2004 Isuzu npr brakes

If it has any drum brakes, adjust them.

It's got drum brakes is there any fluid leaking on the inside of the Rear wheels? Was it low on fluid?

The brakes have been bled, and there is no fluid leaks

Was the o-ring on the master cylinder swapped over when booster was changed? Does the vehicle stop after pedal returns Or does it seem to lack ”power boost”? Can you feel a ”push” on the brake pedal when the engine rpm is increased? I have seen i some cases these o-rings left out on occasion. Make sure to check the ”one-way“ or check valve on the brake booster cable as well.

Hi Chris, The first thing to do is make sure the system is bled correctly. The check for drum brake adjustment (If it has rear drum). If drum brakes are worn out it takes multiple pumps to make the shoes contact and provide braking. The only other thing I can think of at the moment is the master cylinder is bypassing internally due to worn cup seals. Brake booster light? I have not seen that…

It's a low vacuum light, for the booster

That points at the booster vacuum source or the booster, but it would normally have a hard pedal and not be a pump up issue. Did you change the vacuum booster vacuum valve or use the one that came with the booster? I have had bad ones on reman boosters before.

Hi Mitchell, It just dawned on me, that if you pump the brakes multiple times, it depletes the vacuum and the light activates. In that case, do what others and myself recommend and adjust the rear drum brakes.

Yes it seems the brake booster light is for low vacuum,

I had one last week with the exact issue. It had drum brakes and a few clicks on the four adjusters and pedal was perfect.

I will re check the brake adjustments, but pretty sure that was done first

Since it a diesel it requires a auxiliary vacuum pump as there is little to no vacuum at idle. Verify that the pump is providing the required vacuum and that the check valve is also working

smoke test your vacuum lines, thinking I have run into this in my younger day.

Chris If it has disc brakes in front jack up each side and make sure wheel bearings are tight. Also make sure caliper slide pins (assuming disc in front) are not in a bind or seized

Long story short I have to agree with everyone that has responded to your posting. A brake booster is merely a assistant. A soft or low pedal on a hydraulic brake system is always fluid related or surface contact related to the brake lining not mating thr brake drum or rotor . And as mentioned by several other Technicians rear brakes out of adjustment will very well cause that low pedal that you…

I have rebuilt many of those alternators and yes the alternator pump could be bad….Have seen them clogged with fluid….Easy to remove & check…..might pull vacuum hose off and see if it pump pulls any vacuum …..

Ok thanks, Will the vacuum cause the pumping the brake 2-3 times issue?

Adjust the rear brakes

Thanks, I am going to recheck the brakes, but thought that was done first, along with bleeding them.

just try clamping flexible hose to rear brakes first, will prove if excessive movement there. can also clamp fronts, pedal should be solid with no movement. regards Steve.

Extra pump/s is either excess travel in that case brake lining adjustment, worn out drums OR internal loss of fluid pressure in the master once there are no external leaks. The vacuum light is indeed low vac levels which as someone said correctly could be an exhausted supply or insufficient vac at all from a worn or otherwise less productive pump. I think everyone is well on point here so give…

Update, Most everyone on here was correct, It ended up being the rear brake cylinders where bad, plus it did need rear brakes, thanks for all the input

Hi Stephen, The only thing about that method I would add it to plan on replacing the flex hoses in the future. While it is a viable ‘go No go’ test method, it often collapses the inner liner of the brake hoses. Most brake hoses fail internally from what I have dealt with.

we have several fleets of these trucks they have 2 adjusters on each rear wheel the pin must be depressed to adjust. Also they are hard to bleed i can not remember bleeding process . Look it up if you can not find it pm me and i will look it up WE have factory program which has repair procedures in it Luck

The drums will have 2 adjusters and usually they are way out of adjustment I find most do not adjust both

Chris- Why would you replace a booster for a low or sinking brake pedal?

I had one recently with an internal problem within the ABS HCU causing a soft/low pedal issue. Could block off all the lines to the wheels and still have the low pedal, block off hoses from the cab and it was solid.