No brake boost
02-19-2011, 08:18 PM
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2005 S55 1982 300dt
No brake boost
This morning it was 15 degrees F when I went out and started my 300dt. It started right up on the first try. I backed into the street and basically had no brakes. The only way I could stop was by pushing hard on the brake pedal with both feet. Even then braking was very poor. I drove slowly around the neighborhood for about 20 minutes before normal brake function returned. Has anyone else had this happen? Is my brake booster going? Is my vacuum pump not making vacuum until it warms up? Any ideas? This never happens when temps are 30 plus. Only when it get down below 20 degrees.
02-20-2011, 01:50 PM
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83, 300D Federal
Right under where the Main Vacuum Line attaches to the Vacuum Pump is a Check Valve.
Remove the Vacuum Line and unscrew that Valve and see if the inards are still intact. If they are still intact clean it out with WD-40, I would not use Brake Cleaner as it can soften some plastic parts.
You can buy a new check valve but I believe they are close to $50 each.
There is also an O-ring between the Master Cylinder and the Brake Booster. If it is not good you will lose Vacuum.
Of course Vacuum leaks in other parts of the Vacuum System can effect the Braking.
If you have a Vacuum Gauge I would use it directly on the Vacuum Pump to see if it is putting out enough vacuum.
For leaks there is one or 2 small plastic nipples coming off of your maing Brake line. The nipples are brittle and break easily so you want to look for disconnections and breaks in the Vacuum line.
While the Master Cylinder is off see if any Brake Fluid has leaked into the Booster.
while you did say your pedal was hard to depress you did not say if the Pedal was dropping to the Floor before you got braking. If it is dropping more than it should that is a symptom of not enough Brake Fluid or a Master Cylinder going bad.
The above things are not the only things that can cause the issues but a place to start.
Remove the Vacuum Line and unscrew that Valve and see if the inards are still intact. If they are still intact clean it out with WD-40, I would not use Brake Cleaner as it can soften some plastic parts.
You can buy a new check valve but I believe they are close to $50 each.
There is also an O-ring between the Master Cylinder and the Brake Booster. If it is not good you will lose Vacuum.
Of course Vacuum leaks in other parts of the Vacuum System can effect the Braking.
If you have a Vacuum Gauge I would use it directly on the Vacuum Pump to see if it is putting out enough vacuum.
For leaks there is one or 2 small plastic nipples coming off of your maing Brake line. The nipples are brittle and break easily so you want to look for disconnections and breaks in the Vacuum line.
While the Master Cylinder is off see if any Brake Fluid has leaked into the Booster.
while you did say your pedal was hard to depress you did not say if the Pedal was dropping to the Floor before you got braking. If it is dropping more than it should that is a symptom of not enough Brake Fluid or a Master Cylinder going bad.
The above things are not the only things that can cause the issues but a place to start.
Last edited by Outre; 02-20-2011 at 01:57 PM.
02-20-2011, 03:25 PM
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2005 S55 1982 300dt
Ok, I'll start with the check valve and the oring seals at the booster. I have about 15-20 vacuum at idle, although I never checked it when this problem was happening. When the temps are warmer, this problem never happens and the brakes are very good. I'll get back to you with my findings when i get the chance. Right now we are being dumped on with about a foot of snow. My investigation will have to wait until tomorrow.
02-21-2011, 11:54 AM
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83, 300D Federal
Ok, I'll start with the check valve and the oring seals at the booster. I have about 15-20 vacuum at idle, although I never checked it when this problem was happening. When the temps are warmer, this problem never happens and the brakes are very good. I'll get back to you with my findings when i get the chance. Right now we are being dumped on with about a foot of snow. My investigation will have to wait until tomorrow.
Ounce you Engine is going a while you should get 21-22 of vacuum.from the other nipple on the main Vacuum Line.
