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65 Super Duty: Brakes Working Too Well.

Blue-Truck-Nut97

crank polisher
551
18
So here's a noggin scratcher-
Vehicle in question is a 1965 F-850, Napco Powr-Pak 4x4, fire truck. 534 V8, 5 spd direct. (1000 gallon tank, pump and roll, pretty sweet setup.) Truck has vacuum assist brakes, with the vacuum booster unit remote mounted under the drivers running board.

Truck sees seasonal use, we keep it running, and it's full of water in the summertime for fire prevention, tree watering, and trailer wash outs. As goes with most vehicles with hydraulic brakes that sit a lot, I usually have to bleed them once a year or so. About 2 years ago I changed the master cylinder when it developed a leak. Not much else to report.

Here's the puzzle, washed out a trailer a couple days ago, truck was sitting running powering the pump, I walked around back stretching the hose out and noticed the brake lights on. Hmmmm, bad switch apparently, but alas, it can't be that simple. Got in the truck to park it, and the brake pedal was hard as a rock, and the brakes were locked. By shutting it off and bleeding the vacuum off, they would release. Drove it back in the yard, brakes were getting tight by the time I got parked. Again, shut it off, bled off vacuum in the reservoir, and they released.

Initially, I thought the booster was failing, but, it never sucked all the brake fluid into the intake like they usually do.
AND
The hard brake pedal, all toe board clearance is gone now, pedal comes up hard against the stop. I distinctly remember having 3/16"-1/4" when I put the master cylinder on. So after studying on it, I'm wondering about that master cylinder. I'm thinking about cracking the line on the master and seeing if they release next time it's running.

Anybody seen this before?
Thoughts or suggestions?
Know a good exorcist?

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dustybumpers

don't play well w others
Sorry I missed the thread, never hit my phone.
Been almost 50 years since I had a Huff huff system in a truck...... LOL

my 67 f 850 did that. vacuum boost was bad. Next time it locks the brakes, go under the running board, or behind the seat, where ever the unit is, and pull the vacuum line off it. if it releases the brakes, your unit is bad.

Once I replaced mine I tore it apart, the rubber inside was so disintegrated it turned my hands black just touching it.
to solve the problem on the new one a old guy told me to use a desiccant chamber hose from a r 12 a/c line in the vacuum supply to keep moisture from the chamber.
 

Blue-Truck-Nut97

crank polisher
551
18
Follow up question, s,
Did your toe board clearance go away?

On the desiccant line, never thought about that, wondering how it would be effective if moisture doesn't exist in a vacuum. Going to have to ponder on that one.

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dustybumpers

don't play well w others
It did.
Not sure on moisture in the vacuum can , but later when I opened the system it was dry inside. No crunchy rusty parts.


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Blue-Truck-Nut97

crank polisher
551
18
After considering it a while, it kinda makes sense, when the vacuum goes away outside air containing moisture fills the void. Especially on your end of the country. So I bet it would work well. Then, every time you start the truck it would evacuate the desiccant, so theoretically it should last indefinitely.

I'll report back when I get to mess with it.

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