Vacuum booster

I'm at my wits end with this customers 1999 chevy silverado pickup. Comes in with a broken brake line. Of course gm had to put the abs module under the can so you have a bundle of lines running along the frame rail and that's where it's leaking. Put in new lines and bled it. Took it for a drive and the pedal was hard with little braking effort.

Bring it back in. All the calipers move nicely and I cleaned up the pads so they float.

Took it out again same story. Doing some investigation I find that I have next to no fluid coming from the abs to one of the front wheels. Seen this before abs fails. Got a junk yard abs and now I have good fluid flow to all wheels using my pressure bleeder.

Drive it again and same thing. I've run out of things to check except for the master and the vacuum booster.

Pull the vacuum line and the booster woosh's. So I know it holds air. Start pickup with line off and you have good vacuum.

My question is can I have a booster that's junk but yet still holds vacuum? What are your guys thoughts

I was leaning to the booster but I was unsure if you could have a bad booster but yet have no leak or poor driveability.

All the bad boosters I've ever come across would not hold vacuum.

Thank you for the reassurance.