vetracer

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.

bogus

it sounds like the booster is bad... if disconnecting it works and using it kills it...

why? I will have to think on that one.

billybonesmusic

what about brake lights? short in the wiring maybe?

vetracer

It does it with the switch disconnected, too.

Keystring

Bad vacuum regulator in the booster.

It's the little value that plugs into the booster that the vacuum line is connect to.

Even new ones can be bad.

staugur

What happens if you pull the vacuum line and plug it and then hit the brake?If the engine dies then it has to be electrical. It's not the brake lights or cruise control circuits.Did 85's have ABS?Could be there.

vetracer

If that part is "bad", then it is either preventing vaccum flow or it is allowing it. If it is preventing it the idle should remain the same, if it allows vaccum air to be sucked freely into the engine, it should go lean and increase in RPM. 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.

vetracer

If I plug the vaccum line and push the brake the engine idle is normal. No ABS in '85, that came in '86.

Keystring

Then your engine works different than mine does because if I disconnect the vacuum booster line my engine runs rough and the idle drops.But your finger doesn't have a diaphragm that moves in and out when vacuum is applied or removed. If the check valve sticks open, applying the brakes would make the engine idle rough and drop the RPM's because of the sudden drop of vacuum in the booster.

You physically separated the master cylinder from the booster and worked it manually?

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.

CentralCoaster

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.

vetracer

Thanks for all the replies so far. This is what I replaced it with:



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.

staugur

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?

CentralCoaster

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.

ztrips

Did the new booster fix the issue?? It would be interesting to see if the check valve from the old booster (installed on the new booster) would bring the problem back....

vetracer

Idle is normal with vacuum line plugged off and brakes are applied.

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.

The Clevite Kid

http://forums.corvetteforum.com/show...1786894&page=5


See post # 91