issue with brake vaccum, need help
10-09-2017, 04:23 PM
#1
issue with brake vaccum, need help
In my new turbo race car (and another S1 car), am having brake issue:
after a long straight, if I immediately lift and stomp on the brake. Brake goes HARD, as if there is no assist. If I do it slow (wait a second before applying brakes) or if braking zone is NOT immediately after a long straight, brake works as normal.
I have Wilwood front, ABS, NB chassis. the other S1 car also have a Wilwood front, but NA chassis. Already replaced the one way check vacuum valve, same thing.
2 cars are doing this. but 3 other S1 cars don't have this issue. wondering if anyone here had a similar issue and found a fix. thanks.
after a long straight, if I immediately lift and stomp on the brake. Brake goes HARD, as if there is no assist. If I do it slow (wait a second before applying brakes) or if braking zone is NOT immediately after a long straight, brake works as normal.
I have Wilwood front, ABS, NB chassis. the other S1 car also have a Wilwood front, but NA chassis. Already replaced the one way check vacuum valve, same thing.
2 cars are doing this. but 3 other S1 cars don't have this issue. wondering if anyone here had a similar issue and found a fix. thanks.
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10-09-2017, 09:52 PM
#3
What is your idle speed set at? Turn it down as low as possible to create the most vacuum during high rpm lifts.
Consider disabling the ISCV and setting idle with throttle body screw (assuming it has one). This eliminates the ECU trying to catch the motor on a dip.
The whole idea is to create a high vacuum condition between shifts. I would also log vacuum if possible or mount a big gauge in camera view. Should help give you a better idea of what's going on.
Consider disabling the ISCV and setting idle with throttle body screw (assuming it has one). This eliminates the ECU trying to catch the motor on a dip.
The whole idea is to create a high vacuum condition between shifts. I would also log vacuum if possible or mount a big gauge in camera view. Should help give you a better idea of what's going on.
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10-10-2017, 07:32 AM
#4
Retired Mech Design Engr
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The check valve between the manifold and brake booster chamber could be bad and letting the vacuum in the chamber get replaced with air.
If it is OK, and you need more volume of vacuum for more braking sessions between evacuations, then adding an auxiliary reservoir could help.
A way to tell if you have either a leak or a bad check valve is to be running along at high vacuum. Then shut off the engine (XMSN in neutral) and see how many applications of brake (on-off-on-off-on) you get before you are out of assist. Do that on a good car and a bad car and compare.
Or, From the Interweb:
Checking the brake booster:
If the booster vacuum hose and fittings are in good shape, it's time to move to the brake booster itself. A common and simple way to test the brake booster is by using the brake pedal.
1. Sit behind the steering wheel, set the transmission to Parking (automatic) or Neutral (manual), set the Emergency brakes, and start the engine. Let it idle for two minutes and then shut if off.
2. Pump the brake pedal at normal foot pressure four times and hold your foot on the pedal pressing down slightly on it.
3. Start the engine. As you start the engine, you should feel the brake pedal moving downward slightly, about an inch or less. Otherwise, you don't have enough vacuum in the brake booster. To locate the fault, do the Engine Vacuum and Brake Booster tests described in the following sections.
4. With the engine still idling, remove your foot from the brake pedal and turn off the engine.
5. Depress the brake pedal (using normal foot pressure) four times. If you notice the pedal rising after the second or third you depress it, the booster is more likely holding vacuum. Otherwise, a vacuum leak is affecting booster performance. To locate the problematic area, test engine vacuum and brake booster performance using a vacuum gage and a hand held vacuum pump as described in the next sections.
6. Start the engine and let it idle.
7. Now, push down the brake pedal and turn off the engine, but hold the pedal depressed for about 30 seconds after shutting off the engine.
The pedal should hold its position, if not, there's a leak in the brake booster, valve, vacuum hose or intake manifold. Check the vacuum hose, booster check valve, engine vacuum and brake booster as described in the following sections.
If it is OK, and you need more volume of vacuum for more braking sessions between evacuations, then adding an auxiliary reservoir could help.
A way to tell if you have either a leak or a bad check valve is to be running along at high vacuum. Then shut off the engine (XMSN in neutral) and see how many applications of brake (on-off-on-off-on) you get before you are out of assist. Do that on a good car and a bad car and compare.
Or, From the Interweb:
Checking the brake booster:
If the booster vacuum hose and fittings are in good shape, it's time to move to the brake booster itself. A common and simple way to test the brake booster is by using the brake pedal.
1. Sit behind the steering wheel, set the transmission to Parking (automatic) or Neutral (manual), set the Emergency brakes, and start the engine. Let it idle for two minutes and then shut if off.
2. Pump the brake pedal at normal foot pressure four times and hold your foot on the pedal pressing down slightly on it.
3. Start the engine. As you start the engine, you should feel the brake pedal moving downward slightly, about an inch or less. Otherwise, you don't have enough vacuum in the brake booster. To locate the fault, do the Engine Vacuum and Brake Booster tests described in the following sections.
4. With the engine still idling, remove your foot from the brake pedal and turn off the engine.
5. Depress the brake pedal (using normal foot pressure) four times. If you notice the pedal rising after the second or third you depress it, the booster is more likely holding vacuum. Otherwise, a vacuum leak is affecting booster performance. To locate the problematic area, test engine vacuum and brake booster performance using a vacuum gage and a hand held vacuum pump as described in the next sections.
6. Start the engine and let it idle.
7. Now, push down the brake pedal and turn off the engine, but hold the pedal depressed for about 30 seconds after shutting off the engine.
The pedal should hold its position, if not, there's a leak in the brake booster, valve, vacuum hose or intake manifold. Check the vacuum hose, booster check valve, engine vacuum and brake booster as described in the following sections.
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10-13-2017, 08:06 PM
#6
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Could be a small leak in the booster. Down the straight the manifold is in positive pressure, so the check valve is closed. Leak in booster means that ambient air leaks in and you lose the vacuum.
If you lift and wait a couple seconds before getting on the brakes, the engine sucks all the air out of the booster, faster than it can be replenished by the leak, so you get vacuum and boost assist.
Tee a MAP sensor or boost/vacuum gauge into the brake booster line on the booster side of the check valve and watch what it does in various throttle positions.
--Ian
If you lift and wait a couple seconds before getting on the brakes, the engine sucks all the air out of the booster, faster than it can be replenished by the leak, so you get vacuum and boost assist.
Tee a MAP sensor or boost/vacuum gauge into the brake booster line on the booster side of the check valve and watch what it does in various throttle positions.
--Ian
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10-13-2017, 08:50 PM
#7
My my money is on bad booster. Given the age of the diagram it's not surprising. Most cars would not show symptoms due because usage and motor type would mask problem.
Best of luck William
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