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Lamp
Lamp, ASE Certified Technician
Category: Car
Satisfied Customers: 200
Experience:  Ase certified Electronics, Brakes, Steering and Suspension
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Why does my car shake violently when I brake? The following

Customer Question

Why does my car shake violently when I brake? The following items are new: I do not feel any movement in the brake pedal Brakes- rotors, pad, calipers. Engine-remanufactured. Transmission inspected-trans mounts replaced. rear differential replaced. body mounts replaced. new radiator, new flywheel. I have a 1978 cadillac coupe deville 7 litre engine rear wheel drive. 425hp 4 barrel carb. I had a remanufactured carbuerator installed but another mechanic told me that it needs to be replaced. Specifically the flow of gas is not right so possibly the floats are broken. I have had many mechanics drive it none can find the problem. I was also told that I need the egr valve replaced. it was cleaned throughly but it is broken. the temp sending unit for the coolant needs to be replaced because it has a slight ground coming off it and shows up on the dash. there is nothing wrong otherwise. the engine does not run hot.
Submitted: 5 years ago.
Category: Car
Expert:  Lamp replied 5 years ago.

Hello my name is Hector

 

Looks like you have replaced almost all the items on the vehicle. By the way you are describing the issue when you hit the brake the entire engine and all shake. I take it you have already done the preliminary checking for any loose parts in the suspension and all the rotors are not warped. I would really look into the vacuum assist of the brake booster. If the booster is leaking when you press the brake pedal it creates a HUGE vacuum leak that would be the same as disconnecting a vacuum line. Try looking at that first.

 

 

Customer: replied 5 years ago.
rotors are brand new. i had the motor mounts checked out too.

tell me more of this brake booster. i had the brake lines replaced along with the master cylinder. but this booster. i have not had replaced. if it was not working explain this in more detail.
Expert:  Lamp replied 5 years ago.

They way it works is that the master cylinder is mounted onto the booster. When you push the brake pedal the master cylinder using Vacuum form your engine will multiply the force and assist with braking. If it is leaking when you press it the vacumn that your vehicle is producing is leaking out. Think of it as pulling off one of your vacumn hoses off the carburetor it will start shaking like a champ.Look over the hose to make sure not leaking. If the rotors are new that does not necessarily mean they do not have to be turned. If the steering wheel is shaking have them double check maybe even turned. If the entire vehicle shakes feels like the engine shaking, then check the booster and the hose to it.

 

Customer: replied 5 years ago.
wouldnt there be brake fluid all over the place if that was the case. my mechanic found leaks in the master cylinder and breaks in the lines and fixed them. the rotors are all very recently new like not even 500 miles on them. they were brand new out of the box

how could the brake booster vaccum go so easily unnoticed?
Expert:  Lamp replied 5 years ago.
Not necessarily the brake fluid does not go into the booster only the master cylinder. The only thing in the booster is the vacumn (air) no fluid. Just make sure you check the line and the little valve on the line.
Customer: replied 5 years ago.
this would and could cause the body to violently shake?
Expert:  Lamp replied 5 years ago.
It would cause the vehicle to shake as if you had a misfire, maybe twice as bad. It would be very noticeable because the vacumn leak would make the engine think and act like it had a HUGE vacumn leak.
Customer: replied 5 years ago.
if it is not that, what else could it be?
Expert:  Lamp replied 5 years ago.
That would be it the booster, line, valve or the rotors. Nothing else would make the vehicle shake, if nothing is loose in the suspension.
Expert:  Lamp replied 5 years ago.

One more thing you may want to look at the drums in the rear. If they are warped will also cause the vehicle to shake the body. You can check the booster and the vacumn line by pressing it all the way down if it causes a leak the engine will shake.

Customer: replied 5 years ago.
the drums were either replaced new or turned.
Expert:  Lamp replied 5 years ago.
A bad torque convertor will also make the vehicle shake when coming to a stop. May want to have that part checked again.
Customer: replied 5 years ago.
You can check the booster and the vacumn line by pressing it all the way down if it causes a leak the engine will shake.

The whole car shakes also when I put my foot on the brake and shift from any gear to any gear. park neutral drive reverse.

does that info help?
Customer: replied 5 years ago.
suspension brakes rotors and drums have been checked inspected and replaced withing the last 6 months


and the torque converter was also checked and inspected about 5 months ago
Customer: replied 5 years ago.
if you help me figure this out I wouldnt mind giving you an extra 30 bucks. stay with me here.
Customer: replied 5 years ago.
You can check the booster and the vacumn line by pressing it all the way down if it causes a leak the engine will shake.


elaborate this- pressing what all the way down?
Customer: replied 5 years ago.
so if i have all these things check inspected and or repair any of these items if they are bad the shaking when braking and shifting from park neutral and reverse and drive should all stop?
Customer: replied 5 years ago.
answer all the questions i last sent and i will give you an extra 30 bucks.
plus the original 20 so thats fifty
Customer: replied 5 years ago.
untill i feel i have a comprehensive check list I am not going to accept this answer.
Expert:  jwtruck76 replied 5 years ago.
If ou have a way to do this test, try this: jack the drive wheels of th e vehicle up. In this case the rear wheels. Run the vehicle up to about 45 M.P.H and hold down the brake pedal. If the vehicle stalls (dies) there is an issue with the torque convertor.
Customer: replied 5 years ago.
how i check for an air leak in the power brake booster, lines and valve?
Expert:  Lamp replied 5 years ago.

Here let me start from the top I will document all the things it could be and all the things that it may be.

 

If the vehicle when braking is shaking and the wheel shakes then the issue is in the front end. Start by checking the rotors, even though they are new out of the box them may need to be resurfaced. Also check the calipers to make sure one is not binding and causing the vibration. Second check all the ball joints and bushing in the front end if one is loose this will cause the shake through the front end and steering wheel.

 

If the vehicle is shaking and you feel it in the pedal or on the seat and rear of the vehicle then the drums may need to be resurfaced. Have them check all the hardware and make sure that all the drum system is in working order. This may be a stretch but since you have tried everything look over the differential make sure any bushings and the rear suspension are all tight and nothing has play.

 

If the entire vehicle shakes then you may have a torque converter issue. Even though it was checked previously may want to try the check provided above. What happens is that the torque converter when braking will still be cranking away and causing the entire vehicle to shake. When you make it shake kick it into neutral if the vehicle stops shaking then that will condemn the torque converter.

 

The booster can only be checked using a vacumn pump, you pull a vacumn and it should hold. Also check the hose to make sure that it is not dry rotted. The valve should be on the side of the side of the booster and should have a huge vacumn line to it.

 

Other than that the only thing you can do is recheck the mounts and all the items previously checked something had to have been missed. You stated that the carburetor need to be addressed it may also be contributing to the issue but it is highly unlikely it would make the vehicle stall.

 

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Lamp, ASE Certified Technician
Category: Car
Satisfied Customers: 200
Experience: Ase certified Electronics, Brakes, Steering and Suspension
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