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Bad c6 vac modular=poor power brakes?

Discussion in '6.9L IH & 7.3L IDI Diesels' started by Cubey, Feb 11, 2019.

  1. Cubey

    Cubey Full Access Member

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    So, my C6 is still acting like crap after a new vac pump and power brakes still aren't what they should be. They are better than before but it acts like it is having a problem recovering the vac pressure.

    Only the throttle/trans hose and brakes are hooked up to the distribution block. It does go from there->throttle->c6 correct?

    It could be a bad booster, but I'm thinking it's something else. I'm not braking while accelerating so there should be no load on the booster, but it's still acting like the trans isn't getting vacuum.

    Could a bad modular valve cause other vacuum problems, especially if its comstanly sucking air?

    This makes me think it could be the problem. Not my video, but that's not normal right? If one does this, I think it could definitely cause poor vac:


    Anyone had such an issue where the modular caused power brake problems?

    I can manually shift from D to 2 just fine. Going to D at a high enough RPM/speed keeps it in 2nd. If RPMs/speed is too low, it goes back into 1st. 3rd is VERY late, around 45mph I'd guess. My healthy F250 C6/vac shifts to third around 27mph.
     
  2. RossCo

    RossCo Registered User

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    Ok just going off of my experience. My C6 was acting up also. I can't say for sure about your modulator valve being good or bad. It does seem odd that it bleeds down. It tells me there is a leak in the diaphragm but, I may be wrong. Did tyranny fluid come out when you pulled the vac line? I had a VAC leak in my brake booster just enough to cause shifting issues but, not enough to cause braking issues. I'll see if I can find my thread maybe it will help you.
     
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  3. RossCo

    RossCo Registered User

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  4. Cubey

    Cubey Full Access Member

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    I haven't checked the hose, it could just be rotten too. Finally getting some clear days again and now I have the motorhome out on the road again, so I can more easily crawl under on pavement in parking lots. The hose between the new vac pump and the distribution block helped improve vacuum after replacing the pump but still having poor vacuum with the old hose that looked fine.
     
    Last edited: Feb 12, 2019
  5. Cubey

    Cubey Full Access Member

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    It downshifts to 1st when rolling to a stop without touching the brakes at probably 15-20mph, just letting off the throttle.

    Trying to get the vac hoses loose from the throtte valve thing is going to be a nightmare without breaking it. It's so cramped in the van engine bay.
     
  6. riphip

    riphip Supporting Member Supporting Member

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    You have a bad vacuum leak causing shifting & braking problems like that.
    Do you have cruise control on that van? If so, check the 'coffee can' bottom side for rusted spot(s) where it mounts.
    Would also be best to change out all vacuum hose since the engine creates so much heat. Rubber hose does not like heat. I went with silicone lines on my trucks. Better heat resistance.
     
  7. Cubey

    Cubey Full Access Member

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    I have cruise disconnected and a vinyl cap over where it would tie in.

    I think it is the booster, at the very least.

    On my truck, if i hold down the brake pedal at idle, vacuum recovers back to around 22-23 in Hg on my test gauge while flooring the brake pedal.

    On the motorhome, it does not recover while holding the pedal, it flutters around 2-3 in Hg. Even having the vac pump hooked directly to the booster (no gauge), it still has a poorly operating booster.

    Either its a bad booster or a bad new vac pump.

    Thoughts?
     
  8. Cubey

    Cubey Full Access Member

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    Weel, I ordered a Cardone rebuilt booster off Ebay for $104 shipped. I guess I'll know if thats the problem next week.

    I thought about trying to run a LONG hose from my truck vacuum pump to the motorhome to see if it made a difference but eh, I'd have to go waste money on 10ft of hose.

    I did go underneath and replace the short hose between the modular and the steel line. It was pretty bad off.

    What makes me think its either the booster or the new vac pump is that even with nothing else hooked up, its still crap.

    The C6 is shifting itself if the throttle is open far enough. Not sure if its an adjustment issue or what. I'll look into that later.
     
  9. franklin2

    franklin2 Full Access Member

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    I hope you get a good one. I am having less and less success with Cardone rebuilt parts.
     
  10. RossCo

    RossCo Registered User

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    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.
     
  11. Cubey

    Cubey Full Access Member

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    Except I already have a new master cylinder as part of the $1228 of emergency brake work I had to get done immediately from buying this thing without a test drive and without checking the brake fluid. I'd have been better off with a lower mpg gasser in better condition. But I'm a chump at times.i guess we all are.
     
  12. Cubey

    Cubey Full Access Member

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    The vacuum pump is Cardone too but it's supposedly brand new.

    Edit: no idea why the pic is upside down, shows rightside up on my phone ...

    20190213_115324.jpg
     
  13. Cubey

    Cubey Full Access Member

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    The ebay seller is a middle man and can't get a booster for 2 months! I told them cancel the order. I ordered from a different seller. Wish me luck that they aren't doing the same thing.
     
  14. Cubey

    Cubey Full Access Member

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    The vacuum modulator on the transmission is not adjustable, unfortunately. I checked it today.

    It's actually always shifting, if I put my foot down hard enough. I'm not sure if it's a vacuum issue or just how the transmission is setup. I think it was used to tow a car or trailer behind it, due to the really old trailer brake controller. It just shifts very late. Second comes around 25mph and third comes around 37mph. Being light footed gives me poor results, it stays in 1st and 2nd too long. Maybe whoever wanted it that way for having a trailer in tow.
     
  15. franklin2

    franklin2 Full Access Member

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    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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