Riddle me this

Hey folks,
Gotta dilemma that has stumped a regular ASE shop, the dealership, and another entirely different ASE shop for over 2 months now. I have a 1994
GMC 2500 2 wheel drive standard truck, no AC with a 305 in it. My brake light keeps coming on, and the pedal gets spongy. No lines are leaking, I have replaced wheel cylinders twice, it's had 3 different Master Cylinders put on it, and the rear anti-lock has been replaced. Not that this matters, but I've had 3 break jobs done on it in the same 2 month period, pads and shoes. They, (the mechanic's) have tried power bleeding, and manual bleeding, thinking that some valve gets in the way if you try to power bleed it, so then they manually do it, to no avail. But they DO get air, they think. After they bleed it, it's good for a day or 2, then reverts back to: Brake light and spongy pedal.
I am at wits end with this thing, and all I know to do it start switching parts, what's left. Distribution block, power assist, and calipers. I think that's all that's left. Any insight to what I might try next or what it might be??? This is what I told each mechanic and they all say the same thing, "It's very very rare that a distribution block goes bad", and "The power assist wouldn't cause air to get in the line", and "The calipers would show something wrong if they were bad". What the He** is wrong with this thing???
Any and all suggestions are appreciated!!
Thanks guys!
Doug
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I would suggest getting under it and carefully inspecting every single steel brakeline running from the master cylinder to the front and back. A 305 in a 1994 2500 series?
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Additional info: All the lines have been inspected, front to back, and I was under it myself for over an hour inspecting, and there is nothing even damp anywhere. If it were a line, it would HAVE to be hidden as Steve suggested. As far as a brake depressor as others suggest, I have never even heard of one and didn't know they existed... I will talk to my mechanic about it and see if they have one... ?? Also, Augustus, it does indeed have a 305 in it, is that odd??
I've never experienced a brake problem on a truck to this extent before and I am indeed perplexed as well as everyone that's looked at it thus far. Crazy!
Could there be a way that the power booster could draw in air somehow??
Any other suggestions are, as always, appreciated.. I will check back if I get this problem solved and tell what happened or what it was, or if I drove it to the bone yard or not. ;)
Thanks,
Doug
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I used one often when I was towing, to check brake lights without help..
Mine was a 3" long broom stick, one end on the brake pedal, the other on the driver's seat, holding the pedal down.. "low tech" lol
mac
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wrote:

LOL, you know Mac, I get so caught up with doing things with the proper tools or going thru the proper channels, the simplest things, such as using a broomstick, just didn't dawn on me.... I was kind of thinking though, they might have a tool that is spring loaded or something that would keep a constant pressure on it all the way to the floor though. So I was definitely thinking specialty. But a broom handle would indeed be a low tech solution!
Keep the suggestions coming, for nothing is beyond consideration at this point!!
Thanks guys!
Doug
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Doug wrote:

My latest "specialty tool" is made from a modified ratchet style bar clamp with a spring and pad replacing the end and a hook on a cable that holds it to the seat frame.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?ItemnumberF809
Set it up as a spreader. Replace the fixed end with a flat pad with a hook on one side (that way you can hook it over the pedal arm or set it on the pad). For constant pressure you can install th fixed end into a tube with an internal spring OR use a slotted tube and external springs.
--
Steve W.
Near Cooperstown, New York
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Sounds pretty slick, but I'll "stick" with the broom handle and moving the seat up... I know, I'm lazy..
mac
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Doug wrote:

Except I think its more than 3" from the seat to the pedal, My legs are at least 6" in diameter and I'm skinny. I would think its closer to 3' from the seat to the pedal :)
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Ok, Eugene, you're now my official proof reader... and I NEED one... lol
I sure hope nobody cut a few 3" ones out and tried them!
mac
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No, not really rare. I just found my '94 K1500 uburban to be fairly gutless. I just imagined a 2500 series with a 305 TBI unit would be worse. But you could get them with the 4.3 V6 too....
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Put a brake pedal depressor on it for a few hours, if there is a leak, it should show up. Be sure to pull the brake lamp fuse.
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What about the flex lines at the wheels? 94, the rubber may have rotted in some way that allows air in the line.

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Had a similar problem on a Mazda sedan - kid car. No leaks, but brake pedal went soft within days of bleeding brakes. Brake hoses were not holding shape. New hoses resolved the problem.... after nearly everything else in the system (except tubing) was replaced.

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