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Troubleshooting braking issues with vacuum pump

2413 Views 9 Replies 4 Participants Last post by  Tom Mobley
Hi all,

After replacing the rag joint on my 67 SS, I went to take it for a test drive. Good news is that the steering is working fine. The bad news is that the brakes were barely holding with me literally standing on them at each stop sign. Needless to say, I ended the test drive early and have the car safely back in the garage. I have an SSBC electric vacuum pump kit installed (previous owner). I'm suspecting the vacuum switch, but is there a way I can verify that? I want to avoid ordering unnecessary parts if possible.

Thanks as always for the great advice!
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Ok, I used my jumpers and a couple of wire leads to directly power the vacuum pump and it seems to be working fine. That means it's either the relay or the switch. Is there any way to test either of these components to see if they're working properly? It looks to me like the relay has already been replaced once, as the SSBC provided one appears to be from Nagares and the one in my car is from Bosch. The wiring into the vacuum switch is not quite as clean as I'd like, but it seems tight enough.
I tried this additional experiment this morning. Matt, I think this is what you were suggesting. My understanding is that the wire leading from the top of the vacuum switch to the relay completes a ground when the switch is closed, and if everything else is working properly, should complete the circuit to start the pump. So what I did this morning is replaced the vacuum switch connection on the relay with a wire attached directly to a grounding point. After doing this, I started the car and cannot detect the vacuum pump running (it's hard to hear the pump, as the engine is pretty loud with the cam and headers. I did not feel any exhaust from the exhaust port either, but not sure if I would or not). At this point, I'm assuming, if my experiment is valid, that it's the relay switch, which is unfortunate as I don't see an easy way to bypass this safely and drive the car...looks like I'll have to order and wait for the part...
You should only test a switch with a bypass wire. That's not a permanent solution. If that makes the pump run, than you know the switch needs replacement. Also, you need to pull the vacuum line from the pump and put your finger on it when the pump runs to be sure the pump creates vacuum and doesn't have some kind of internal leak, but still runs.
Yep, totally agree that the bypass is not a permanent solution. I will do as you suggest and ensure I'm pulling vacuum with the pump, but the pump is definitely not running with the bypass in place, so I suspect the relay is at fault. Thanks for the great advice!
I pulled the vacuum line as Matt suggested and ran the pump. Definitely pulling a good vacuum, so there's pretty much no doubt it's either the relay or the switch. My other test with the ground wire indicated a possible issue with the relay, so I'm going to go ahead and replace that. I guess it's possible both could be bad, so let's see what happens with the relay replacement first.
Hi all, finally have an update on this problem after a slight diversion to deal with an unexpected ignition issue...

Turns out it was the relay and after installing a new one from Summit, full braking capability has been restored. Thanks everyone for the great help as always!
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