The brake warning light in the instrument cluster serves double duty. It can be illuminated by the closing of either one of a pair of unrelated switches.
First, there is a switch on the e-brake pedal linkage that closes and lights the warning when the e-brake is ratcheted down - that's just a simple reminder to pop them free before driving.
The other switch is located on the body of the BCPV. It closes to alert you to a possible major failure of the primary braking system. Presumably a severe loss of hydraulic pressure in one of the brake circuits (when the brakes are applied). And it follows that this failure to reach normal pressure would most commonly be due to a failed brake line/hose.
The switch is closed when the BCPV's spool valve shifts a certain distance from it's normal (i.e mid-point) position. This shift is caused when the brakes are being applied and a large pressure differential exists on either side of the spool. As noted above, the worst case scenario is a failed brake line.
But it doesn't have to be a burst hose or fractured section of tubing that causes the spool to shift. There are other reasons for a severe pressure difference to develop in the two hydraulic circuits. A common cause is badly worn brake shoes - or shoes that are just way out of adjustment (along with non-functioning self adjusters).
These brakes have self-adjusting shoe mechanisms that assure correct lining-to-drum clearances at all times. The automatic adjusters operate only when the brakes are applied as the vehicle is moving rearward or when the car comes to an uphill stop.
Although the brakes are self-adjusting, an initial adjustment is necessary after the brake shoes have been relined or replaced, or when the length of the adjusting screw has been changed during some other service operation.
Frequent usage of an automatic transmission forward range to halt reverse vehicle motion may prevent the automatic adjusters from functioning, thereby inducing low pedal heights. Should low pedal heights be encountered, it is recommended that numerous forward and reverse stops be made until satisfactory pedal height is obtained.
If a low pedal condition cannot be corrected by making numerous reverse stops (provided the hydraulic system is free of air) it indicates that the self-adjusting mechanism is not functioning. Therefore it will be necessary to remove the brake drum, clean, free up and lubricate the adjusting mechanism. Then adjust the brakes, being sure the parking brake is fully released.
Since you have thoroughly inspected the hydraulic system for leaks (and found none), and you are also certain the the system is properly bled - I would take a close look at the clearance between the brake shoes and drum surfaces. Excessive clearance in the drum brakes (accompanied by tight clearances in the disc brakes) can mimic a loss of pressure in the rear circuit.
The spool shifts, closes the switch and you get a warning light.
The function of the BPCV is more complicated than just to cause a warning light however. When the spool shifts to one side, it also shuts off flow of hydraulic fluid into the damaged circuit. This serves to preserve braking capability in the undamaged circuit.
It gets complicated, but this may also explain why your braking capacity seems to come and go (as does the warning light).
