1992 Pontiac Lemans Pedal to Floor

1992 PONTIAC LEMANS
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SQUASHTOWNER
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Customer complained about poor brake pedal. Exact symptoms of a bad master cylinder. Replaced the cylinder, bled the brakes at each corner, and at all the ports on the master cylinder, and when not running, the car has PERFECT pedal...better than that of my 2005 matrix. Start the car, and the pedal goes to the floor when you push it, and the car will hardly stop.
We suspected a faulty reman MS (it happens) and replaced it, and did the whole bleeding process again. Same pedal.
Car has new pads, rotors, and recent rear shoes. Rear brakes are adjusted, caliper slides are perfect, wheel cylinders move freely.

Is this the booster? Or could it be those 2 little proportioning valves on the brake lines, attached to the MS?

Thanks for any help!
Feb 6, 2006 at 8:27 AM
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CHEVY350GUY
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no abs?
Feb 6, 2006 at 9:11 AM
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SQUASHTOWNER
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No ABS.
Feb 6, 2006 at 9:51 AM
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MVANNESSJR
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Check the vacuum lines and make sure they look good. Is there a hissing sound? I would suspect a possible failing power brake booster. There isn't much left if you've already replaced the master cylinder.
Feb 6, 2006 at 7:37 PM
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SQUASHTOWNER
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[quote:664159f9a7="mvannessjr"]Check the vacuum lines and make sure they look good. Is there a hissing sound? I would suspect a possible failing power brake booster. There isn't much left if you've already replaced the master cylinder.[/quote:664159f9a7]With the booster vacuum line plugged in, or disconnected, there's a very noticeable"SSsshhhhtttt" sound every time you pump the brake pedal.
Feb 6, 2006 at 8:49 PM
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SQUASHTOWNER
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Oh, and those little proportioning valves re unavailable anywhere, apparently. Our local GM dealership has onle ever had one Lemans in for work in all the time they were sold, and they told us that the car is now classed as "vintage" and parts are hard to come by, if not impossible.
Feb 6, 2006 at 8:51 PM
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MVANNESSJR
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Without actually being there and listening to it myself, it would appear that the power break booster has seen better days and should be replaced.
Feb 7, 2006 at 12:04 AM
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Suggested Repair

Inspect the brake booster vacuum lines, then replace the power brake booster. The pedal is firm with the engine off but drops to the floor when running, and a hiss is heard when the pedal is pumped, which points to a failed booster rather than the master cylinder or proportioning valves.

Conversation Summary

A 1992 Pontiac LeMans with no ABS had a soft brake pedal only with the engine running, despite two master cylinder replacements, thorough bleeding, new pads and rotors, recent rear shoes, and free-moving calipers and wheel cylinders. The pedal was firm with the engine off. A hiss was heard each time the pedal was pumped with the booster vacuum line connected or disconnected. Proportioning valves were reported unavailable. The expert advised checking the vacuum lines and concluded the power brake booster should be replaced.

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