My truck doesn’t want to stop

Question:

I really enjoy the concise and informative responses to your subscribers.

The brakes on my fully restored 1986 F-150 4X4 have become a problem following the exchange of the 4-speed transmission for an automatic C6 and the installation of an Offenhauser 4 bbl. intake and Edelbrock 390 cfm carburetor.

Before swapping the aforementioned parts the truck would stop on a dime and give you nine cents change. After the swap the truck will stop but only with every amount of pressure I can apply. Every brake part is new. I mean everything except for the original brake lines. I installed a vacuum booster pump thinking the new manifold didn’t allow for the power booster to work with the new manifold and carb.

I repeat, every brake part is new. I even used Earls braided lines. The pump causes the booster to draw 18 lbs. as measured on a vacuum gauge. I ran a hydraulic pressure check between the front and rear brakes and it was within specs. I also checked the proportioning valve and it is working properly. So, the only thing I didn’t change was the original pedal and the steel brake lines.

Why did the brakes perform so well before the exchange of parts? What can I do to improve braking so I can make a “panic stop” to pass state inspection?

My only and last guess is that I didn’t bed the brakes properly when I installed them earlier.

Answer:

First, failing to bed (also called burnish) the brakes properly would not cause the hard stopping problem you’re experiencing. Bedding the brakes when they’re new is a technique that causes the brake linings and rotor or drum to have more precise mating surfaces, which leads to somewhat better brake performance and longevity. But this has no real impact on how much pedal effort is required to stop or the vehicle’s ability to make a sudden (“panic”) stop.

You indicate that you measured manifold vacuum at 18 in.-hg (well, you said 18 lbs. but I’ll assume you meant 18 in.hg, which is the standard way of measuring vacuum). But you don’t say whether you’ve determined for sure that the vacuum is actually reaching the booster’s diaphragm. There should be a check valve at the booster—make sure that the valve is there and functioning properly. If sufficient vacuum is reaching the booster and it’s still not providing power assist there’s likely something wrong with the booster. An internal leak, mis-matched parts, improper assembly, a clogged or defective atmospheric breather or another problem may be preventing the booster from doing its job.