Brakes Diagnosing Brake Booster

boostfrk · May 16, 2013

I have the exact same problem as this guy...http://www.stangnet.com/mustang-forums/threads/hissing-sound-coming-from-brake-pedal.733836/

I checked the vacuum line at the switch on the brake pedal; everything looks intact and not broken.

Under the hood, I disconnected the vacuum line from the brake booster and plugged the line. Started the car, and no hissing at all. To me that seems to point to a bad brake booster.

Could is be as simple as the check valve on the front of the brake booster (the valve that the vacuum line attaches to)? I'll replace the brake booster if that's what it takes but I'd rather not. I'm getting ready to drive the car from FL to CO in a few weeks so I have to make sure it's as reliable as can be.

jrichker · May 16, 2013

Power brake booster test:
1.) Check engine vacuum you should have 14"-18" of vacuum at 625-750 RPM, 18"-21" vacuum at 1000 RPM.
2.) Vacuum checks good, start car and run engine up to 1000 RPM for 30 seconds.
3.) Cut off the engine and then press the brake pedal 3 times. It should get harder to press and travel less each time you press it. If it doesn't, then the brake booster is suspect.

boostfrk · May 17, 2013

jrichker said:
Power brake booster test:
1.) Check engine vacuum you should have 14"-18" of vacuum at 625-750 RPM, 18"-21" vacuum at 1000 RPM.
2.) Vacuum checks good, start car and run engine up to 1000 RPM for 30 seconds.
3.) Cut off the engine and then press the brake pedal 3 times. It should get harder to press and travel less each time you press it. If it doesn't, then the brake booster is suspect.

I checked the vacuum and it's within the acceptable ranges listed above.

I started the car, ran it for 30-40 seconds at about 1k RPM, cut it off, and the brake pedal got harder by the second push. By the third push it was nearly solid and on the 4th push the travel was minimal.

Based on this, and the above, that would mean the booster is OK, right?

What else should I check? The check valve perhaps? It has to be something related to the brake booster since the hissing stopped when I removed vacuum to the booster and plugged the line.

FoxMustangLvr · May 17, 2013

How many miles is that trip from FL to CO? What kind of temperatures do you think you might encounter along the way? Many steep grades to climb and descend? I'm of no help with your problem but cooling and braking must be perfect for the trip.

Good luck! Report back with the solution when you hopefully find one.

jrichker · May 17, 2013

Try replacing the check valve grommet, the only clue is that it is bad is that would hiss anytime the engine is running. Use a length of vacuum hose as a stethoscope and probe around the booster, check valve grommet and check valve to hear any hissing that a vacuum leak would cause.

boostfrk · May 20, 2013

FoxMustangLvr said:
How many miles is that trip from FL to CO? What kind of temperatures do you think you might encounter along the way? Many steep grades to climb and descend? I'm of no help with your problem but cooling and braking must be perfect for the trip.

Good luck! Report back with the solution when you hopefully find one.

Just under 2,000 miles I believe, but it's starting to look more like I may ship the car rather than drive it. Our house is in Colorado Springs so we're on the eastern edge of the Rockies. No steep grades from here to there.

jrichker said:
Try replacing the check valve grommet, the only clue is that it is bad is that would hiss anytime the engine is running. Use a length of vacuum hose as a stethoscope and probe around the booster, check valve grommet and check valve to hear any hissing that a vacuum leak would cause.

I replaced the check valve and the grommet. No change.

When I turned the car off and pump the brakes the pedal is hard within 3 pumps, so that sounds like an indication of a good brake booster to me.

I'm not sure what else to check now. The hissing seems to be getting worse but my braking doesn't feel affected; the pedal isn't harder or softer, just the same. Considering the hissing goes completely away when the vacuum is removed from the booster it has to be something associated with the booster. Is it possible to have a failing booster but still good pedal feel and have the booster pass the "pump the brakes as soon as the car is turned off" test?

1200gt · May 21, 2013

I'm not so knowledgeable about these issues, but problem solving follows the same basic rules. If it stops when you disconnect the booster then it is associated at-least.

Could be a seam around the booster and not a grommet or hose. Be good if there were a smoke test that could be devised....

Wrap Duct tape around the booster, anywhere there is a seam and see if the sound decreases. I'm just thinking of things I'd try, now that Jrichker seem to be short on answers; dumb stuff that sometimes works out.

boostfrk · May 21, 2013

1200gt said:
I'm not so knowledgeable about these issues, but problem solving follows the same basic rules. If it stops when you disconnect the booster then it is associated at-least.

I agree with this 100%. I figured I'd disconnect and plug the vacuum line for the cruise control just to rule it out. I know the BB and the C/C valve aren't connected but just to be sure it's a quick, easy test and a check box to check off.

There is the area where the rod from the brake booster and the brake pedal meet. The BB rod obviously then travels back through the firewall into the B/B, so I'm guessing that's where the leak is. I thought I might try and get a flashlight up in there to see if I can see anything visible. As big as this leak sounds I hope something visibly wrong could be seen.

1200gt · May 22, 2013

boostfrk said:
I agree with this 100%. I figured I'd disconnect and plug the vacuum line for the cruise control just to rule it out. I know the BB and the C/C valve aren't connected but just to be sure it's a quick, easy test and a check box to check off.

There is the area where the rod from the brake booster and the brake pedal meet. The BB rod obviously then travels back through the firewall into the B/B, so I'm guessing that's where the leak is. I thought I might try and get a flashlight up in there to see if I can see anything visible. As big as this leak sounds I hope something visibly wrong could be seen.

If the leak is in the area of the rod going back to the pedal thru firewall you would hear the sound moreso inside the car, I would think.

I know exactly why you're not jumping into taking out the booster right away, they are monsters to get in in the first place if you have SN95 unit. I'm about to begin my Cobra Booster install into the 86GT!

Interested in hearing the solution - another fix to have knowledge about...

Don't you just LOVE these Foxes??? After all, you could be trying this fix on say, a Nissan 300ZX Turbo where a credit card cant slice thru the engine bay anywhere without getting stuck!

jrichker · May 22, 2013

Google is your freind - see http://auto.howstuffworks.com/auto-parts/brakes/brake-types/power-brake2.htm for a very good description of how a power brake booster works.

If the only time you hear hissing is when you step on the brake pedal, and it passses the test I posted eariler, you are OK. The hissing is the sound of air or vacuum entering the booster by way of the valving inside the booster

boostfrk · May 22, 2013

jrichker said:
Google is your freind - see http://auto.howstuffworks.com/auto-parts/brakes/brake-types/power-brake2.htm for a very good description of how a power brake booster works.

If the only time you hear hissing is when you step on the brake pedal, and it passses the test I posted eariler, you are OK. The hissing is the sound of air or vacuum entering the booster by way of the valving inside the booster

I'm hearing the hissing even when I'm not on the brake pedal. Just sitting in the driveway idling, car in neutral and feet on the floor I can hear the hissing. It gets louder when I push on the brake pedal.

It didn't do this 3-4 weeks ago, so something has changed.

I'm thinking my only option at this point is to replace the booster.

jrichker · May 23, 2013

The booster is somewhat challenging to change. The bet way is to open the driver's door, sit on the ground between the open door and the body. If you use a ratchet and a universal joint socket, you can get to all 4 nuts without too much trouble. One of the top nuts is a little difficult to get at. If your socket isn't so loose that the universal joint wobbles all over the place, it isn't too bad

boostfrk · May 23, 2013

jrichker said:
The booster is somewhat challenging to change. The bet way is to open the driver's door, sit on the ground between the open door and the body. If you use a ratchet and a universal joint socket, you can get to all 4 nuts without too much trouble. One of the top nuts is a little difficult to get at. If your socket isn't so loose that the universal joint wobbles all over the place, it isn't too bad

Sounds fun. I've also heard of people removing the driver's seat to allow a bit more room. I looked under there yesterday and all 4 nuts appear fairly easy to get to. I can't imagine this is any harder than replacing the heater core and I've been there and done that already. I'm hoping I can get this swapped out in a day or two of work.

Any huge difference between a booster from Advance -vs- Napa?

boostfrk · May 28, 2013

So I picked up a new booster from Advance last week and installed it this morning. Really not too bad of a swap; the biggest concern was making sure not to kink the brake lines when I pulled the master cylinder out of the way to remove the booster. The worst nut was the upper left (viewed when sitting in the driver's seat). I was able to remove the knee bolster piece and use a combination of a 14mm socket, 10" extension, 6" extension and 3" extension to reach that nut. I snaked the extensions just to the lower left of the steering wheel and right through the wiring. Fit perfect and I was able to remove the nut.

I also removed the driver's seat. HUGE help.

Started the car and no more hissing, and the hissing is barely audible when I push the brake pedal.

Here's a couple pics of my old booster.

jrichker · May 28, 2013

Hidden under all that rust is probably the pinhole you heard hissing at you...

boostfrk · May 28, 2013

Exactly what I thought after pulling it out.

After coupons and returning my core I paid $65 for the booster. So for that amount of money, the 2 hours it took me to swap it out, and not hearing the hissing anymore it's definitely worth it.

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