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Thanks tjc, I am going to a guy I know who owns a garage and have him test it also to confirm.
The other curious thing is that it seems like the brake light comes on as I loose my vacuum and then goes out after it builds back up. Is this sensing the decreased pressure on the fluid because of the loss of vacuum assist and if so why would it stay on until the vacuum build back up even after I have let the pedal come back up?
i am not up on the electrics of the newer trucks, so i will not comment and lead you in a wrong direction.
but i think you are looking in the rite direction with the pump if you can only get 11-12 inches.
what i do is hook the vacuum gauge up to the vacuum "T" on the firewall near the driver side valve cover and take a reading. then i step on the brake pedal a few times to monitor the pump. if will then disconnect the booster and plug the booster line and see if the vacuum goes up. this will rule out a leak in the booster if the vacuum rises drastically.
A little update. I replaced the vacuum pump this weekend, now the vacuum is up to 22-23 inches and the brake light does not come on from repeatedly steeping on the brakes. I haven't test drove it but I think that problem may be fixed. I still have what I consider a falling brake pedal that falls faster then what I feel is acceptable. I did get another master cylinder to replace the other new one on it now that is leaking out the back some, so may be that will take care of some of it.
Has anyone ever left the vacuum gauge on and then pumped the brakes? I did and by pumping the brake rapidly, I could knock the vacuum down to 0 or close to it but it would recover pretty quick. Is that normal or would it indicate a leak in that. I did shut the truck off with the gauge still on it and did not see any drops over a minute, held 22-23 inches.