Main spring sag...

[rsingler]

I just rebuilt a USD Conshelf IV with OEM parts but struck an annoying IP problem as it was like new inside...I pulled it partially down again and put a tiny bit of lube on the push pin shaft and changed the backup washer from split to dished solid type in the crown.

Reassembled with no other changes and it snapped to 125 psi and stayed rock steady ever since!

If I had to pick one of the two, I'd say the backup Teflon washer is the culprit. Folks sometimes forget that it's not just the seal at the HP seat, it's also the seal in the balance chamber. A distorted Teflon backup washer may have also distorted the adjacent HP seal o-ring. Nice pickup!

The diaphragm's pushrod is not much of a dynamic item. Unlike a piston shaft, the diaphragm pin just transmits force, and has no contact along its length with the regulator; only at each end. So I wouldn't really expect lube to change anything.
But nice fix with that cup-shaped backup washer! That design change was a real improvement over the split Teflon washer.

 

[guruboy]

Wouldn't a leaking HP piston stem oring cause leaking from the ambient chamber ports and not related to the IP?

Nevermind, you're talking about a diaphragm reg

 

[rsingler]

Wouldn't a leaking HP piston stem oring cause leaking from the ambient chamber ports and not related to the IP?

Yes, in a piston reg you're exactly right.
So without an IP gauge (and you REALLY should have one - see @couv 's Regulator Checklist), the way you differentiate between an ambient leak due to HP shaft o-ring vs. HP seat or balance chamber is as follows:
Connect your leaking reg to a tank and pressurize it. See the bubbles/hear the air coming from ambient holes. Find out what opening of the tank valve is required to generate just an occasional bubble (may be just a crack open if leak is bad).

1) if the HP shaft o-ring is leaking, when you put the reg and tank in the sink with the piston side lowest, no bubbles will appear for awhile. Then if you look closely, air will appear at the TOP of the ambient hole and begin to bubble out. IP will not change.
2) if the seat or balance chamber is leaking, one of two things will happen. A submerged second stage will begin to bubble as it starts to freeflow due to rising IP. However a balanced second stage may tolerate quite high IP's. In that case, the piston o-ring, which is much softer than the shaft o-ring, may start to leak. You can detect that by having the regulator almost horizontal under water, with the piston side slightly down, and an ambient hole highest along the cylinder of the regulator. As air escapes past the piston o-ring, it will bubble right out the ambient hole, and looking closely you can see that it appears from the piston side.

Of course, if your reg tolerates REALLY high IP's, and your o-rings seal well, the first indicator you have of severe IP creep is a ruptured hose. Which leaves you with 90 sec of air or less if you're diving.
Which is why I check IP EVERY TIME I first fire up my reg on the boat. I use my cheap $20 gauge attached to my BCD hose, which doesn't have to be very accurate. Quick lockup? Roughly within spec for a cheap gauge? Good to go!