As a thought. The problem is showing up at all 6 wheels. The only parts that affect all 6 wheels. Is the master brake cylinder, and the perportioning valve. His GMC doesn't have a P valve. ( PO removed it.) The problem is the P-30 MC. I had one once for about 5 months. Then removed it. That MC for our application has issues. Not going into the details of those issues here. You can PM me.
Bob Dunahugh
78 Royale since 2003.
4 COPO Yenkos
Bob Dunahugh
78 Royale since 2003.
4 COPO Yenkos