One of my auto shop employees, who is also a machinist, suggested tiny dowels which we just ordered. They are 0.093 diameter and 0.125 long. Drilling the holes a bit bigger, I think they will fit and can be pressed in. I was reminded these plugs need to withstand line pressure.

@V8fan I now understand your suggestion after disassembling a spare pump. Also non-trivial as you have to press a plug into an inner passage and the only room you have is through a 3/16" passage after drilling out the plug near #237. My plan A will be to plug the two holes in the boost valve sleeve with the tiny dowels we are getting. Your pump modification will be plan B. Thank you!

 

Air Compressor at 300 is not the same behavior as oil at 300+
But sure you can try if you want

FWD is also statically applied and not dynamically applied. Which contributes greatly to its level of distress

Simply put...you're messing with something you appear to not fully understand and from those of us who have messed with it before...you're asking for more trouble than anything else. And what you think is there to be gained...isn't really there

But hey if you wanna break some stuff in the name of tuition...by all means

 

Pressure is pressure
If leakage not so great to lowering it or source of air can produce enough flow to maintain pressure - resulting force will be equal
I can make oil pump to achieve such pressure -just don't want to at this moment

I thought this is the way how knowledge earned and learned

 

SUCCESS!
I blocked the reverse feed holes in the boost valve with tiny dowels, inserted back into the pump and just tested it with a pressure gauge.
At idle I have 75 psi of line pressure. (Cold start, 65F.)
Moving into Reverse, I see a brief jump to 100 psi, but then settles back down to 75 psi. Engine has a hiccup but does not stall. (Not sure if the hiccup comes first or the 100 psi spike, but previous Reverse pressure was about 180 psi.)
In Drive at idle pressure remains at 75 psi.
To ensure I reassembled the boost valve correctly, I blip the throttle and pressure jumps to 150+ psi.
Overall, this seems to be a success. I may not be able to drive the steep hills of San Francisco in reverse, but I only need to crawl in reverse on flat roads.

In detail:
* I purchased a new Sonnax 4L60E-LB2 on which to make the mods. I also had a spare stock boost valve for experimentation.
* We purchased an assortment of tiny dowel pins from McMaster-Carr in Aurora OH. Determined that their #98381A180 dowel pin of 3/32 diameter and 3/32 length worked best. (Bought 4.)
* Determined that a 0.089 drill (not a basic size) worked best by first testing on the spare stock boost valve. Ran the drill back a forth a few times to open the hole a bit more and clean it.
* The dowel has a tiny bevel. Maneuvering this tiny dowel was difficult. My shop machinist stuck the dowel onto a channel lock plier with some RTV and that worked like a charm. Moderate pressure pressed the dowel in via bevel side. Checked that the dowel did not protrude into the inner area nor past the outer lip of the boost valve. The second one was a bit more difficult because I didn't want to press against the first one while pressing in the 2nd. A flat piece of steel helped.

This picture shows the result of testing on the spare and then the actual mods to the Sonnax boost valve. It also shows how small and short the dowel is.



* I then verified that the dowel did not stick out too far with a machinist ruler and by inserting into a spare pump I have on hand.
* As suggested above, I filed a flat on the larger diameter part of the inner valve to exhaust any reverse oil that got through.
* I already had the trans draining for a few days; about 9 quarts came out. Fortunate that I have an aftermarket pan with drain plug.
* Getting the C-clip off the boost valve was a bitch as even completely compressed with the appropriate snap-ring pliers it didn't want to clear the opening. Finally compressed it with pliers, stuck a long small screwdriver in and bent the C-clip which then came out with extended needle nose pliers. The same needle nose pliers then removed the boost valve, also a Sonnax LB2. I set the boost valve, inner valve and spring aside as I had new parts.
* Inserting the modified boost valve from underneath was a challenge as I wanted to be sure both springs were centered correctly. While fiddling, the pressure control valve finally fell out too. I then used a lot of Vaseline (couldn't find TransGel) to hold the pressure control valve and the outer spring in place. Then pushed the boost valve and inner spring up and in. Fortunately I had a spare C-clip and it went in easily.
* Cleaned the cast aluminum pan and the gasket and re-installed; torqued bolts to 10 lb/ft.
* Reused the Dex-VI fluid as it only had maybe 2000 miles on it.
* Installed pressure gauge on the left side pressure port.
* Started it up to test and report results.

 

Would be interesting just for a test to give a hard hit to it in reverse to see if any adverse affect. I do not think they would be but just for verificaton, Did you note any delay in P-R engagement ?

 

I am still very interested in this as some day I hope to have a dyno for testing. But for my idea of a controlled bleed to the rev boost with 3rd clutch oil to raise the line 10-20 pounds average at cruising speeds in 3 and 4 only . Mostly for my HD builds as unlike performance guys who could just do it in tune.Many of my HD guys towing etc dont have that access and would be nice to add it Mechanically / Hydraulically for them and in such a way as rather than happening during the shift making then harsh , Happen after the shift, But would take a dyno to practically cailibrate such a thing.

 

Correct and adjustable.

 

Took the car for a short road test today with a trans pressure gauge taped to the windshield. Wonderful (!!!) that I could reverse it out of the garage without it stalling a bunch of times.
The line pressure stayed at 75 psi when idling in Park, Reverse or Drive.
I gave a little throttle in Reverse and it took off with no apparent slippage or delays. I guess 75 psi is enough on a basically new trans. (Of course the stock 50 psi might not be enough.)
Sorry Frank, I could not drive briskly in reverse and watch the pressure gauge. I'm confident it was the same as it would be with that much throttle in Drive.
As expected any significant (forward) acceleration bumped the line pressure past 100 psi.
With the engine now warmed up, when I moved from Park to Reverse there was no longer a change in RPM or line pressure. (There was a tiny blip in pressure when I first tried this with a cold engine and wheels off the ground.)

BTW - I have Hydroboost braking which puts an additional load on the engine when you depress the brake. Therefore, previously, backing up with a foot on the brake was even more likely to stall the engine when cold.

 

Yes I expect you would see the line rise with throttle in reverse . I can only imagine GM has the rev boost due to the HIGH rev ratio and possibly backing up with heavy loads like a trailer etc? But In normal use ina car I would not really expect anything maybe a slight slowing of rev engagement ?
Good it worked out for you. I have over the years spoken to a few customers over similar issues with IDLE and reverse an could offer little soluton other than higher idle to compensate. So a new possible solution that issue it seems and easy to impliment.