Brake pedal whistles

1996 chevy s10 pickup with 96500 miles. Have been dealing with this problem for more than a year. Replaced master cylinder once, brake booster twice and the abs pump (cost more than the transmission). My new mechanic has bled the lines twice, replaced the master cylinder and brake booster and apbs pump and is now at a loss. The brake pedal will go down half way and stick with a loud vacuum leak ‘whistle’. I can pry the pedal back up with my foot and then it will operate find for a few days…then whistle again. I love this truck, the engine is strong (new tranny 9 mos ago) and the body in great shape. I don’t and can’t afford to get rid of it yet. My mechanic is just out of ideas…help.

Unplug the ABS (pull the fuse) and see if the problem goes away.
Post back with the results.

If it has a vacuum line to the booster I would replace it.

Barky, you have reactivated my brain cells. Good post.
There should also be a check valve in that vacuum lime to allow one-way passage of air. My money is on the check valve.

Like the idea of the just pulling the ABS fuse…drive this truck only a couple of thousand miles a year (less than 5,000) But you’d think my mechanic would have done that before replacing that gold plated abs pump. He did say he could always look at the check valve again but said it might be in the brake pedal assembly and should replace it. What do you think?

A whistling sound would suggest air passing through a hole. I know of nothing in a brake pedal assembly that could whistle.

venting and containing the vacuum above and below the diaphragm controls the brake booster and a faulty control valve can bleed off vacuum from above the diaphragm allowing vacuum to accumulate below it, causing the booster to apply pressure when the pedal is released. A sticking brake pedal will cause the same condition. Someone familiar with the operation of the booster needs to check that out.

At first read, it sure look like something associated with the power booster. Maybe the mechanic can use an engine stethoscope to isolate exactly where the whistle sound is coming from. There’s at least some possiblity the whistle is a vacuum leak somewhere else, which is somehow affecting the power booster’s ability to boost. That could result in other engine related symptoms, like rough idle, missing, etc. Do you have any of those?

Try applying and releasing the brake pedal several times without the engine running. After a few pumps the pedal should get quite hard to push. If the pedal travel feels normal and returns up normally I would say the booster is bad.

I assume he thoroughly inspected the brake pedal and pedal hanger assembly when he replaced the booster the first time?

I guess he looked at the brake pedal assembly; I called and let him know the problem still exists and he is now reluntant to consider the brake pedal assembly but he also never came up with a leaky check/control valve. The engine runs great; no misfire, no rough idle. I can’t believe this is a problem with the power booster. This truck has had 3 power boosters installed in the past year. The first one wasn’t OEM and that guy thought maybe the part was bad and replaced it with a new “Chevy” brake booster (second) the third one was installed with the new mechanic, he installed the master cylinder and said the brake fluid was black, thick and probably demaged the brake booster. New brake fluid; bled lines etc. When you do pump the brakes with engine off, the pedal does get harder to push…as if building pressure. This whistle doesn’t happen everytime…fine for days then going down hill and you’re praying you can pry the pedal back up and stop then fine for days after. My mechanic has driven the truck for days and said he couldn’t get to happen for him but the next week it happens with me. Again, thanks for any input; you guys are a real help.