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Dad Mechanic
Dad Mechanic, Mechanic
Category: Car
Satisfied Customers: 202
Experience:  I have worked on everyting from Model Ts to 2012 Autos,
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Nissa 280zx: I put on a new disc brake system along with a

Customer Question

I put on a new disc brake system along with a new engine in an older auto. My brakes are very very sensative and I have been told it is because I have too much vacuum going to the brake power booster. I am also told the tell-tale sign is the fact that the brake pedal takes a dive when the car engine is started and returns to normal when the engine is shut off.

Do you think this is the problem and if so, how do I reduce the vacuum going to the power booster? OR if you think it is another issue - what do think it is and how do I fix it.
Submitted: 4 years ago.
Category: Car
Expert:  Dad Mechanic replied 4 years ago.

Dad Mechanic :

Sorry for your problem. I am here to try and assist you.can i ask you a few questions first. did you own the car you are working on before the engine & brake change?

Customer :

Yes I bought it to do these changes

Dad Mechanic :

Had you driven the car before you made the changes?

Customer :

Yes and the problem did not exist. The original engine was a 150HP 6cyl standard engine for this car. I also have not changed the power booster - it is the stock booster.,

Dad Mechanic :

I was wondering because if the booster gets a crack in the diphram they can go down to a point and the crack will seal amd the brakes will apply quickly almost like a panic stop sometimes.

Dad Mechanic :

There is no more vacuum on your 350 than the orignal engine so your problem must be with the new brake system. did the nissan have 4 wheeld drum brakes or rear drum and disc front?

Customer :

Rear drum and front disc

Dad Mechanic :

I would try your stopping on dirt and see if the rear brakes are grabbing, you will be able to see better in dirt with light brake presure?

Dad Mechanic :

are your new rotor and calipers much larger than the old rotors?

Customer :

Yes they are. I also did put a new master cylinder on - this is a Wilwood system - I was told the booster would be OK with the new system.

Dad Mechanic :

did you try disconnecting the vacuum to the booster ato see if you even need to have it in the system?

Customer :

NO

Dad Mechanic :

Iwould try that to see if you need the booster in the system. I don't know of any thing that would allow you to reduce the vacuum in the system like a air presure regulator.

Customer :

Give me one minute and I will let you know

Dad Mechanic :

You may be able to get a 12 volt vacuum pump and a resirve tank they would not produce the amount of vacuum that the engine produses. would that be something would be intersted in tring?

Customer :

I just tried removing the vacuum line. That is not the answer.

Dad Mechanic :

it would seem you have too much presure to the calipers it dosen't just brake normal and just grab, it just seems grab from the start

Dad Mechanic :

This timed out whill i wasn't watching

Customer :

Yes - unless I really pay attention to the foot pressure I could project myself through the windshield.

Customer :

Sounds as though this problem remains unsolved.

Dad Mechanic :

Maybe a simpler solution to your problem would be to chang the fulcrum on the pedal by making the pedal arm longer so you have to use more foot presur to apply. would you have room to make the pedal arm longer from the master cyl to the pivot arm? I'll have to change this to answer mode because the will get on my case about it . you don't have to accept re give me a rating untill you are satisfied with my answers please don't rate me at a 1 or2.

Dad Mechanic :

why would you rae my service as bad as we haven't exhaused all the avenues of possibilitys?

Dad Mechanic :

are you still there

Dad Mechanic :

Welli'll still try to help you even though you have rated me bad. Another possible solution is to use a larger bore master cylinder. This will reduce the presure to the wheels and you will have to apply more pressure to the pedal. you can also change my rating.