We had issues with the rear brakes not bleeding and isolated it to the pressure regulator, so went out and pulled a backup off the parts cars.

the swap fixed the issue and brakes are bled.

So we decided to take apart the bad one. Here is what is looks like a part. Those o rings were pretty gummed up preventing the piston from moving.

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I always wondered what one of those looked like inside. Thanks for sharing ... Are they rebuildable or is that dangerous (because brakes ...).

Rebuildable. PMB offers the service or just the kit

Rebuilding looks easy, but how do you calibrate? Do you need to?

https://pmbperformance.com/products/pressur...7779c&_ss=r

Yes they need to be recalibrated. You do not want a car with too much rear brake bias. Rear brake lock is inherently unstable, and dangerous both to you and others that share the road with you.

The curve is in the factory manual. The reality is most folks don’t have the appropriate high pressure gauge(s) or the bench rig to supply the pressure to set it.

If simply taken apart and reassembled without changing the adjustment screw, it will be close.

Send to PMB for a full restoration is the way to go.

Just curious, how is it adjustable? Seems like the bias is set by the spring?

You set the preload on the spring.

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Oops, almost forgot this one.
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so if you just took it apart and put new seals on it do you have to set the preload again?

Theoretically no if you did not touch the spring adjustment. But then again that is a 50 year old spring, not sure it is as good as new. I think I would get it calibrated again.