There's a fault in the ABS system.  You need to get the code read from
the ABS system (not the same as the main computer) to see what's wrong.
It could be a sensor or a bad connection.

If the problem started right after the brake job you might have damage
to a wheel rpm sensor or the wiring to it.  If those are OK, check the
fuse and the wiring to the ABS controller (should be located about 12
inches directly aft of the driver's fanny) -- open each connector and
look for corrosion -- these had typical Bosch copper contacts without
plating to forestall corrosion.  If you see green corrosion on the
contacts, gently clean and reassemble.

IIRC there were two lights on the '86.  An ABS Warning light and an ABS
Active light.

If it's the warning light, is it on before start? After start but before
the car is placed in motion? or, comes on after the 5 mph test sequence?

Early C4 ABS was 'vanilla' Bosch and was common to Corvette, Chrylser
New Yorker and other cars. The better independent brake shops have the
test set for early Bosch ABS--call around--usually less expensive than
going to the Chevy dealer.  This wasn't an OBD compatible system so you
can't read 'codes' with a typical scanner.

PJ