GM trans rebuild, low pressure and slipping in R and 1ST related?

Third gear started slipping in my 93 Pontiac Bonneville. The car has
about 146000 miles. I decided to fix the trans while the problem was
still minor.

Sure enough the third clutch showed signs of burning and for unknown
reasons the clutch plates with splines on the inside had all the splines
broken off.

Trying to follow the rebuild manual (ATSG 4T60E) as closely as possible
I got the trans back together and installed. The test drive revealed
that reverse and 1st were slipping, not so much as to make the car
undriveable, in fact it seemed to shift at the right points and did not
seem to slip in the other gears. But it was slipping enough that it had
trouble getting up a steep driveway.

From the manual it seemed that my problem was with the input clutch.
After some thought I figured that I might have put one of the input
clutch seals in backwards. Out and apart comes the trans. Seals in the
right way. Back together again and back in the car, same problem.

Now I do what the manual wanted me to do first. A check of the trans
fluid pressure finds that it varies from 60 psi at idle and goes up to
70 psi at higher engine speed. Changing the gear, or disconnecting the
vacuum modulator does not change the pressure from the above range.

Question, would 75 psi be enough pressure to get the car to move forward
but to slip in 1st gear?

Question, from the manual it appears that pressure is regulated in part
in the valve body, would you guess I have a sticking valve? I did take
apart the most of the valve body but some parts (pressure regulator)
would not come apart. I think the line boost valve bushing retainer
deformed the bore enough so that the parts could not be easily removed.

I am ready to give this trans one more weekend, do I have any hope of
fixing this? Any ideas what is causing the slipping?

Thank you for any help.