Chevy Brake Issues

About 6 months and 2,000 miles ago I had a complete 4-wheel brake job done on my truck. For the last week or so I was noticing the pedal being soft and at times going to the floor. Today the red Brake Warning light is coming on in the lower right-hand corner of the cluster. Any ideas what could be going on here?

Need more info here. Year, ABS, 4 wheels disc? Hydroboost or vacuum? I assume youve checked the fluids and they are still at a good level?

It's a 1994 Silverado Ext. Cab V-6 with a little over 153,000 miles.

Mechanic shop says bad "ABS Dump Valve", a rare part costing $325.00.

Is it safe/okay to drive this regularly with the ABS not working?