The Stovebolt Site Forums The Parking Lot Big Bolts Dual Hydrovacs???

Eventually Id like to switch the single master cylinder out in my COE. The only reason being is that I do intend to haul with her and I like the increased safety margin. Two problems, the first being I tend to lean towards more original equipment and the second being that I dont have the room under the floor to mount a booster without moving quite a bit of stuff.

So Im inclined to think that I may be able to upgrade to a dual master in the stock location and run two Bendix Hydrovacs, one front, and one rear.

Does anyone have any experience with building a similar setup? I really cant see any reason why it wouldnt work, so long as I stay with a master that has equally sized resevoirs putting out the same pressure.

Any ideas?

Thanks, Jeff

Many of the later trucks with hydraulic brakes do just that if you look at C60s from the 80s in big truck wrecking yards you may even find a whole system like that you could use.

Wouldn't it be cheaper to buy one small diameter aftermarket brake booster that two hydro's?
Scott

Thanks Grant, I figured there must have been something like that around.

Scott, they arent that expensive to overhaul, just time consuming. I have a pile of old hydrovacs to chose from. The kits are usually about $50 for most of the seals, and for a complete kit that has everything it shouldnt be more then $100. I already have one that I rebuilt last year and an extra kit on the shelf, so all I have to lose is my time involved.

Thanks, Jeff

I seem to remember a dual Hydrovac setup on some motorhomes from the 70's and 80's. You might consider plumbing your system like the newer "dual diagonal" brake systems that apply one front, and the opposite rear brake with one master cylinder reservoir to minimize mismatched braking effect between different-sized wheel cylinders front and rear. That setup also allows at least one front wheel to continue braking even if there's a catastrophic failure like a blown wheel cylinder or burst line on the other system. Since front brakes do the majority of the stopping due to weight transfer except in max-payload situations, it's a good idea to keep at least one front brake capable of functioning at all times.
Jerry

That diagonal approach scares me. Especially with a truck that might have a different load on the rear each time you drive it. Sometimes it might go straight, sometimes it might bend right, and other times dart right...

I've noticed on some pickups starting in the late 80's the rear brakes are huge, and they have anti-lock systems just on the rear. I assume that's because with the variable load on the rear sometimes you really need more braking power there than in the front.

On a 2-ton truck like Jeff's COE, complete with dump bed and an occasional load of gravel, there will be times he needs those rear brakes much more than the front, and the diagonal setup would turn the truck if one half the system failed.

Would it be better to have NO rear brakes, if that happened to be the system with the problem? I've driven 3-axle air brake rigs without brakes at all on the front, supposedly to maintain steering control on slippery roads, and 2-axle rigs with a brake isolator to select between "dry road" and slippery road" conditions. The DOT mandated brakes on all axles some time ago, and I got the job of retrofitting several trucks in the early 1990's. I think I'd rather have at least one set of rear brakes working, even on a loaded rig, since the rear duals will have much more traction, and braking power, than a steer axle with both wheels locked up, and no steering control. The right pull from a rear hub braking will be at least partially offset by the left front trying to stop.
Jerry

I've never driven a loaded truck with no rear brakes (except around the yard), or one with diagonal brakes, so I have no experience with that. When I last drove a semi tractor there were still rigs on the road with no front brakes, and I may have driven some of them. I no longer remember that.

I have driven a light truck with a bad seal allowing grease to make the front left brake fail. That was scary, with the truck pulling hard to the right every time I touched the brake pedal, almost jerking the wheel out of my hands and the truck off the road.

Is a diagonal arrangment factory on anything?

I do think I'd prefer to have my brakes divided by axle, even if it meant having very ineffective brakes on one end.

Volvo had a diagonal braking system on there model 240 cars for years. 4 wheel disc brakes. split between front and rear and then l/f to r/r and r/f to l/r. there was a lot of brake lines running from the front to the rear.

John Gott

Instead of the conventional dual brake arrangement where the front discs are on one system, and the rear drums are on another master cylinder chamber, several cars in the mid-1980's went to diagonal braking. On cars, not necessarily trucks, up to 90% of the stopping effort is handled by front brakes, due to weight transfer toward the front during normal driving. Rear brakes on automobiles are just there for stability, to prevent the rear end from trying to pass the front on a hard stop. That's why rear brakes, even disc setups, are so small and ineffective, to prevent rear wheel lockup.

On cargo-carrying vehicles, the rear brakes must be big enough to handle a max payload, but there needs to be a proportioning valve to cut back on the brake application on a lightly-loaded rig. Most of the time, it's a simple squat valve between the rear axle housing and the frame that responds to suspension deflection. Heavier load, more spring deflection, more brake effect!

Since the rear brakes on a medium-sized truck will probably have larger wheel cylinders in the rear than in the front, it makes sense to me to split the power boost (and fluid displacement) equally between both hydrovacs by connecting one front wheel and one rear to the same booster. Do it on opposite sides of the vehicle, and any tendency to pull in case of a failure will be minimized if one system runs dry. If you do that, try to use a master cylinder with two fluid reservoirs the same size, like the ones used on dual-brake, 4-drum systems in the early 1960's.
Jerry

If it were me I would find the latest model truck with hydraulic drum brakes front and rear and the dual hydrovacs like you want of the same or higher GVW and copy that system. I am sure that the manufacturer put way more time and money into the engineering than we could ever hope to.

HL I have seen and worked on a lot of medium and heavy duty truck braking systems from hydraulic too air and have never run across a load sensitive system for the rear that is a new one to me.
My old 57 Kenworth had no front brakes and still had the brake limiting switch on top of the dash for bobtailing in the rain. Those limiting switches were band and shortly after brakes were required on all axles.
The rear brakes on both my 56 and 64 two tons are a good four times the size as the fronts

I agree 100 % with using late model components because manufactures put a lot into research. I would get a complete system from a late model bobtail with hydraulic brakes to make sure everything is meant for each other. As far as the old limiting valves. That was from stories before my time and I bought my first semi in 1964. All the trucks I had with front wheel brakes disabled were dangerous. We took the brakes off for weight savings, but it made them more dangerous on slick road. The drivers locking up is what causes a jack knife. I'll take the trucks we have now for safety that limit all of the tractor brakes when there is no trailer and have anti-lock.

i have heard of the load sensing type of brakes where more breaking power will be applied to back breaks when there is a load on the vehicle funny thing is i have only seen this system on late 80s early 90s toyata pickups.... as if there gona have much of a load on 'em

with the hole diagonal style i'm not really sure how it would handle on a vehicle with a heavy load its main thing like Hotrod said the rear brakes on most modern cars are so small that they really don't do much breaking, if you had to stop a new car with just the back breaks... you better bring along another pair of undershorts

me personally i think i'd just run the split between each axle that design has been used for years and years, and far as i know most big trucks with hydraulic brakes still run that design so....

Nate

My 87 Ford F-350 ran rear wheel antilock brakes, and a load-sensing proportioning valve. When my wife's nephew ran it out of rear axle lube and roasted the gears and bearings, I replaced it with a GM 14-bolt axle. I had to disable the RWAL, but the load-sensor stayed put. Even without the antilock in operation, it was difficult to lock up the rear wheels, even when running empty.
Jerry

My only concern would be with two separate hydro's. If one work better, or differently, like they all do, you could create one wild braking problem. I myself wouldn't consider it, I like my single booster/dual res master set-up,
Scott