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Hydro-Vac booster brake problems


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  #1  
Old 02-03-2012, 08:21:00 PM
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FWurth FWurth is offline
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Default Hydro-Vac booster brake problems

Used this truck for years without any real problems. The Hydro-vac booster finally went,now we cannot seem to get a reliable replacement. In the past year and half have 3 different reman units and only one was useable,the others wouldn't work right out of the box! The one that worked only lasted 9 months, it started leaking brake fluid into the engine intake. The Napa store got me another one no charge this morning,put it on only works occasionally. Will get another hopefully tomorrow. What gives? Can't these outfits supply reliable replacements? One rebuilder insisted that it had to be something else but when I got one from the last parts store, It's the one that worked for 9 months, the brakes worked ok besides we already replaced every thing on the truck the first time when it failed. We tried to get a factory new one but aparently on longer avilable, thats why we went to the rebuilders. Tthe truck in question is a 1975 GMC tandem drive 6500, we regularly haul 45000 lb gross, it's low mileage farm truck. Brakes are kind of nessary, it's starting to annoy to say the least and I need the dammed thing! Anyone else have any experience with these units? F.J.W.
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Old 02-03-2012, 08:52:32 PM
WillGen WillGen is offline
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Default Re: Hydro-Vac booster brake problems

We use Power Brake Exchange 1-800-580-5729. Harold is not the smoothest customer relations person to talk to, but when dealers and Napa failed us they came through.
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Old 02-03-2012, 09:04:29 PM
xplor xplor is offline
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Default Re: Hydro-Vac booster brake problems

It sounds like you are not changing the master cylinder the source of the leak. Brake fluid eats up the parts of the booster.
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Old 02-03-2012, 09:34:45 PM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Hydro-Vac booster brake problems

Hydrovacs are also used by American La-France fire trucks. They are problematic, as you have found, there are no new ones out there. Largest problem is that the rebuilders are not rebuilders. They are parts changers, who do not know how to troubleshoot or diagnose problems. They change a few parts, and throw it back in the box, not repairing the original problem. They just pass it along. The problems you are having are not only you, but about 60% of the guys trying to get reliable items that work. Kinda like the old days, when you tried to get a rebuilt Rochester Quadrajet carb. Sometimes took 4 or 5 times to get a good one.

By all means, replace the master cylinder, if the hrdro does not come with one already attached. Putting a bad cylinder on a good unit is an invitation for failure. If there is fluid between the units, or if the engine is sucking brake fluid, the master must go. If the seals in the hydro go, it will pull fluid from the master. Once that happens, the master is shot too - the leak will not re-seal, the cups inside go bad. Good luck with your next unit - hope you get a good one!
Andrew
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Old 02-03-2012, 10:26:51 PM
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Default Re: Hydro-Vac booster brake problems

Ugh, nasty hydrovacs! Keeping the brakes adjusted helps them live a little longer, so if you replace it be advised.
You might see if you can find a newer junkyard truck and replace the vacuuum system with a hydroboost system. They were much a better and more reliable system. Works off the pressure from the power steering with an electric back up if the engine should quit while you are trying to steer. I think they started to appear about 1977.
Regards
Mike
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Old 02-03-2012, 10:43:58 PM
Straight Arrow Straight Arrow is offline
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Default Re: Hydro-Vac booster brake problems

Have had really good luck with Alretta truck parts in Mass. 508 788 9409. Very knowledgeable about brakes. Helped me w/47 international and other odd stuff.
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Old 02-04-2012, 01:04:14 AM
Axtion Jim S Axtion Jim S is offline
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Default Re: Hydro-Vac booster brake problems

Why are you shopping at a car parts store for truck parts, talk to truck people they know what the problem is and know how to set it striaght
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Old 02-04-2012, 01:08:21 AM
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Default Re: Hydro-Vac booster brake problems

Thanks for the suggestions, yes have put on new master cyl, brake hoses,wheel cyl kits. Replaced about every thing except drums and shoes, but their good. Have them adjusted close but the truck has 12 wheel cylinders, so there is still a good bit of pedal travel. I added a extra vacum resevoir too and a vac gauge to monitor things,it helped quite a bit with pedal effort. When the last unit failed we kept losing brake fluid, took of vac tank and drained out about a half gallon of fluid, so absolutely know Hydro-vac failed. Thought about changing it to full air brakes which it should been equiped with in the first place,too much truck for hyhraulic brakes anyways, but thats pretty major surgery and don't have any suitable donor unit. Nappa is supposed to have another one tomorrow hope that one works. In reading some of posts above I should clarify the type of setup this has. The main master cyl pressurizes the Hydro unit down on the frame under the cab that has another power cylinder that feeds the wheel cylinders, a double diaphram vac chamber is mounted on that to help multiply the pressure at the wheels. There isn't any vac assist on the first master cyl. The manifold vac is connected to the control assy on the Hydro vac unit. It's not like the power brake unit on the smaller cars or light trucks. F.J.W. The Truck parts Suppliers have already struck out, Nappa does supply heavy duty truck parts as well.

Last edited by FWurth; 02-04-2012 at 01:28:31 AM.
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Old 02-04-2012, 08:58:10 AM
pegasuspinto pegasuspinto is online now
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Default Re: Hydro-Vac booster brake problems

My first suggestion would of been to never let your original unit go-it was probably in MUCH better shape then any of the rebuilds you got. Some of them might have millions of miles and been on a dozen trucks and are just so smooth wore out and rebuilt so many times that they are truly scrap. You might look up 'hydrovac booster system' on google, I found several interesting links, parts are available and seems easy to work on. Try and find a truck junkyard and maybe find a better unit to start on-sounds like these were used on motorhomes, I'd look for one there if an interchange is possible as many of these will be under 100,000 miles.

If you keep using the parts store units, you will have to keep swapping till a good one finds you. But it's definitely the junk units and not your fault-seen it before....

Robert
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Old 02-04-2012, 09:35:10 AM
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Dale Barnett Dale Barnett is offline
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Default Re: Hydro-Vac booster brake problems

Back a few years ago when I was turning the wrenches we had a costumer put 3 or more of these on his truck and none were good according to him.
He brought the truck to us to fix.
We had a old time wrench turner that used to work there and stopped by quite often just to shoot the bull and see what kind of new stuff they were putting on cars these days.
He heard what the customer was telling us and ask two questions first was did the unit come with the master attached or not and two how are you bleeding the system?
Come to find out the units did not have master attached and the customer was breaking the unit before it was put on by pushing and pulling the activation rod in the unit and there by ruining the diaphragm.
His answer to the second question was he was bleeding it with the brake pedal at the wheel cylinders like a car.
Long story short the second cylinder was dry when he started
he never bled it then when he thought it was bled well enough he would start the truck and because there was still air in the system the hydro would over extend and again ruin the diaphragm.The travel is very short.
We ordered a new unit with master attached and bled it as the old wrench told us to an it was fine.
To bleed the unit you first do not hook up vacuum to unit then fill unit with fluid then put on pressurized bleeder then start at first master bleed at line out then go to second master and bleed line in then the bleeder then shortest line out then next shortest an so on till no air is found or seen.
Only then hook up vac. and start and try.
He said that in the 70s Gm had a lot of trouble with these and he went to a GM school to learn the right way and told he ruined a few before he went to school.
Hope this helps.
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Old 02-05-2012, 07:30:16 PM
John C Walker John C Walker is offline
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Default Re: Hydro-Vac booster brake problems

When ever we had to do any work on these type of units it was a nightmare. That being said we always use what was called a pressurized bleeder ball and replaced the vac. chamber and master cyl. as a unit and had good results. If using a bleeder ball always make sure it is filled full of brake fluid before you start. If it runs out of fluid half way through the job you might as well take everything off and start replacing.;faint:
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Old 02-06-2012, 12:26:49 PM
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Default Re: Hydro-Vac booster brake problems

scrap the vacum setup and install a hydroboost that gets its power from the pwr steering pump.
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Old 02-24-2012, 12:54:46 AM
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Default Re: Hydro-Vac booster brake problems

Want to thank everyone for their help, finally got the thing to work. What I ended up doing to get it to work was I took the reman unit totally apart the problem seemed to be that (1) it was put together dry,(2) it had some rust in the area under the control diaphram,(3) the control valve plunger was sticky. The booster would lock up during application ( not good) We cleaned it up, got the valve assy limbered up and put a special grease on the large connecting rod between the vac chambers to aid in free movement and help the O-ring seal the vac in, this really helped reduce the pedal reistance, much easier to press the pedal now. we pulled all the hubs agian to limber up the brake adjusters so we could get them adjusted closer, a couple were stiff and not allowing the self adjusters to work, I thought I had that cured the last time but aparently not well enough. For now the brakes are the best since I've had it, I can see that I'll have to keep the brakes adjusted real close and not rely on the self adjusters and I'll also keep a spare unit on hand and not rely on the store supplied ones without going thru them myself. These units all came with the vac chambers and the hyd master preassembled as a unit, I didn't find any mechanical damage inside the unit so bleeding the air out wasn't the cause of any of the problems. F.J.W.
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