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Old 03-04-2006, 08:32 PM
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brake light on, could it be a bad master cyl

hi all, having a delima here. got an 80 trans am se with rear disc brakes did a tune up and tested it on the but dyno it ran really good but when i came back to the house it was smoking really bad. found that the intake gasket had been sucked in, fixed that problem and noticed that the brakes went to the floor and the brake light on the dash was on. seems like i have also been losing brake fluid somewhere also, my engine speeds up a little, about 200 rpms when i try to pump up the brakes. also it smokes a little , something it did not do before. could it be my master cyl[ or my booster or possibly both how do i troubleshoot this any help would be greatly appreciated thanks ahead of time TURBO

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Old 03-04-2006, 09:54 PM
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First find where that brake fluid is going to. It may be leaking on the exhaust system, hence...the smoke. Take care of the fluid loss of the braking system, then put brake fluid in the master cylinder and bleed the system. Hopefully that will fix the problem without having to replace the master cylinder!
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Old 03-05-2006, 04:00 AM
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Yes, a brake light can mean a bad M/C. The brake light comes on with the parking brake, and when the pressure differential switch is tripped. If the M/C goes bad, one side will provide more pressure than the other. That trips a switch that kicks the light on warning you of the situation. It can be caused by a bad M/C or by leaking brake fluid that let air in the system.
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Old 03-05-2006, 04:19 AM
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Doc here,

You could also have Brake fluid leaking past the master Cylinder, into the power booster, and into the induction system (smoke) ..

You are getting an increase in RPM's when you step on the brake, (diaphragm leaking)

You are losing fluid (leak into the booster) and not enough pressure to hold the brake light off..

You have smoke ( Brake fluid passing all the way through the system into the vacuum hose, then into the induction system, only to burn off making smoke)

The clues say the cylinder and booster are shot..

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Old 03-07-2006, 07:04 AM
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I have my brake warning light staying on, not everytime I drive though.
I asked a friend in the business about the problem and he gave me this reply:

Yes Malc, if it is the same type of system that was used since back in the 1970 years, I do know about those systems.
Typically the same warning light served to remind that the parking brake was ON, but also warn about a problem with the hydraulic brake system. A switch at either the parking brake or the switch in the hydraulic system grounded the light to turn it ON. Basically the hydraulic system warning is that either there is presently or there has been unequal pressure between the front and rear systems, which would indicate a problem with a leak or low fluid.
The hydraulic switch for the Warning Light is in the "junction block" that has a bunch of plumbing lines going in and out of it, and sometimes this junction block is actually a front/rear proportioning valve as well. What happens Malc is that if there is a loss or not enough pressure either at the front or at the rear brake system, a plunger in the block moves either toward the front or to the rear side of the junction block--which then makes contact to ground, and turns on the warning light by grounding it.
It was common for those warning lights to come ON (light-up) after servicing the hydraulic brake systems (like changing rear wheel cylinders or front calipers or brake lines or a master cylinder or anything that involved draining or loosing fluid.

To get the light to go back off after the plunger has moved to either front or rear, we have to use hydraulic pressure to once again push it back to the center. The only technique that I know of is very effective, but it requires two people working carefully together, and bleeding the brakes while a lot of pressure is applied and the bleeder valves are opened only ever so slightly during this work.
Here is the method Malc...
One person will be inside of the car, have the ignition switch ON so that the Warning light will be ON (glowing brightly), and pump the brakes a couple of times, and hold the brake pedal down with a lot of pressure as would happen in a panic stop. You almost cannot apply too much pressure--and it will require a lot of pressure to move that plunger in the junction block. (Without enough pressure, that thing will not budge).
Then, while the one person inside of the car is pushing and holding the brake pedal down HARD, the other person will crack open a bleeder valve just barely enough to let a little fluid leak out of the bleeder screw. Either do one of the fronts first, or one of the rears first, since it is impossible to know which direction will be needed to push the plunger back to center. It will be a guess as to which end of the car to bleed first.
What is very important is that the person inside of the car must watch the Warning Light closely, and follow the brake pedal down while keeping a lot of pressure on it as the fluid slowly escapes out through the bleeder valve. If the Warning Light goes out, STOP PUSHING on the pedal, but do not release the brake pedal either, just maintain it where it is and instantly call out to the person who is at the bleeder screw "OUT" or some brief call so that the person at the bleeder screw can close the bleeder valve at that very instant that the warning light is out (which is when the plunger is pushed back to the center). This is a little tricky for the person inside of the car if they have never done it before, and I have found that it definitely works best to explain to the person what we are actually doing (applying lots of hydraulic brake pressure, and then bleeding off a little pressure from either the front or the rear system to cause that plunger in there to move back to the center).
If the person inside of the car does not stop applying pressure and just hold the pedal where it is and call out to the person at the bleeder screw, the plunger will move all the way to the opposite side of the junction block and once again the light will be ON. When that happens, we go to the other end of the car and repeat the procedure to move the plunger back in the other direction. For two people trying to work together have never done this before, expect that they will miss it once or twice before success is found.
If the light never does go out the first time that you try--go to the opposite end of the car and try releasing pressure over there (which would try to push the plunger in the opposite direction).
One secret to making this operation successful is NOT to open the bleeder screw to far--just enough to let a little brake fluid slowly escape (which will let the pedal creep down very slowly). You can imagine that if the bleeder screw is opened too far, the pedal will go down very fast, and then the light will go out only for an instant and then be ON again before the person inside of the car has a chance to react.

I hope all the explanation above makes sense Malc. This is something that I have successfully done many times, and with many makes and models. It does work--and the key to making it work is understanding that there is a plunger in there that will move to on side or the other when there has been lack of pressure at one side. So to move the plunger back to center we must cause un-equal pressure at the other side.
It was something that was much easier to do with manual brakes, or with a stock engine that idled very smoothly and quietly if the engine is running for power brakes. With a Hot Rod that idles rough and noisy, and with a woman in the driver's seat helping, the procedure might be difficult.
By the way Malc, I learned this procedure from a brake mechanic who worked at a GM dealership, and once he explained to me how it works I understood perfectly and have used it for years, and passed it on to other people who I have worked around. Too many mechanics will simply unplug the wire from the switch at the junction block--and I don't like that. These lights can be a good thing, they can warn of a hydraulic brake problem (but don't count it, they are not absolute, although most of the time this switch system does work pretty good).
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