Chevy Blazer Brake Problem

I have a 95 Chevy S-10 Blazer with 4WD. When I initially come to a stop the pedal is nice and firm but after I'm stopped in traffic, the brake pedal will slowly sink to the floor. I checked for external
leaks and found none. The master cylinder has been replaced 4 times, the last time with a new (not reman) unit. The BPMV for the anti-lock system has also been replaced. But the problem remains. Are combination valves known to leak internally? Is there air in the system or BPMV? Can the BPMV be bleed without a scan tool? Bleeding the system manually does not show air and does not correct the problem. Any other ideas? Thanks in advance!
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Taken a good look at the power booster and vacuum source?
Doc
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No I have not, but why would poor vacuum cause the pedel to sink to the floor? I would think that would cause more pedel effort from the lack of power boost.
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Think about what the vacuum booster actually does and you will figure out the reason. Hint--- Higher RPM = Less vacuum. Lower RPM = More vacuum
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Steve W.
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"Steve W." wrote

Enlighten us....I don't get your reasoning either.
Ian
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yeah, as Grandpa used to say................'This oughta be good........'
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Lower vac. when driving down the road do to higher rpms. This means less help at the booster the first time he applies the brakes. Then when he stops the engine rpms drop and produces a higher vac. That is why the pedal drops a bit, the higher vacuum makes the booster work better. Problem could be a small vac.leak or the check valve may be starting to go bad.
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Steve W.

"Gary Glaenzer" < snipped-for-privacy@mchsi.com> wrote in message
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"Steve W." wrote

What do you think happens the instant that you move your foot from the gas pedal to the brake pedal? And what the hell is the purpose of the check valve in the vacuum line to the booster?
You might be right, but I've never seen it.
Ian
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Never owned a Jeep that had a bad check valve or vac. reservoir have you. My 88 did exactly this with a small leak in the res.When sitting still the vac would get higher and the pedal would drop. Took me three weeks to find it since I had gone through the brakes twice.
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