Query about brake problem

y4kas, Sep 9, 9:14am
I have a early 90's Nissan Bluebird (Diesel).Every so often the brakes go very soft and I end up using heavy force to slow up the car.Brake idiot light also flashes occasionally on heavy braking (however no real pattern to it so may be unrelated).Once the problem occurs it goes away again for minutes/days/months.

Does it sound like its a faulty booster! Cheers

franc123, Sep 9, 9:23am
More like an issue with the vacuum supply to the booster, or the vacuum pump itself on the alternator.Check for any disconnected vacuum lines in the engine bay, the pump can't keep up the supply to the booster if there are leaks, especially when the engine is running slowly and you are applying the brakes repeatedly, such as coasting down a hill for instance.When this happens the pedal goes hard, not soft though,an intermittently soft pedal is something you'd associate with a master cylinder fault.

mugenb20b, Sep 9, 9:24am
I would be checking vacuum hoses between the vacuum pump and booster, make sure they aren't split, oily, etc. Also check the alternator belt tension.

snoopy221, Sep 9, 9:27am
Brake idiot light also flashes occasionally on heavy braking (however no real pattern to it so may be unrelated).
Alas NOPE you ACTUALLY mean the LOW Vacuum warning pressure/vacuum switch is bringing the ***warning***** Light on as it should.

snoopy221, Sep 9, 9:29am
Incidentaly-whilst agreeing with other posters.
Have seen sender warning light units fail.[leaking fail.]

poppajn, Sep 9, 9:30am
Low brake fluid!, it,s just a thought.

dent, Sep 9, 9:45am
possibly two diffrent faults. Low vacume issue and maybe bypassing master cylinder.

next-to-normal, Sep 9, 9:53am
could be old fluid is now in the line---flush dont work to well when fluid hot

monzaman, Sep 9, 10:35am
Low or no vacuum to booster will make br pedal much harder,fluid level,cond or m/cyl bypassing!

sr2, Sep 9, 12:23pm
If there was an intermittent problem with the m/cyl (i.e. bypassing or not compensating) you would probably loose your pedal whenever the problem occurred; further more intermittent m/cyl problems are usually few and far between.I??

snoopy221, Sep 9, 12:39pm
I'd politely suggest posters READ post 1 at this point.
I.E.

early 90's Nissan Bluebird (Diesel). Every so often the brakes go very soft and I end up using heavy force to slow up the car. Brake idiot light also flashes occasionally on heavy braking

heavy force.heavy braking.
Now when brakes go **soft** and you have to use heavy force to stop.
well the **soft** is akin to sumfink **soft** dat don't work as well.or limp even.[Irrespectivley heavy force on pedal and lazy limp slowing and a low vacuum warning light on.mmmkay.
-Oh Dat's Right.!Idiot Light!-i forgot.

snoopy221, Sep 9, 12:42pm
The diagnostic !tip! here sr2 is poster one starts is diesel and pumps the brake pedal about five or six***idiot light**.
Will come on.Beyond Doubt.
shall we guess the number! 4! 5!6!

y4kas, Sep 9, 11:34pm
HI all, thank you for the advice.

I started the car with my foot on the brake and definately felt the brake pedal "give" as the motor started.With no revs pumping the brake pedal once brings up the idiot light for 3-4 seconds, which can be repeated endlessly.Holding the motor constant with gentle revs and pumping the brake it takes about 18 pumps to bring up the idiot light, once again 3-4 seconds before it goes off, to come on again after 1-4 pumps.Giving it a break to recharge and it once again is back to about 18 pumps.

Hoses and claims look fine.Time to look at the diesel's vaccum pump!

Many thanks.

skin1235, Sep 10, 12:14am
pop the oil drain pipe off and check if there is actually any oil getting through the vac pump, if not be prepared for quite a cost, and I'd suspect there is not, they are not a high oil flow system and block quite easily, then the rotors wear out and the shafts wear out, eventually they will take out the alternator as well

snoopy221, Sep 10, 1:47am
re post #13.a vacuum gauge is a handy tool here-however it IS a nissan circa 1990.they do NOT run a vacuum tank and it is common for the **idiot light** sensor to fail by leaking-obtain a good secondhand-or new **idiot light switch-or to further test-follow the small vacuum line from the alternator to the idiot light switch and remove switch and insert appropriate threaded bolt and repeat procedure in post #13 and the pedal **feel** will soon tell ya if the switch was leaking-K

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