loss of power assist on brakes.

Sierra3

Sierra3

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Had a new booster replaced at dealership thinking that was the issue. Brakes function properly with plenty of assist under normal above 10MPH driving on and off. But it looses asist when holding brakes for a long time in bumper to bumper traffic at idle in drive. It's similar to tractor trailer brakes when you keep pumping the brakes and air pressure goes down on the gauge. The pedal gets really hard to push with no power assist at all. Its a sobering experience when comming up on a car ahead of you and you have to leg press the brake pedal when you expect it to be alot eaiser.
What im looking for if anyone can take a pic of the Mark VIII set up particularly the directinal valve and vacum hose routing. Im suspecting i have it hooked up wrong or a leak in my vacuum canister, and at idle not producing enough vacuum to compensate for the leak? Suggestions or things i should look for?
It only happens when im at very low parking lot speeds and engine is in drive and basicly idleing in trafic or stopped traffic. Then the next time you apply brakes you get a surprise of no assist.
I have the 2001 SVT motor in there but tried to keep everything else factory Mark 8. Only emmisions that are missing is EGR which is blocked off with plate. Did the Cobra have a bigger vacuum canister because of the larger cams? Or should i go with a new aftermarket canister from like summit racing or jegs?
 
RE: loss of power assist on brakes.

Whoa!!! Hold the phone...

What am I missing here??? Thanks for providing the diagram.

Okay... from the brake booster check valve, a hose goes to the intake manifold, then to another check valve, and then to a canister?

What the heck is the canister for? What the heck is that check valve for? Where is the check valve going to the canister located?

Before I remove my brake booster which may be bad according to answers to my post about loss of power brakes, if these other components and hoses are involved, I need to know where they are and check them first.

Thanks
 
RE: loss of power assist on brakes.

The 'canister' is a vacuum resevoir used to maintain system vacuum during changing driving conditions where the system vacuum normally fluctuates. The check valve isolates the vacuum resevoir and downstream vacuum components from these same fluctuating vacuum conditions.

On my '93 I believe the check valve is positioned near the EGR and IMRC vacuum solenoids on the drivers side. above the head, back towards the firewall. It is black and blue in color with one inlet and two outlets.

EDIT: These components should NOT be a issue with the normal operation of the vacuum booster since there is a check valve at the inlet of the brake booster. BUT if these components have a leak and cause low vacuum, it is possible the vacuum levels are never high enough to properly maintain vacuum levels at the brake booster.
 
RE: loss of power assist on brakes.

Found the problem last night at the track. Only made one pass and just watched my buddies for the rest of the night.
The straight up aluminum nipple on the upper intake that is directly feeding the power booster has a 90 degree rubber boot that is heating up and getting soft. The rail of the fiberglass hood sits directly on top of it. When it gets soft the hood rail is putting pressure on it and choking it off.
I have to cut a hole in the rail on hood or cut the nipple down some on the cobra intake to get clearance. I dont want to modify the hood for fear of weakening it when it gets slammed. So im looking for options for the 90 degree elbow. A thick plastic one is what im really looking for like i had on my Trans Am intake. That would hold off the pressure of the hood. It's only like 1/4 inch of clearance i need, but its definetly pinching it.
My old booster was probably still good. Well it needed a new one anyways with my old engines milage around 98,000 before she blew.
 
RE: loss of power assist on brakes.

Also if anyone is running the open air elements like this one out of their valve covers like i am, could add to the problem of low vacuum for the power assist. Lonnie told me about this when i went down there for another reburn. JMS Racing put it in there so i left it alone. It's one of those things where you get side tracked with all the other little projects all the time and you get used to seeing it there, so it looks normal. Fine for a carb engine, but it's unmetered air and a heck of a vacuum leak.
I'll be removing mine and plumbing it back into the brake booster side asap. I will be cutting a notch into the backside of hood to clear the elbow. It will allow more hot air to escape onto the windshield also.

1995 DK Blue Lincoln Mark VIII
1998 DK Blue Chevrolet 1500 P/U

http://www.lincolnsclub.org/forum/user_files/1657.jpg
 
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