I just got a 88 Suburban 4X4 with 12K miles on a GM
re-manufactured 700R4 (1993 model). It was installed in 2002 and then sat for 2 years. All forward shifts are great,
even pulling a boat, but it slips in reverse over 3-5 mph without the boat and much worse with it. I changed the fluid
and filter and cleaned and lubed the low reverse value but no
change. Anyone have any ideas on what I can do next???

I am curious, what are you calling : "cleaned and lubed the low reverse value "... or even if that is a typo.... what are you calling the low/reverse valve?

Does the tranny have good engine braking in manual low gear?

Accelerate from a stop in first gear with shifter in manual low gear.... at about 25 mph let completely OFF the gas pedal. Does the engine braking happen?

Or does the the vehicle continue to travel forward and the engine drops to near idle

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I'm sorry..."reverse boost valve and sleeve in pump cover" I've heard they can stick and cause problems but it checked
out OK. But you are right...braking in manual 1st gear is very different from my 82 Chevy PU 700R4...Its clearly NOT
braking like it should... What does this mean???

poor engine braking in manual 1st gear indicates there is a problem with the low/reverse clutch pack at the rear of the main case.

you could run a pressure check of the tranny to see if a problem is indicated
by low pressure.

0-300 psi guage hooked to the pressure port on the drivers side just above the shift lever bolted to the tranny

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ran the pressure test and got :
120 psi idle in park
160 psi in D @ 2000 rpm
180 psi in R @ 2000 rpm and got a severe pulse/ vibration in pressure gauge hose...it would almost shake out of my hand. At idle and at any rpm in reverse but not in any other gear...what does this mean???

Can i rebuild a 700R
then put on a engine test stand and hook up pressure gauge to tell if everything is good before i install in vehicle.
I seen it by a d.i.y. trans tech and wanted to know.....
thanks j.
I just hate to r and r twice

the pressures are too high at idle. 180 psi in revrse with a large pulsing indicates a problem to me. Slight fluctuations are normal.

You need to scan the PCM as the vehicle is operating ( live data stream) to look for abnormal inputs

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Save the earth , choke a tree .... save a squirrel