pump I would suggest changing to a 4l60e pump but that is my opinion as i have had great results with that set up both presure and line rise wise, Just be sure and make the hole in the case missing in aux cases for the fwd and block the D2 feed at top right of case "square hole round inside" vb check ball works well.
But to me short of a very badly worn pump or PR valve bore (sonnax does make Oringed boost valves or you can lathe cut in your exisiting for that ) The early shifts and low line point to an issue with the TV LIMIT VALVE or spring or leak in that circuit.
I posted full instruction on changing to the 60e pump in the 700r4 tips and tricks post by Marroon monster. (NOTE THE 60E PUMP CAN BE ANY STYLE OR YEAR AS LONG AS SHORT STATOR PWM OR NOT MAKES NO DIFFERENCE)

If it was ME. I would, if i pulled unit, Change out to use a 4L60E pump with a .500 boost valve and the small spring I have shown in my post before, I would verify the TV LIMIT valve and spring and modify it to allow removal and install without removing VB and or change the VB all together with a known virgin one I then modified blocking the 3-2 , Set feed sizes suitable to what you are looking to acheive shift wise. I would probaby ditch the TG stuff as I dont really care much for it personally (GM CALIBRATION OVER ALL AND FUNCTION WORKS PRETTY WELL IMO) . Use the sonnax TV spring in the throttle valve along with a solid spring to pin the throttle valve at WOT . Install a very stiff spring or block motion of the line bias valve.

Just my unsolicted thoughts on the matter, Note with the 4L6OE PUMP SET up as I have previoulsy described i have never seen less than 200 psi at FULL TV ussually in fact closer to 230 PSI, The only down side I have found is N-D engagement is a little harsh with stock stalls but just fine with stalls over 2500.

Also note quick easy way to raise overall shift point across board, remove GOV gear you will see a pocket in the valve, Place spring here , Make sure it fits pockek and fully compresses into it . You can find three different stiffness that will work here in a harbor freight assorted spring kit but can find even more with other sources, The stiffer the spring the later the over all shifts , I use this to fine tune over all shifts works well overall and is quick n easy unlike grinding or changing weights and from experience keeps the stock overall shift spacing and kick down functions.

Originally Posted by dixiebandit69

Thank you for your insight, Frank.

I used the Sonnax boost valve and spring, and used the Transgo TV valve/ spring setup (shown here).

Your idea of using a 4L60E pump is great, but if I'm going to pull this unit out, I'm ditching the TH700 entirely.

I've been thinking about grabbing the ECM and wiring harness off a GMT400 pickup, and installing a 4L60E. I have a 4L60E out of a '93 1500 that needs to be gone through.

In the meantime, I'm just going to keep driving this one until it craps out. This was only supposed to be a temporary vehicle anyway.

I'll try that Governor spring mod you mentioned, though.

Originally Posted by dixiebandit69

I also blocked the 3-2 downshift valve like @vorteciroc said, but since I didn't have an appropriately sized cup-plug, I peened a nut to go in its place.
IT WORKED WONDERFULLY!

Originally Posted by vorteciroc

I would however absolutely avoid using a PCM prior to 1996...
(The processing power is severely lacking in regard to control of Line-Pressure, and is why 4L60E Vacuum Modulators were once popular)
(Line-Pressure Rise and Accuracy are rather TERRIBLE!)
Ideally use something from 2000+ for a PCM.

Originally Posted by dixiebandit69

Okay, thank you for the advice, but what GM vehicle was still using TBI in the early '00s? I'm guessing some medium duty trucks?

Originally Posted by vorteciroc

Meh... Not for TBI, No.

You'll be fine though unless you really need a High-Performance Transmission in this particular Vehicle.
Stock and Mild Performance will be fine with the Normal Hydraulic Upgrades...
Line-Pressure can be slow to rise and recover though.