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Discussion Starter #1
I have a 68RS small block car. Used to be a daily driver until about 2000 when I parked it. Now I put a new engine in it finally and started driving it and have a couple issues.

One, the "BRAKE" light is on, and won't turn off.
Two, the brakes suck unless I put it in neutral and rev the engine to get the booster to help out.

Here's some more background.
The car has '69 Chevelle calipers, and I think a Chevelle booster. I've had the car since 1992 and drove it daily for 8 years like this without any trouble. I had to redrill the pedal to fit the booster on it (IIRC) to get the rod to line up correct. The brakes were very touchy, very good. It had a basically stock 327 in it back then. After sitting for 10 years, I put a new 327 in it with 11:1 and huge solid cam etc, it has about 7 pounds of vacuum at a idle now, I was wondering if the low vacuum would cause the light to come on? I also thought the booster should retain vacuum once I drive it up to speed and let off the throttle, shouldn't it hold the vacuum created during deceleration and use it to help it stop? If so, it doesn't seem to work this way at the moment. maybe the booster has a leak? I dunno how to check it, maybe by sucking on a hose and seeing if air keeps going through it? or blowing through it with compressed air? Should I just put a manual master cylinder on it?
 

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First off, I'm no expert. But I have some experience. You may also want to try a search on here if you don't get more/better responses...

Check out: http://www.pozziracing.com/brakes.htm A lot of good info there and in another link at the top of that page. Make sure you have the correct set up installed in your car: master, valves, etc.

As far as the light, it comes on when there is a problem sensed at the pressure differencial switch. From that page:

"2. PRESSURE DIFFERENTIAL WARNING SWITCH -same as drum, senses differences in front to rear pressure, warns of leak by turning on a brake warning light on the dash."

Check for leaks. If you can't find any, a good bleed should help?

The brake feel does sound like it's a vacuum issue. I say that because of the improvement when revving the engine...

Might want to flush the fluid out for new stuff while you are at it.
 

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Discussion Starter #3
The rear brakes were dry after sitting for 10 years, I had my kid hold the pedal down while I was tightening the u-joint bolts, and that could have triggered the pressure differential switch, since the rear had little/no fluid in it. It doesn't leak though, so I don't know why it would have drained out..
 

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The parking brake and it's switch are both working as they should,switch is good and the wiring is good? A problem here will also light up the brake light.
 

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Boosters need about 13" of vacuum. Decell creates the most, but only until speed/idle catch up. Brake light only comes on with parking brake set and a leak in an axle set.

Underneath the master cylinder on the frame, is a portioning/metering valve with a tan wire going to it. Dsiconnect the tan wire and see if BRAKE light goes out. If it does, then the valve is set to one side - blocks off the non-working/leaky side.

The valve should have a notched stem sticking out towards the front about a 1/4 inch. If not, open the opposite bleeder and push hard & quick on the pedal to reset the rod to the middle.

Suck on the booster with a hand vacuum pump, and no brakes applied, it should hold vacuum until pedal pressure is applied - opens up the atmoshere valve inside the interior to move the rod forward into the m/cylinder.
 

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An easy way to see if the booster is holding.
pull the check valve out - if no air rushes to escape- your booster is bad.
It should hold air for a few days.
 

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Discussion Starter #7
The booster valve has no vacuum at the grommet after the engine is shut down (the grommet is working correctly, already checked that). Guess I need a new booster, is there an inexpensive one that would fit? I think mine isn't a camaro one anyhow, Like I said I think it is a Chevelle. Are they all pretty much the same from the late 60's / early 70's? I could get one from a boneyard pretty cheap (like free) if i knew what would fit.

The other choice would be to change it to manual brakes I guess.
 

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The booster valve has no vacuum at the grommet after the engine is shut down
Engine running creates vacuum.

Booster should have a shot of vacuum after shutdown as there is, or should be an anti-backfire valve to protect the diaphram, more like a check valve engine vacuum opens. One last ditch effort of assisted brakes after engine is shut off.
 
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