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Q: ABS System on 1990 Toyota 4Runner

I have an issue with my 1990 R-Runner. When I'm at about 50-65 mph and apply the brakes, the brake pedal goes all the way down & if i hold it steady it eventually comes back and then it breaks. My mechanic tells me that it may be the ABS system. This usually occurs when I'm getting off the freeway. The master cylinder has been tested and has brake fluid. Anyone know what it can be?
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I also have a 4 Runner (1995) 4X4 and it had a very similar problem, it was the power brake booster. There is a diaphram inside that can go bad and can cause a vacuum problem with the ABS. This is very easy to fix and inexpensive. Did it myself!

Did I happen to mention that I am a Female!!!
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I have seen the master cylinders to leak down or leak down intermittently, it would require removing and disassemble to inspect the seals in the brake master cylinder. It is certainly cheaper than investigating a fault in the ABS valve block. Any problem with brakes needs to be investigated.
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