1995 Chevy Cheyenne Very Hard and Spongy Brake Pedal

1995 CHEVROLET CHEYENNE
116,118 MILES • 6 CYL • 2WD • AUTOMATIC
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OMBENA
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The M/Cylinder has been replaced, drums and rotors turned, wheel cylinders and front calipers have been replaced also. The system has been bled several times and the fluid in the reservoir is clear. I have checked the brake rubber hoses while someone pumps the pedal and there does not seem to be a problem there. When I step on the pedal it feels like stepping on a brick. When I step on the pedal slightly I hear a hissing sound that seem to be coming from the boot at the power booster. When I apply medium pressure to the brake pedal the engine advances about 400 RPM. Please give me some guidance.

Thanks, OMB
Jan 5, 2010 at 5:08 PM
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MERLIN2021
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There are two types of brake power boosters used un chevy trucks, by what you describe, yours is a vacumm operated unit, the other is a power steering (hydraulic) unit, either the booster is no good, or the check valve in the large vacuum line that plugs into the booster, remove the check valve, try sucking air thru each end, it should flow in only one direction. Replace the valve if anything else happens, if valve passed the test, make sure there is engine vacuum in the hose, if there is replace the booster.
Jan 5, 2010 at 5:25 PM