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The pedal was nice and firm this morning. Went for a 15 mile ride and the brakes worked fine.

Thanks, that is what I will do. That original master cylinder is a horrible design. What meathead came up with this? I had to use a crawfoot wrench to get the vent line fitting off as nothing would work. No room for any other type wrench. I will probably get one of those remote reservoirs...

I replaced the brake light switch and flushed the brake system with new brake fluid. I used three quarts of DOT 5 fluid to do this. When I first tried the brakes after bleeding the brakes, the pedal was a bit spongy but it is firm now. Is this normal? On passenger cars and trucks the pedal...

A fine threaded bolt with some teflon tape on it which I partially threaded in by hand only to keep the brake fluid from leaking until I replace the switch. I did not have a pipe plug with the right threads.

Wow, you are making a bunch of conclusions that are just wrong. I did read the post that you linked and it took me a while to figure out that these switches are actually different - the one that works off of pressurized fluid is fluid activated, the other is just an air pressure switch. They...

It is located on the front end of the air pack. Thanks for the explanation.

It looks like the air pack is leaking brake fluid through the damaged switch (still installed at this point). I tried to unplug the electrical connector, but the whole switch body broke off inside the connector, as it was partially melted from heat. If there is brake fluid leaking from the...

Can you explain the switch upgrade? The present switch is an air pressure actuated switch with two electrical leads. I think the switch had an electric short in it and burned, causing an air leak, enough to cause a soft brake pedal for a moment. What does the "upgraded" switch look like?

I got under the truck, found the brake light switch on the air pack, it sure enough failed, there is burned smell in that area. I found a new switch on eBay, hopefully that is all it was. Since it is a pressure switch, if it fails internally, it will cause an air leak from the air pack.

I found that the brake light switch is located on the air pack itself. It looks like it is a pressure switch. Is it possible that it failed and caused the air leak I heard? I hope that is all it was. Where can I find a new stop light switch?

I have had no problems with brakes on my M109 for many years until today. I drove the truck around my yard, when suddenly two things happened: 1. The pedal started sinking to the floor and when I pumped it, it became firmer again, but I heard hissing noise from under the truck which sounds...