Low brake pressure warning

[Wallace White]

You know the 5k's auto-check light for the hydraulic brake system--the
flashing red triangle? I've gotten it sporadically for a few seconds
over the last couple of years; since the this week's A/C clutch repair,
however, it's come on on most of my drives and stayed on till shutoff.
Sometimes turns on while at speed, sometimes when braking from speed,
sometimes I don't remember. I've noticed no problems with steering and
braking.

I've gathered that there are three triggers for this warning light:

(1) Pentosin fluid level switch
(2) brake fluid level switch
(3) brake servo pressure switch

I've confirmed that (3) is the source: unplug it while the warning is
active and the warning quits and the switch shows continuity.

The Bentley explains that the switch triggers at 87~127 bar (normal
operating pressure is 140 bar). Pretty broad range...

The hydraulic system in the car is in good shape overall. Haven't had to
add Pentosin in more than a year, since putting in a rebuilt pump (from
the pick-n-pull!). Bomb is four years old and still gives 25 pumps. Rack
and pressure hose are three and one years old, resp.

The Bentley tells me it could be one of two problems, then:

(a) Faulty pressure switch
Lacking the special pressure gauge VW 1441, what do I need to check the
pressures? Looks like you can screw into the fitting where the pressure
switch normal resides. Maybe it's cheaper to just buy a new switch?

(b) Leaky brake servo, causing low pressure.
I'll do the drip test. I understand a few drops are normal.

Any other tips here? Guesses at the risk of driving it at present?

Thanks,
Wallace
'87 5kcstq 190k