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Old 08-06-2006, 03:39 PM   #1
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ogden , North Carolina
Join Date: Jul 2006
Posts: 13
another hydra vac question

the valve that opens to let the vacuum in that applys the brakes has two wires running to it on our camper. A yellow wire that ran to the valve was cut...well we hooked everything up and i temporarily hooked the cut wire to the turn signal on out flat 4. well it works...so i wired up our little pigtail connector that the airstream uses, and reconnect both ends of the yellow wire, hoping that it was mistakingly cut. all the lights work and when you hit the brakes you get NOTHING....i thought it may have been our brake controller being the problem, so i hooked ip up to our third brake light on our suburban to see if it was or not. STILL NOTHING....any idea on why it would be doing this?? everything on our truck is ok, the valve is in working order also...im guessing the wire that connects to the valve runs into the camper and connects to the round female connection for the lights is where the problem may be?? any other ideas on how to trouble shoot this?
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Old 08-06-2006, 04:35 PM   #2
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1977 31' Sovereign
1963 26' Overlander
1989 34' Excella
Johnsburg , Illinois
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My 77 had no wires attached to the vacuum hose. Previous owner may have invented something. You can not use a flat 4 connector with an Airstream. You need to get a Airstream type round connector or a modern flat pin round connector like the ones on the modern trucks. Wiring diagram should look like this link: http://www.airforums.com/forum...ams-13217.html

If the brakes do not work, there may be something wrong with the vacuum actuator assembly as well. (Lots of luck finding an RV shop to help you with that) . Sounds like you are over your head since you do not have a service or owners manual. (owners manual is only mariginally helpful)
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Old 08-06-2006, 05:13 PM   #3
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1978 31' Excella 500
Venice , California
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HydraVac info

May this will help. The hydra vac unit is operated by a 12V. solenoid. It is designed to apply hydralic effort in proportion to the electrical signal from the brake controller (not from the brake light signal-although they occur simultaneously). One side of the solenoid goes to ground the other side to the yellow wire which goes to the connector (see attrached PDF)
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File Type: pdf hydra vac.pdf (152.5 KB, 104 views)
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Old 08-06-2006, 05:31 PM   #4
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ogden , North Carolina
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what did you use to open that file?? there are no wires going to a vacuum line, just to the solenoid that opens when you apply the brakes to let the vacuum apply the brakes. one of the wires that ran to that solenoid were cut, and im guessing for a reason.....im trying to figure out why they were cut since i re-connected the wire and they arent working.
thanks!
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Old 08-06-2006, 05:38 PM   #5
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1986 25' Sovereign
Southern Middle , Tennessee
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A synchronizing valve responds to the current sent by your brake controller and energizes the brake booster. White and yellow wires go into a solonoid which attach to the synchronizing valve. The solonoid coil can be checked by disconnecting the 7 way plug to the trailer then checking the resistance across the yellow and white connector pins. Resistance should be 1.5 to 3.0 ohms. If the reading is improper, disconnect the solonoid wire connector and take a reading directly from the wires. If coil resistance is within spec, the 7 way trailer plug, trailer wires, terminals, splices are defective and should be replaced. If the solonoid coil resistance is improper, the entire synchronizing valve assembly must be replaced.

Normal synchro valve operation can generally be determined by listening for ingestion of air into the valve filter as the brakes are activated. Vehicle should be running obviously when making this test. If no air flow occurs when brakes are applied, listen for valve buzzing or clicking to assure that solenoid is operating. '77 EXCELLA MANUAL

The upper right pic arrow points to the synchro valve under the booster, bottom left shows another picture and bottom right shows a bottom view of the synchro and booster.
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Old 08-06-2006, 05:45 PM   #6
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1978 31' Excella 500
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The file is pdf (Adobe Acrobat portabale document file) and there are free reader copies on line.

The Hydra Vac is a three part system. The hydralic brakes are activated by a vacuum muscle which is controlled by an electrical signal from the tow vehicle controller. You are correct in saying that the vacuum line has no electrical connection. Manifold vacuum is supplied through a check-valve from the tow vehicle intake manifold (separate pump on diesels) through a quick-connect fitting at the bumper. The mating hose on the trailer leads to the chamber of the vacuum diaphragm via an electrical solenoid which applies vacuum in proportion to a signal from the tv brake controller. When you apply the brakes on your tow vehicle, the brake contoller sends a voltage to the hydra-vac soleniod through the appropriate wire in the trailer's electrical umbilical cable. This voltage allows vacuum to exhaust air from the diaphragm chamber and apply pressure to the hydralic master cylinder and thus apply the brakes. I'll see if I can figure out how to attch the pdf as thubmnails so you can see the on the post.
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