Brake booster check: FSM procedure??? (1 Viewer)

[e9999]

before changing my brake fluid, I went through the series of tests listed in the FSM.
There is something wrong or that I don't understand in there.
This is the 97 FSM.

Coming to the brake booster operational test, the first part says to
"Depress the pedal several times with the engine off and check that there is no change in the pedal reserve distance"

but then in the second part "air tightness check", it says to turn the engine off, and depress the pedal several times slowly. If the pedal goes down furthest the 1st time, but gradually rises after the 2nd or 3rd time, the booster is air tight"

Is it just me, or are these 2 tests contradictory? In my case, it did indeed go up each time when pressing, which satisfies the second test, but not first.

It could be consistent if by "no change" in the first test they really meant "no decrease".

Ideas?
What do you see?

thanks
E

 

[Cruiserdrew]

There are basically 2 different tests. In the first one you need to bleed off all the vacuum-thats why you depress the pedal several times. Then with your foot on the brake, start the truck. As vacuum is reestablished and you get power assist, the pedal should go down further without additional pedal force on your part.

The second test trieds to establish that your booster will hold vacuum by establishing a full "charge" of vacuum in the booster and shutting off the truck. Then you use a bit of vacuum with each stroke so the pedal won't go as far each time because it has less assist.

Boosters can fail in another way as well that is not detected in this test. Sometimes they leak at full pedal travel only. This gives normal results in the above tests, but unpredictable behavior when you really romp on the brakes, like to hold yourself on a steep hill. If you fully apply the break and hear any hissing near the firewall inside or outside the truck, you need a new booster.

I had to replace the booster on my FJ40 but has anyone ever heard of an 80 series booster going bad?

 

[Landpimp]

carefull, you will hear a hissing sound even with a good booster, just a different hiss, I think it comes from the bellows around the master push rod in the cab.

I suspect your booster is fine from what you say.