Engine dies when brake applied - Why?
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 2,168
Likes: 16
From: Phoenix AZ
Engine dies when brake applied - Why?
Hello everyone -
It has been a while since I posted here, the C4 has been doing well and modding the C6 has been giving me most of my tech challenges so I've been spending a lot of time over there. My question is does anyone know why the motor would die when the brakes are applied? I just changed my master cylinder and vaccum booster and when I started it up and pressed the brake the motor stumbled and died. When I take the master cylinder loose from the booster and press the brake everything is normal. If I disconnect the vaccum hose the motor speeds up.
So I am asking myself, how can this be happening? If the booster is bad, the only thing it can do is let more air in and speed up, same as pulling the hose. What could be killing the motor? I checked the new master cylinder for leaks, looked for signs of wetness (brake fluid) in the vaccum line and found none. I put the old brake booster on and found no change. Please help!!
BTW my car is a carbuerated '85 with a race motor.
It has been a while since I posted here, the C4 has been doing well and modding the C6 has been giving me most of my tech challenges so I've been spending a lot of time over there. My question is does anyone know why the motor would die when the brakes are applied? I just changed my master cylinder and vaccum booster and when I started it up and pressed the brake the motor stumbled and died. When I take the master cylinder loose from the booster and press the brake everything is normal. If I disconnect the vaccum hose the motor speeds up.
So I am asking myself, how can this be happening? If the booster is bad, the only thing it can do is let more air in and speed up, same as pulling the hose. What could be killing the motor? I checked the new master cylinder for leaks, looked for signs of wetness (brake fluid) in the vaccum line and found none. I put the old brake booster on and found no change. Please help!!
BTW my car is a carbuerated '85 with a race motor.
Drifting
Joined: Apr 2007
Posts: 1,339
Likes: 2
From: port charlotte fl
Hello everyone -
It has been a while since I posted here, the C4 has been doing well and modding the C6 has been giving me most of my tech challenges so I've been spending a lot of time over there. My question is does anyone know why the motor would die when the brakes are applied? I just changed my master cylinder and vaccum booster and when I started it up and pressed the brake the motor stumbled and died. When I take the master cylinder loose from the booster and press the brake everything is normal. If I disconnect the vaccum hose the motor speeds up.
So I am asking myself, how can this be happening? If the booster is bad, the only thing it can do is let more air in and speed up, same as pulling the hose. What could be killing the motor? I checked the new master cylinder for leaks, looked for signs of wetness (brake fluid) in the vaccum line and found none. I put the old brake booster on and found no change. Please help!!
BTW my car is a carbuerated '85 with a race motor.
It has been a while since I posted here, the C4 has been doing well and modding the C6 has been giving me most of my tech challenges so I've been spending a lot of time over there. My question is does anyone know why the motor would die when the brakes are applied? I just changed my master cylinder and vaccum booster and when I started it up and pressed the brake the motor stumbled and died. When I take the master cylinder loose from the booster and press the brake everything is normal. If I disconnect the vaccum hose the motor speeds up.
So I am asking myself, how can this be happening? If the booster is bad, the only thing it can do is let more air in and speed up, same as pulling the hose. What could be killing the motor? I checked the new master cylinder for leaks, looked for signs of wetness (brake fluid) in the vaccum line and found none. I put the old brake booster on and found no change. Please help!!
BTW my car is a carbuerated '85 with a race motor.
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 2,168
Likes: 16
From: Phoenix AZ
Again, oddly enough when I take the master cylinder off and push the brake pedal (which moves the booster through its cycle) the idle stays normal.
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 2,168
Likes: 16
From: Phoenix AZ
If I plug the vaccum line and push the brake the engine idle is normal. No ABS in '85, that came in '86.
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Drifting
Joined: Aug 2006
Posts: 1,624
Likes: 2
From: Canandaigua New York
That's what happens when I put my thumb over the vaccum line that attaches to the booster - idle is normal. If I remove my thumb the air gets sucked in, the engine leans out and RPM's increase. I cannot in any way duplicate the condition of stalling the engine by increasing/decreasing the flow of vaccum from the booster hose to the engine. It makes no sense.
Again, oddly enough when I take the master cylinder off and push the brake pedal (which moves the booster through its cycle) the idle stays normal.
If so, then the booster would maintain the vacuum because the diaphragm would not move and cause a pressure difference between the 'intake' side and the 'vacuum' side.
When you hit the brake pedal, the rod cracks open a valve, allowing air to enter the booster on one side of the diaphragm while sealing off the vacuum. This increases pressure on that side of the diaphragm so that it helps to push the rod, which in turn pushes the piston in the master cylinder.
Without the check valve, the vacuum line would appear to the system as 'wide open' or disconnected from the booster.
This may not be your problem, but is one possibility.
Team Owner
Joined: May 2002
Posts: 24,337
Likes: 25
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Your brake booster is bad. No need to overcomplicated it, just replace it. Don't get a cheap sh*t one either, I and others have wasted plenty of time installing ones that were bad right out of the box.
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 2,168
Likes: 16
From: Phoenix AZ
Funny thing is the old one used to work fine. I only replaced it because I replaced the master cylinder and thought it would be a good idea to replace the booster since they were both original and this car has been raced agressively for the last 10 years. I was having intermittent pedal travel and loss of brake pedal firmness and had troubleshot and eliminated the rest of the brake system as a source of the problem. Now neither one of them work.
Drifting
Joined: Apr 2007
Posts: 1,339
Likes: 2
From: port charlotte fl
Lets go back to my post.You plug it(vacuum line) and hit the brakes engine idle's-yes or no? you attach it to the booster and hit the brakes engine dies,yes or no? If you just leave it off does the engine speed up and stay running?And just for fun what happens if the MC is disconnected from the booster as well as the vacuum and you hit the brake?
Team Owner
Joined: May 2002
Posts: 24,337
Likes: 25
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Was the brake warning light coming on during any of these episodes of excess pedal travel?
Make sure to adjust the pushrod length on your new booster. I don't know if too short of one could overextend the diaphram and cause a vacuum leak, but it's possible. And it will put slack in the pedal 100% of the time. Perhaps the piston on your new master cylinder has a different piston cup and requires a different booster pushrod adjustment.
Last edited by CentralCoaster; Jun 10, 2007 at 02:01 AM.
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 2,168
Likes: 16
From: Phoenix AZ
Yes, engine dies when booster vacuum line is attached and brakes are applied.
If you disconnect it while the engine is running the engine speeds up because of the lean condition it is causing. That's why it doesn't make sense that the engine dies when the brakes are applied. If the booster is bad, the only thing it CAN do is allow a vacuum leak, which should increase engine RPM. If the MC is disconnected and the brakes are applied, the booster rod moves accordingly and there is no change in engine RPM.
Safety Car
Joined: Jan 2006
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From: Miami FL
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