Sienna 1998 Vacuum coming to booster is low, making pedal to be little hard vacuum hose is not leaking, nipple on manifold is also slow, nipple had no blockage. hissing sound from air filter is louder

Call parts store for price of part. Replacement is not difficult so expect $100 to $150 labor.

You can only have whatever the engine vacuum is
at any time.
Is there vacuum to the booster check valve
& is the valve working,assuming you have the
old one or a new one with the new booster.
Does fluid flow out all 4 corners?
Are the Rubber Flex Hoses Collaped
Calipers Rusted & frozen?
Did you remove the master & clean out
the moisture from around the piston
That is where you start,then flush & on to
every component in the system, if they are
on the vehicle for many years
You do realize brake fluid gets changed every
2 years on all vehicles or you got big problems with
moisture in years to come

NO. the vacuum is used to make the brake pedal easy to push, power brakes. the pedal would be hard without vacuum, that is why the pedal gets hard with the key off and engine not running. the problem you are having is more of a hydraulic problem.
Have the brakes checked asap. the brake system may have a leak causing the pedal to go low. check the fluid level and if it is low or empty then there is a problem, could be a rotted brake line leaking or a wheel cylinder leaking.

Problem with master cylinder will cause that sort of problem some times.

Have you tested your brake booster and your calipers?Your brake booster doesn't make any noise, and it doesn't use any electricity or gasoline, but it ensures that you can stop your car with only a light touch of the brake pedal. Things weren't always like that: before the invention of the vacuum brake booster, cars still stopped. It's just that you had to really stomp on the brake pedal. The modern brake booster is an ingenious device that operates using something that your engine generates whenever it's running: vacuum. The brake booster takes engine vacuum via a rubber hose that runs from the intake manifold, and the brake booster uses that vacuum to amplify the pressure you put on the pedal. A light application of the brakes is translated by the brake booster into significantly more pressure on the brake master cylinder, ensuring that your car stops quickly. So what happens to the brake booster if your car stalls, resulting in a loss of engine vacuum? Early designers realized that gas engines were hardly foolproof, so they designed a little check valve into the brake booster circuit. The brake booster stores enough vacuum to provide full boost for two or three pedal applications even after the engine dies. The check valve on the brake booster is what keeps that vacuum from leaking out. And speaking of leaks, that's the reason most brake booster units have to be replaced. As your brake booster ages, the rubber seals and diaphragms that hold the vacuum tend to wear out and crack. If the brake booster can't hold vacuum (despite the check valve's best efforts), its time is up and you'll need a new or remanufactured new brake booster.

Hello donmicca,

On the back side closest to the fire wall the brake booster vacuum line runs from the intake manifold to the booster. It may be that it was dislodged when you changed the spark plugs, just reconnect it and that should solve the problem.

Regards,
netvan

It sounds like the vacuum booster control unit that gives you the power assist check the main vacuum hose that clamps to the unit it mabey leaking engine vacuum or the other end of the hose where it attaches to the intake manifold if there is now leak the unit maybe bad

I would check the medium sized rubber hose that goes to the brake booster from the inlet manifold as it can swell internally and prevent vacuum getting to the power booster. Replace if necessary. I would also check all those pesky little rubber vacuum hoses that are used for pollution control. They tend to go hard and split where they connect to the metal piping if they are over 10 years old, allowing air / vacuum leaks into the inlet manifold. It could also mean that the rubber diaphragm inside the brake booster has developed a leak and causing loss of power assistance.

The vacuum booster is mounted on the fire wall behind the brake pedal. the master cylinder is mounted to it. there is a black hose that is connected to it. that is the vacuum hose. it goes to the intake manifold (engine). check to see if it has come off or is cracked or broken. if everything checks out check vacuum at the hose. if tis there then the diaphram in the booster is bad. replace the booster let me know if you need help with that

It sounds as if the brake booster is leaking. When you press the brake pedal, vacuum is drawn from the engine supply, if the booster is leaking, it allows too much vacuum to leak, stalling motors & triggering sensors. There is a check valve located between the line that comes from the engine manifold over to the brake booster & that can also cause problems.

Plug that line & use your brakes, if it cures the problem, you've found your issue.

Other than that, if you have another vacuum leak, the use of the brakes could just make it so bad that it then triggers symptoms so you shold check your vacuum lines. Vacuum leaks can be identified by usiing a can of carb. cleaner with a nozzle, spraying a small amount in bursts near vacuum lines, connections, and plugged lines. If there is a leak, the engine will stall or slow when it draws in the carb cleaner and you should be able to identify where the leak is. Go slow, spray only a little at a time & allow it to dissipate as it is a flammable product.

good luck