Hum from brake master cylinder?

[kargo27]

All,

I have a 2000 Ford Contour with 144,000 miles and recently it's been doing this "hum" from under the hood.

If I rev the engine it goes away, but the hum usually happens at idle or a little above idle and is random.

I know that the power steering pump is powered by the belts but what about the brake master cylinder? Is there a pump that operates it?

I had the power steering pump replaced about 2 months ago b/c it had a small leak so I don't think it's that.

However, I had the brake system flushed by the dealer just the other day and it started doing that hum not long after that. Also, this hum had occurred about 3 times in the past, usually when the engine was cold and just started. I'd tap the brake pedal hard and it would stop.

Could it be the antilock brakes motor activating?

Also, I asked the service writer at the dealer, who I've gotten to know pretty well, unfortunately, due to having to replace most everything on that car except motor and transmission, and he said that the brake fluid looked decent and that it wasn't terribly bad.

Thanks,

Karl

 

[mechtech2]

Tap the brakes and it goes away?
Probably just the vacuum valve to the brake reservoir.
Should be rather cheap and easy to fix - check it out first.

 

[kargo27]

Mechtech2,

Thanks for the tip, I'll check it out.

Yeah, it goes away when I hit the brakes or if I rev the engine a bit. It's a random event. I posted earlier that it does it when it's cold but this morning it didn't do it until the car warmed up.

Karl

 

[Merkava_4]

Quote:
I know that the power steering pump is powered by the belts but what about the brake master cylinder? Is there a pump that operates it?


That would be the power brake booster which is operated by your foot.

 

[kargo27]

Originally Posted By: Merkava_4
Quote:
I know that the power steering pump is powered by the belts but what about the brake master cylinder? Is there a pump that operates it?


That would be the power brake booster which is operated by your foot.

LOL! Yeah, true! But you know when the engine isn't running and you hit the brake it gets firm really quickly. When the engine is running it doesn't have that firm feel.

So it must be powered by something else, isn't it?

 

[Merkava_4]

Quote:
On cars with a vacuum booster, the brake pedal pushes a rod that passes through the booster into the master cylinder, actuating the master-cylinder piston. The engine creates a partial vacuum inside the vacuum booster on both sides of the diaphragm.

When you hit the brake pedal, the rod cracks open a valve, allowing air to enter the booster on one side of the diaphragm while sealing off the vacuum. This increases pressure on that side of the diaphragm so that it helps to push the rod, which in turn pushes the piston in the master cylinder.

As the brake pedal is released, the valve seals off the outside air supply while reopening the vacuum valve. This restores vacuum to both sides of the diaphragm, allowing everything to return to its original position.



How Power Brakes Work

 

[punisher]

I will try something different........IAC. When you hit/release the brakes, or rev the engine, the IAC changes to compensate. IACs, FoMoCo IACs, do cause oddball noises at idle on occasion. Sometimes described as a "hum", or by one customer or a "horny bull moose under the hood" by another.

Also check for any cables/harnesses laying against the booster, they can transmit vibrations at certain frequencies to the pedal or firewall.

 

[kargo27]

Thanks Merkava, that helps explain it and now I understand.

Punisher, it wouldn't surprise me. I had an IAC go bad in a Ford F150 I used to own. Thank you, I'll look for that, too.

Well, I started it up this morning and got it to make the "hum/horny bull moose" noise and got out quickly as I could to try to get my hand on the vacuum booster but I was about a foot away and the hum stopped. I couldn't get it to repeat.

I sprayed some WD-40 on the vacuum lines and seals where they enter the intake manifold as well as on the vacuum booster itself in case oxidation may have caused them to be brittle and not seal effectively.

I'll drive it around the block to see if I can get it to hum.

Thanks again, all for suggestions.

 

[kargo27]

Update:

Well it's 83 degrees outside and the hum hasn't occurred today. The hum did occur when the weather was cooler, like 50-60 degrees.

I don't know if it's because of the warmer temps or because I lubed the vacuum fittings because they may have not been sealing properly.

If it happens again, I'll post.

Thanks for the help, all!

KG

 

[kargo27]

Update no. 2:

Well, the hum is back. Again, I tried to pop the hood and see where it's coming from but it doesn't hum long enough.

KG

 

[kargo27]

Well, the hum has now been replaced by the "check engine" light.

There is a surge/stall upon acceleration but the engine idles fine when not in gear and under power.

Ford incorrectly named this car "Contour". It should have have been called the Ford "P.O.S."

KG