Bad c6 vac modular=poor power brakes?

franklin2 said:

You can always try tweaking the vacuum thing on the throttle arm of the IP. That was their way to monitor the load demand on the engine since the diesel did not have vacuum.

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Yes, I have thought about that. Funny thing is, the arm isn't at rest in the same position. On my truck, it points straight up. On the RV, its at an angle toward the firewall. The screw on the truck is barely in and on the RV its about half way, but due to how far back the arm is, it makes sense for the screw to need to be in so much, so the throttle arm can contact it.

I guess I can try to take a picture if I can get my phone in there to the side.

Cubey said:

Yes, I have thought about that. Funny thing is, the arm isn't at rest in the same position. On my truck, it points straight up. On the RV, its at an angle toward the firewall. The screw on the truck is barely in and on the RV its about half way, but due to how far back the arm is, it makes sense for the screw to need to be in so much, so the throttle arm can contact it.

I guess I can try to take a picture if I can get my phone in there to the side.

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I think he is talking about the vacuum regulator on the other side of the throttle shaft that is used to adjust what rpm the trans shifts at. It is a small black plastic piece with a couple of small vac hoses attached to it. It also has 2 screws that can be loosened to allow you to rotate it to adjust it.

I am interested to see what you find. I am having a similar issue with my ‘88 e250. Everything was fine until my vac pump froze up. I replaced it (also with a new Cardone) and it never was quite right. Slow to build vacuum and brakes would go hard at idle when cold out. I thought it must be the old hose that connected to the vac pump was leaking, but I replaced it and still having issues. I have a new vac pump on the way and I was also looking to replace my vacuum reservoir (the coffee can thing). I don’t see a stock replacement listed, but am looking for a universal replacement. Just stinks cause it is freezing out, so wrenching in this weather is no fun.


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jayro88 said:

I think he is talking about the vacuum regulator on the other side of the throttle shaft that is used to adjust what rpm the trans shifts at. It is a small black plastic piece with a couple of small vac hoses attached to it. It also has 2 screws that can be loosened to allow you to rotate it to adjust it.

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Ahh, ok. So heck, there's up to 3 places to adjust the transmission then; the kickdown screw by the throttle, the vacuum thing by the IP with 2 hoses, and the vacuum modulator on the transmission itself, if you have an adjustable one.

It is probably easier to buy a $15 adjustable modular for the transmission end and adjust that, rather than trying to adjust the plastic thing, when it comes to a van. You just pull the vac hose off and turn a flathead screw in or out.

C6 - Diesel uses purple stripe modulator..Adjust the regulator or may be bad. Vacuum should hold at 21 or less. Replacing the modulator with different can burn up tranny. My nephew tried to cheap out on his.

Cubey said:

It is probably easier to buy a $15 adjustable modular for the transmission end and adjust that, rather than trying to adjust the plastic thing, when it comes to a van.

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You still may have to adjust the VRV no matter what modulator you have on the transmission.

riphip said:

C6 - Diesel uses purple stripe modulator..Adjust the regulator or may be bad. Vacuum should hold at 21 or less. Replacing the modulator with different can burn up tranny. My nephew tried to cheap out on his.

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I will have to check mine. I replaced it a while back with the same color that was on it and I thought it was a green stripe.


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riphip said:

C6 - Diesel uses purple stripe modulator..Adjust the regulator or may be bad. Vacuum should hold at 21 or less. Replacing the modulator with different can burn up tranny. My nephew tried to cheap out on his.

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I read someplace that green stripe was diesel, not sure where.

People say black in this thread...
https://www.oilburners.net/threads/rebuilt-c6-wot-shift-problem.72679/

Ugh, that VRV screw is a huge pain to reach. It only has one in the front thank goodness. I can feel in the back where one should be but isn't. It would probably require pulling the engine to loosen a rear one! Yes, it's that bad. I'd like to stab the engineer in the throat who thought philips heads were a good choice for an extremely cramped place where normal screwdrivers can't reach. I'm going to have to try some kind of right angle ratcheting screwdriver for the one screw there is.

Oh for Pete's sake, they sent me a totally wrong booster. The studs are very far off. Filed a "not as described" return with ebay and the old booster is back on for now.

That sucks. Sorry to hear it.

RossCo said:

I got my booster and master cylinder assembly at Napa for about 150, took care of the VAC problem and one less thing to worry about with the new master cylinder. Its actually a little less money to get them as an assem instead of by piece.

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Napa seems to be the only possible retail source for one. I called the one here before I tried buying online because they said it'll cost freight. I doubt they'll refund that if it's wrong too. Really, really annoying. I guess I will try going in store there and ordering it. You can't online. I guess that's my last choice unless I try Amazon.

O'Reilly is broken, acting like they are in stock online but it never adds to cart despite saying it was. Advance auto adds to cart, then complains that zero are available. Autozone just says sold out.

The new booster from Autozone should be here tomorrow. Wish me luck that it's the right part, and that it fixes the problem.

What's weird is, the colder it is, the worse things are. When it's been 60 degrees? They seem to be acting better. Makes some sense, rubber expands when it's warmer. I just hope it's the booster and not the bloody new vacuum pump.

Cubey said:

The new booster from Autozone should be here tomorrow. Wish me luck that it's the right part, and that it fixes the problem.

What's weird is, the colder it is, the worse things are. When it's been 60 degrees? They seem to be acting better. Makes some sense, rubber expands when it's warmer. I just hope it's the booster and not the bloody new vacuum pump.

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Mine had the exact same symptoms. I had replaced the vac pump, so I figured it must me a leak somewhere. After replacing quite a few lines without any results I went ahead and replaced the pump agian. Easy job and it was still under warranty. Now it is 95% fixed, so the pump was bad out of the box.

I am thinking that my vacuum reservoir is going out and causing the other 5% of the issue. Going to replace it soon.


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Well, i might try swapping known good vac pump from the truck and see before I install the new booster just to be sure.

That would be a good idea. At least it's free!