Based on our shop experience specializing in heavy-duty diesel brakes since 1985:

• The 6.2L Diesel Rule: Unlike gas engines, these diesels don’t produce vacuum. If your pedal is hard, your hydroboost is likely bypassesing internally or the accumulator nitrogen charge is gone.
• Flush is Mandatory: Power steering pumps on these older trucks often shed metallic debris. Failing to flush the system with fresh fluid will score the new internal seals of this booster instantly.
• Pedal Rod Height: This unit is built for the factory 1983-1991 pedal geometry. Ensure your firewall mounting area is clean and flat before torquing the 10mm studs.