In studying the schematic & checking out the only manual I have on this type booster assy (it's on a Hyster- not sure how it relates to yours) I have a few concerns:

1)- from the schematic it looks like the front of the booster is a sliding valve assy. It's hard to tell but it looks like the hydraulic system doesn't hold pressure unless that plunger is depressed- is that correct- when does the hyd. portion have max. pressure?

2) From my Hyster book (yours may be different) there's an adjustment screw on the front mounting plate of the booster ( it would be covered when mounted), have you seen/adjusted this screw. It sets the pressure of by-pass.

I wish I could be of more help- without a picture of the master cyl/booster assy I'm out of ideas. List the part # of them & I'll get a breakdown from my vendor