I changes brake booster and the brake lights are on all the time

When the brake booster light and brake light come on after replacing the batteries in a 2017 Isuzu NPR Diesel, it could indicate a problem with the brake system or a malfunctioning brake booster.
Here are some steps you can take to troubleshoot the issue:

1. Check the brake fluid level: Low brake fluid level can cause the brake booster light and brake light to come on. Open the hood and check the brake fluid level in the reservoir. If the level is low, top it up with the recommended brake fluid and check if the lights go off.
2. Check the brake system: Inspect the brake lines, brake pads, and brake rotors for any damage or wear. If the brake pads are worn out, replace them immediately.
3. Check the brake booster: The brake booster is responsible for increasing the force applied to the brake pedal. Check the brake booster for any damage or wear. If the brake booster is malfunctioning, it will need to be replaced.
4. Reset the brake booster light: If the brake booster light and brake light stay on even after checking the brake fluid level and inspecting the brake system, it may be necessary to reset the brake booster light. To reset the brake booster light, turn the ignition switch to the "on" position without starting the engine. Pump the brake pedal 3 times, then turn the ignition switch off. Start the engine and check if the lights go off.

If none of these steps work, it is recommended to take the vehicle to a certified mechanic or the nearest Isuzu dealership for a professional diagnosis and repair.

Suspect insufficient bleeding of the new booster/lines.

Booster problems are rare but they do go bad sometimes.
A bad booster generally causes the brake pedal to feel hard to push down, and a lot of pressure on the brake pedal is required to make the car stop.
Rarely, a vacuum brake booster will develop a vacuum leak, which can cause engine performance problems, hissing or whistling noise, as well as requiring increased pressure on the brake pedal to stop.

The first test:
With the vehicle parked, turned OFF, and the parking brake set, pump the brake pedal 5 to 6 times. If the pedal gets harder to push after several times, your booster is probably good.

The second test (To make sure):
After the test above, without making any changes, press and hold the brake pedal with light to moderate pressure.
Now with your foot still on the brake pedal, start the vehicle.
If the booster is good, you will feel the pedal go down when the engine starts.

Hope this helps!

I had my power brake booster go bad and I replaced mine as well as bleed the brakes that when I went and tried them out 'I slammed on my breaks early before getting to close to the light' that my car left skid marks all over that road because the brakes worked like power brakes should and yours sound like your booster was installed wrong or the pads are bad or you didn't bleed all the air out of the line. Your problem you described was like it felt before I replace brake booster ie lousy non power brakes when I had power brakes.

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.

You sure you have the correct booster
Or use the same rod
You should not have to bleed anything,you
didn't open the brake system
Now you need to bleed all 4 corners,never
the master,that is a myth.

Ever consider the engine has to be running for
the booster (vacuum remember) to work,
and you can not bleed the brakes without
the engine running as you have to have
someone push the brake pedal thus the booster

the rear brakes shouldn't be apart if your pumping
the pedal,now you screwed up the brake switch position

from autozone.com:

1. Disconnect the negative battery cable.
2. Apply the parking brake.
3. Unplug all necessary electrical connections.
4. Remove the master cylinder from the booster and pull it off the studs, CAREFULLY! It is not necessary to disconnect the brake lines.
5. Disconnect the vacuum hose from the check valve.
6. If necessary, remove the brake light switch.
7. Disconnect the booster pushrod at the brake pedal.
8. Remove the booster mounting nuts, located on the inside of the firewall.

Fig. Fig. 1: Vacuum booster mounting

1. Lift off the booster and remove the gasket.

To install:

1. Install the booster with a new gasket.
2. Tighten the booster mounting nuts to 21 ft. lbs. (29 Nm) on all except 1996-98 models. On 1996-98 models, tighten the nuts to 26 ft. lbs. (36 Nm).
3. Install all remaining components.

Sounds like there is a fault with the vacuum pump not generating enough vacuum for the brake booster assist, or a leaking vacuum hose. Try pinching off all the vacuum hoses to see if the problem goes away. Could also be a leak in the brake vacuum booster itself

check the vaccum hose that connects to the brake booster under the hood, also check the sensor on the brake booster. it could also be abad brake booster assembly

Mr. Clark:
It would be extraordinary for a technician to recommend a booster and a master cylinder with no previous mention from you of a possible problem in this area. In other words, the technician had no business looking at the m/cyl or the booster, unless you told him that your braking was deficient.
Here is what REALLY happened: Somewhere, I'd bet, your brake light came on, and somebody "topped off" the master cylinder, to shut the light off. Now, please understand that the brake fluid level in the master cylinder reservoir DROPS PROPORTIONALLY to the wear of the brake pads. When the tech replaced the front brake pads, he obviously had to squeeze the caliper pistons into the housings to allow the insertion of the new pads. This action forced the brake fluid from the now-compressed caliper pistons BACK into the master cylinder, and it sprayed out from under the cap, coating the cylinder and the booster. Tell that repair service that you do NOT need a booster and a master cylinder, THEY caused the problem with the fluid being everywhere. I am confident that I just saved you a significent amount of money that you do NOT need to spend. Have a great day.