If you bled the brakes properly.. and there are no leaks.. there are not a lot of reasons why pressure would be low enough to activate the warning lamps.
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The accumulator ball stores pressure behind a gas, with a membrane between them. If this very simple device went bad it wouldn't be long before the ball completely filled with fluid and you would then have no brake pressure at all... never ever again. I think we can rule that out.
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Other than that, the pump might be starving for fluid and can only supply normal pressure erratically. Such starvation might be caused by low fluid levels, a clog in a feed line, etc. Aside from being obvious, a low fluid level would elicit a warning.
A clog would have to be searched for.
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Or, the pump may be overheating and shutting down, and you get periodic episodes of low pressure. The pump relay is notorious for causing this one. It's contacts stick closed, and the pump runs steadily for about 20 minutes (pressure is high all this time, and there are no warnings during this period.)
But eventually the pump's internal protective heat sensor shuts the pump off for about 5 minutes. Pump the brake at this point and pressure drops. There is no pump to restore pressure. The warning lamps light up.
The pump cools off and pressure is restored. Lamps go off.
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I'd first check that relay if i was you.
While parked, run the engine and pump the brakes until the pump goes on. Shut off the engine and open the hood.. wait a minute or so.
Does the pump go off or not? If not, tap the relay's housing. Does the pump immediately shut off? If so, the relay is bad.