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When I depress the pedal, most noticeably at a stop while idling, I hear a very audible whistle (kind of wolf-whistle pitched) that comes and goes with added or removed pressure on the pedal, and sounds like it is coming from the engine bay. This doesn't happen until after driving for a good 15-20 minutes.
I have power brakes, but they appear to not be a stock setup:
I can't see or feel any leaks, and will check tonight with someone else depressing the pedal... but was wondering if anyone has had/heard of similar issues. Thanks in advance for any input.
sounds like you possibly have a vacuum leak in the line to the brake booster, the rubber grommet at the booster or inside the booster itself. you should be able to hunt it down if you warm it up and get your buddy to press the brake pedal while you look for it!
I once owned a '61 GMC Carryall that somebody had swapped a 390/FMX into. They also added the Hydro-vac to it. Those things are a PITA when the check valve fails.
Locked the brakes down, 20 miles from home, in the rain, 6 month old twins strapped into the back seat. Ruined the brake drums and shoes before I got her home.
Ford offered factory or dealer installed power (drum) brakes on 1965/66 F100/250 2WD's.
Page 28 in the 1966 Car/Truck Accessory Brochure sez:
"The answer to increased braking power, responsive pedal action with real brake "feel" extra power for faster, surer straight line stops: C6TZ-2A091-C $47.00.
However, the power booster is mounted in the usual spot...on the firewall.
Autoparts stores, brake shops offered a "universal" power brake kit. The power booster is a Hydro-Vac.
A Hydro-Vac (NOT a Hydro-Boost) is a remote type of power brake booster, basically it can be mounted anywhere.
For one example, 1955/57 T-Birds had these mounted on the left fender apron, because there was no room for a conventional booster to be firewall mounted.
If the OP's truck has a frame mounted booster, it's a Hydro-Vac.
btw: When you get your 1964/72 truck parts catalog look here: Illustrations ~ Section 23 ~ Page 8. The entire setup is shown.