Check the operation and alignment of the brake light switch, located at the upper end of the brake pedal under the dash area
Make sure brake fluid in the master cylinder is at the proper level Make sure parking brake is fully released and switch is working
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DOT3
A 1978 Ford F100 Custom 1/2 ton pickup LWB with no AC sold new for $5179.
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Probably a faulty brake light switch or the brake light switch is installed incorrectly.
Pending on condition, I would say it good condition it could go from 1,500 to 3000 with no aftermarket parts.
Either the power brake booster is bad, or it don't have any vacuum going to it. That is the part that the brake master cylinder is bolted to.
Purchase a Haynes Manual thay is specific to your truck at a parts house or bookstore.
First check the fuse. If it is good, look under the dash and find the brake light switch. It will be mounted on the brake pedal lever or may be mounted so the brake pedal lever contacts it. It is adjustable and may just need adjusting. It the adjustment is correct, then replace it.
Hemmings Motor news? E-Bay? Salvage yard?
Make sure brake fluid is full check that emergency brake is released fully check equalizer valve (where brake light switch connects) remove wire loosen fitting and step on brake lightly to center piston in switch