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Accumulator(?) hum.


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My 89 coupe is seldom used. (81K miles on odo) Lately I hear a low pitch humming/tooting  sound at start up and it stays that way for minutes even if I wait for a warm up before driving. Brakes are soft and there's no such thing as doing a heavy nose dip when pressing hard on the brake pedal.    Am I right in suspecting a need to change the accumulator. It was replaced about 10 years and 30K miles ago, but Father Time kills things whether we use them or not. What think Ye?

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I use a pretty simple test to rate an accumulator.

Clean the brake reservoir so that tape will stick to it and you can see the fluid level.

Turn the key to ON.......do not start the engine and wait until the brake pump motor stops running.

Mark the fluid level on the tape.

Turn OFF the key and pump the brake pedal until it gets hard.

Mark the fluid level on the tape.

A good accumulator will have a difference in the two marks you made of LESS than 1/2 inch

If the difference between the marks is MORE than 1/2 inch you need to order a replacement.

 

NOTE...these measurements are for original style accumulators........newer replacement accumulators often have different volumn than the originals because of that it is just a guess if they are bad.

The logic behind this process........a new accumulator had a 750 psi precharge.......when the pump pressurizes the system it moves forces fluid into the lower section of the accumulator

until it compresses that 750 psi to the 2500 psi shut of point of the pressure switch.      If the precharge is low,  more fluid will go into the accumulator and you can see that in the above test.

 

If you have not FLUSHED your brake system and put in new fluid,   that should be done.    There are a bucket full of reasons for flushing the system and several owners have said that flushing alone

made a noticeable difference in their brake performance.

 

 

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