Soft brake pedal / low vacuum
Thread Starter
Instructor
Joined: Jan 2015
Posts: 124
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From: Cave Creek arizona
Soft brake pedal / low vacuum
1990 L98 pedal has always been soft, tough to lock up / make ABS activate but it can be done. New booster, brakes bled multiple times by peddle and vacuum, new pads, roror, and hoses. Now I have read vacuum below 18 inches can cause this. Running 16 pounds checked with two different gauges.
Thoughts and thanks.
Thoughts and thanks.
Melting Slicks
Joined: Oct 2016
Posts: 2,024
Likes: 363
From: Central Alabama
I'm confused on why you're chasing low vacuum for a soft pedal. Low or no vacuum creates a hard pedal.
Try pressing the pedal 3 or 4 times with the car off. The pedal should get hard to press. If it doesn't you if you have air in the lines.
Now with pressure built up hold your foot on the brake pedal and start the car. Pedal should move down with the weight of your foot. If it does the vacuum system is working.
It sounds like your really saying you have poor braking performance.
Try pressing the pedal 3 or 4 times with the car off. The pedal should get hard to press. If it doesn't you if you have air in the lines.
Now with pressure built up hold your foot on the brake pedal and start the car. Pedal should move down with the weight of your foot. If it does the vacuum system is working.
It sounds like your really saying you have poor braking performance.
Last edited by belairbrian; Mar 17, 2018 at 11:36 AM.
Thread Starter
Instructor
Joined: Jan 2015
Posts: 124
Likes: 5
From: Cave Creek arizona
I'm confused on why you're chasing low vacuum for a soft pedal. Low or no vacuum creates a hard pedal.
Try pressing the pedal 3 or 4 times with the car off. The pedal should get hard to press. If it doesn't you if you have air in the lines.
Now with pressure built up hold your foot on the brake pedal and start the car. Pedal should move down with the weight of your foot. If it does the vacuum system is working.
It sounds like your really saying you have poor braking performance.
Try pressing the pedal 3 or 4 times with the car off. The pedal should get hard to press. If it doesn't you if you have air in the lines.
Now with pressure built up hold your foot on the brake pedal and start the car. Pedal should move down with the weight of your foot. If it does the vacuum system is working.
It sounds like your really saying you have poor braking performance.
That doesn't make sense to me, but multiple brake sites state that 18 to 20 inches of vacuum is needed for the booster to work properly. The car stops alright, it just doesn't feel like true power brakes.
Thank you for your reply.
Race Director
Joined: Oct 2008
Posts: 17,982
Likes: 1,074
From: Lansing MI
Yes i should have said hard peddle. It's like the booster is not doing it's job 100%. I have done the test mentioned above and it did as you stated. I was just wondering if the lower vacuum by 2 pounds would have that much effect on the booster...
That doesn't make sense to me, but multiple brake sites state that 18 to 20 inches of vacuum is needed for the booster to work properly. The car stops alright, it just doesn't feel like true power brakes.
Thank you for your reply.
That doesn't make sense to me, but multiple brake sites state that 18 to 20 inches of vacuum is needed for the booster to work properly. The car stops alright, it just doesn't feel like true power brakes.
Thank you for your reply.
Melting Slicks
Joined: Oct 2016
Posts: 2,024
Likes: 363
From: Central Alabama
The booster stores vacuum so it can work when the engine is in a non-vacuum generating state. Low vacuum would take longer to replace it. But that usually occurs from the engine design. Are you running a stock engine or a modified one?
Do you have any indications of a vacuum leak such as a high idle?
Could be a worn master cylinder, could be the brand of brake pads. Could be the pads were not broken in properly and are glazed.
When my booster was going bad I had a hiss when applying the brakes I sort of had power brakes that faded and made the car hard to stop.
Do you have any indications of a vacuum leak such as a high idle?
Could be a worn master cylinder, could be the brand of brake pads. Could be the pads were not broken in properly and are glazed.
When my booster was going bad I had a hiss when applying the brakes I sort of had power brakes that faded and made the car hard to stop.
Thread Starter
Instructor
Joined: Jan 2015
Posts: 124
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From: Cave Creek arizona
Pedal feels the same in all conditions. Stock engine 115,000 miles. No vacuum leaks that I have been able to find yet. Have not done anything to the master and pads were broke in per the manufactures instructions. No hiss. One suggestion was to add an electric vacuum pump. May have to look on the net to see what is available. I hate throwing money at parts and hoping. Always believed in diagnosing and replace...
Race Director
Joined: Oct 2008
Posts: 17,982
Likes: 1,074
From: Lansing MI
Pedal feels the same in all conditions. Stock engine 115,000 miles. No vacuum leaks that I have been able to find yet. Have not done anything to the master and pads were broke in per the manufactures instructions. No hiss. One suggestion was to add an electric vacuum pump. May have to look on the net to see what is available. I hate throwing money at parts and hoping. Always believed in diagnosing and replace...
The fact that you are saying there's no difference, makes me think there's something wrong with the brake booster.
Race Director
Joined: Oct 2008
Posts: 17,982
Likes: 1,074
From: Lansing MI
Now that I think of it the one way check valve at the booster could do the same thing if its malfunctioning. In fact that part better work perfect if you have borderline insufficient vacuum at idle.
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Thread Starter
Instructor
Joined: Jan 2015
Posts: 124
Likes: 5
From: Cave Creek arizona
I would install a vacuum gauge and tape it to my windshield and drive the car for a bit. I think you are going to find that when you decelerate with the throttle closed and with whatever engine braking is available, the vacuum signal is high enough to operate power brakes once, the first time you depress the pedal, probably not the second.
The fact that you are saying there's no difference, makes me think there's something wrong with the brake booster.
The fact that you are saying there's no difference, makes me think there's something wrong with the brake booster.
Race Director
Joined: Oct 2008
Posts: 17,982
Likes: 1,074
From: Lansing MI
Took it out for a drive with the gauge taped to the window. Never saw it go above 16 inches. I used the actual hose for the booster. The booster is definitely doing at least part of it's job since the brakes really suck without it... Maybe these brakes are just that way. I seem to see a lot of people complain about this. Could the two inches be that much difference. I can get a universal vacuum motor for $145, or maybe try to find one off of a wrecked diesel pickup at the junk yard, but who knows if that would be any good. Only been this way for three years I have owned it so if I cannot figure it out oh well.
Thread Starter
Instructor
Joined: Jan 2015
Posts: 124
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From: Cave Creek arizona
Thanks for all or your suggestions!