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Jim Grant's Tech Tips
'97 Mercury Villager, Brake Vibration
Q: Recently I had rear brakes
replaced on my 97 Mercury Villager. Shortly afterwards I developed
a vibration in the brake pedal. This became worse and during a road trip
to the mountains, we had to go to a repair shop. The service manager there
said that there was a TSB issued about the rear brakes, that they "should
not be adjusted". I have not found any notice, but after he turned
the drums...we haven't had any problems. Is there a TSB listed somewhere
that you know of that I could see?
-
Chris D.
A:
Im not aware of a do not adjust the rear brakes service
bulletin. Im more inclined to believe that your vehicle was suffering
from the over adjusted rear brakes syndrome or the brake drums that were
not machined correctly when the brake job was performed. An incorrectly
machined brake drum is not always noticeable when the brake work is first
performed. The new brake shoes and the newly machined finish on the brake
drum tend to soften or muffle the imperfection at first. Once the two
new surfaces have worn into each other the imperfection makes itself known.
You could drive a vehicle a week or two before a brake pulsation/vibration
develops. The fix would require machining the brake drums again. Adjusting
the rear brake shoes on any vehicle, is not real a science per-say, more
or less based on feel and experience, both of which can cause a problem
even with the best of technicians. Over adjusted brake shoes are not always
noticeable at first (unless really over adjusted), what happens is as
the brakes heat up the clearance between the brake drum and brake shoes
goes away. This is when the problem compounds itself. Without enough clearance,
the brake shoes rub on the brake drum creating more heat. More heat makes
the fit tighter and the brake shoes rub more. This results in additional
heat and can cause a vibration in the brakes that becomes more pronounced
the longer the vehicle is driven. Stop the vehicle and let the brakes
cool and the problem will be gone until the vehicle is driven long enough
for the brakes to heat up again. A brake drum is not made to handle this
constant heat and, over time, the brake drum will begin to distort resulting
in a brake vibration. Once again the fix requires machining the brake
drum to correct the condition.
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