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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Have you checked for a leak somewhere in the vacuum system, especially around the barbs the vacuum hoses attach to. Careful application of small amounts of propane around the hose ends will help diagnose location.
Andrew Zwickl wrote:

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Why would a vacuum leak cause the pedal to sink? I would think a vacuum leak would result in the power brake booster not working and harder pedal effort. Also, what effect would propane have in detecting vacuum leaks?
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location.
Propane will increase idle speed when the vacuum leak sucks it in. I've found that brake cleaner or carb cleaner works a lot better than propane. You can spray it anywhere you want, propane doesn't cover as well.
-Bruce
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You can spray carb cleaner if you want, but I've seen it flash up and burn other mechanics faces if there is a hot enough component or a leaking (arcing) sparkplug wire.
It's easy enough to modify the end of a propane torch with a hose and a pin-point nozzle to use to home in on the source of a vacuum leak.
Better yet, build a smoke machine.
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Andrew Zwickl wrote:

Andrew, I'm not a mechanic but maintain my own cars. Based on my understanding of brake systems, I consider the master cylinder a pump. If you are not losing fluid (the level in the reservoir drops and stays low), the only way the peddle would go to the floor is:
1. The fluid is going into a wheel clyinder that is expanding way too far (can't say I have an example of what would cause this, but if for instance you were to press the peddle with the rear drums removed, the shoes would be free to move much farther than intended and consume much more fluid in doing so. When the peddle is released, the shoes will retrack and the fluid goes back to reservoir. I can't think of anything that would cause the peddle to be hard and then slowly go to the floor unless there is something wierd with the self adjusters).
2. Internal leak in the master cylinder. Maybe you need to replace one more time but wait until the stars are better aligned.
3. I don't think air would cause this
I want to say when bleading brakes, I can pump the peddle to the floor about 3-4 times before needing to add fluid. This said, I would expect I could put a mark on the reservoir and notice a change in the level on a single peddle pump. You may get someone to watch the reservoir to see if the fluid leaves the reservoir between the hard stop position and the floor. If it does, the fluid is leaving the master cylinder and it is probably ok. If not, then either this test isn't sensitive enough or the MC is bad....
That's just my simple logic. I don't think the proportioning valve would be able to store fluid, but I don't know if the ABS could possibly do this.
Good luck, bb
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