The Stovebolt Site Forums The Shop Area Driveline Air over Hydraulic conversions

Morning,
Curios to know if anyone has done an Air over Hydraulic conversion on their ride. I'm looking at options to do away with the hydrovac. Any suggestions would be appreciated.

Thanks

When I was running a one man truck shop back in the late 1970's as a means of keeping from starving to death, back when a Southeast Asia veteran couldn't buy a job, everybody hated air/hydraulic brake systems. They were downright unreliable, and had a tendency to dramatic failures at the worst possible time. I can't imagine why anyone would install one on purpose. For the same amount of effort, and probably less expense, you could convert to a hydroboost system that uses the power steering pump to provide power braking. Some systems even have an electric pump that takes over in case of an engine stall that puts the P/S pump out of operation.

There's absolutely nothing wrong with a properly maintained hydrovac brake system. Millions of them ran untold mileage, stopping max loads (or more likely big overloads) for several decades. My grandfather hauled thousands of gallons of beer over poorly paved (or sometimes unpaved) 2 lane roads from St. Louis and Milwaukee to Nashville in the late 1930's, using a 34 Chevy dual wheel flatbed as a tractor, and a short trailer- - - -with vacuum boosted mechanical brakes on the tractor, and NO brakes on the trailer!
Jerry

I am not sure I am familiar with an air over hydraulic system in this context.

What is the basic theory of operation?

Back to the OP question: why do you want ti get rid of the hydrovac? What problem are you trying to solve?

From what I'm reading by y'alls comments, they can be pretty strong when properly maintained, but finding someone to rebuild/work on can be challenging to say the least. I would guess the air over hydraulic concept would be the same as it was years ago, but I would hope that they have engineered out any of the issues that were around when these first hit the market. This brake rebuild will be on my 451 GMC dump truck. Plan on working it around my property with loads ranging from top soil, gravel and mulch among other things. Loads will be heavy and I just want to make sure I have the best and most easily maintained system installed. I've even entertained the idea of a full air brake conversion. Again, I appreciate everyone's opinions.

Thanks

I've seen two different styles of AOHB systems (there may be more). The first and most common is to have a normal hydraulic brake system with the Air Booster Teed into the system just like the Hydrovac system. The AOHB system does require a air compressor and air tanks.

The second type is a system that looks like a Full Air setup with a treadle pedal and two air chambers (one for each axle) with Master Cylinders connected to then mounted on the frame rail. The down side to this system is if you loose air, you have NO brakes as there is no mechanical/hydraulic connection between the pedal and the drums like there is in the first example.

If I ever restore my '56 9200 LCF that has the second type system I will convert it to Full Air with rear Maxi brakes.

I've driven antique trucks with Hydrovac's for 25 years (40K miles or more) and never needed to service them other than a shot of light weight oil in the can every now and then to keep the leather seal soft.

Mike B