Brake Vacuum Booster,Failing?

[nelson]

Mine is a 93 mu 2.8,I have recently replaced all of my disc pads front and back and all done properly.I have noticed that when it is idling the brakes are not functioning well and this warning light switches on and off as I step on the brakes.The brake pedal slightly reaches the floor and when I pump the brakes it get hard and softens again.
When the vehicle is running the brakes are all o.k and now without this warning light switching on.
My suspect is the brake vacuum booster,if I am wrong what would be the culprit behind this brake failure???

[monty]

If booster was faulty the pedal should be hard to depress as it assists in the brake application, being able to pump up brakes usually indicates air in the brake lines and the light on the dash should be hooked up to a switch on the master cylinder to indicate that the valves in the brake master cylinder are traveling too far indicating a problem with pressure (e.g. air or leak) in the brake lines or cylinders.

[monty]

When you put on brake pedal with engine off and holding it, when you start the motor does the pedal sink a bit further, as in more brake pressure being applied???

[James W]

The warning light operates on low fluid levels in the master cyclinder tank... there is a float switch... check the fluid level real soon before more air gets in... and it will need a bleed anyway by the sounds of it.... also go searching for any leaks

[geeves]

It could be a master cylinder fault but my money is as follows. Theres 2 switches in the master cylinder and one in the booster. One does fluid level as James says. Do also check that the float hasnt stuck down. However if the fluid was low or there had been a loss of vacuum the light would stay on. It also doesnt explain pushing the pedal to the floor.
The other switch on the master cylinder measure the balance between the front and back brakes. This is I expect a true sign of the problem. The pedal being pushed to the floor but able to pump up is the other sign. This indicates a brake calliper with excess travel. Normally I would go straight to sticking slides on the callipers but since you have just replaced the rear pads the adjuster on the rear calliper may also be out of adjustment.
First Remove all the callipers from there mounts ant check and lubricate the 2 slides. One is the bolt the other is a metal sleeve. Reassemble and problem might be fixed. If not recheck the adjuster on the rear callipers. This is the small allen key under the 12mm bolt in the back of the calliper that you wound the piston back with when you replaced the rear pads. Wind it in till the brakes just bind then release a little bit. Problem fixed.
Another possibility is that the adjuster mechanism in the calliper is faulty. This means an expensive rebuild by you friendly brake specialist

[nelson]

when the pedals are depressed and the engine is turned on, the pedals slowly sink,the float to the master cylinder is o.k.The thing that puzzles me is that this only occurs in a stationary state but when the vehicle is running the brakes function very well.I will look on to your recommendations and and see how it goes.Have changed a lot of disc pads but this is the first time that scenario has happened to me.

[geeves]

I did have the same thing in an old Toyota I owned 2 cars before the Isuzu. It was a leaking seal in the master cylinder letting the fluid back to the reservoir. A nasty trick with brake master cylinders is that when they leak it ends up in the booster and doesnt come out anywhere. Only symptom is fluid loss. With my Toyota there was no fluid loss.
The reason it doesnt do it while driving is that theres very few occasions that you brake as hard for as long as the stationary test. To prove this do the stationary test with the fast idle set to 1500 revs. You will get same result

[Demonic]

I agree with geeves, my old toyota had the same problem, leaking past the Master cylinder back into the reservoir. the give away was the fact i could push the brake pedal to the floor.