Air over Hydraulic brakes

I have trailer with air over hydraulic brakes and am looking for info about changing to vacuum over hydraulic brakes. This will match my truck any advise on how and where to get the required equipment would be a great help.

PBR made all this gear and are still in business. They were bought out by Repco who can order the parts you need. Once you have worked out the setup find a Repco shop which still has the old catalogues. Some parts are stored in the same warehouse as hens teeth and rocking horse do do, so you might need to change your plans along the way.

I seriously doubt whether you would be able to find any new vacuum braking equipment. Vacuum braking stuff went out of fashion in the 1970's. Vacuum trailer brakes are slow acting, and not particularly good at stopping.

At one time .. say mid-1960's .. PBR built a massive range of hand controls, vacuum and air braking boosters, tanks, valves, and God knows what else. They now only make a 1/4 of what they built, back then.

I have the 1965 PBR catalogue "Power Brake Equipment for buses, trucks & semi-trailers" (PB3 catalogue). This catalogue runs to nearly 65 pages. There are even trailer boosters with both air and vacuum booster operation.

Virtually none of this vacuum stuff would be available now, it would have nearly all been phased out 30 years ago. You will have to source old vacuum equipment from wrecks and recondition it.

You will need ..

1. A model PV100A combination hand & foot control valve ..
2. A model PV86 oil bath air cleaner with integral check valve (for vacuum supply line) ..
3. A truck-mounted hydro power unit ..
4. A VH5B reaction valve (connects to truck vacuum tank, hydropower unit, and control valve) ..
5. A PV48A combination relay, tank and brake valve. This valve relays the vacuum to the trailer and applies the brakes if the truck and trailer separate.
6. One VH54 vacuum/hydraulic master cylinder for each trailer axle ..
7. A vacuum tank for the trailer (for emergency braking and quicker brake response) ..

I remember that chart in the workshop of a garage where I used to work, well quite a few years back. How did the average petrol motor produce enough vacuum to work those trailer brakes.

653Detroit,

I would be checking with captain kaos down at licencing also before you go ahead and make sure they will let you rego the trailer on vacuum over mild retarding brakes.
I got knocked back here in WA. Well not knocked back but my GVM on the trailer would be the tare...ie cant cart an esky to a show technically. As we cant licence trailers on club licence in WA i would have to pay full reg to cart air. Even to do this I had to write a letter seeking acceptance based on historical interest to the technical committee....rejected. Stupid thing about is if I pull axle out of the said trailer and put under old truck as a lazy I can get a GCM increase. As long as engineer gives it a tick. Which he would cos it was his idea.
What sort of truck is it ?...may be alot easier to add a compressor and the air bits you need and leave the trailer air. Air bits are easy to find.

Basil

653, I have it in the back of my mind that they are now outlawed in Victoria... certainly dont think a new rego or re rego would be possible with Vac brakes, someone who had kept a Vac brake unit current may still be able to keep paying but you might be in trouble.... talk to you roadworthy bloke and take a visit to a Vic Roads office... changing you truck to air might be smarter cheaper and was a common practise way back when...

good luck... and let us know how you go

/me

Atkipete - Well, the vacuum brake system relies on you having your foot off the go pedal, when you're stopping .. so the carby butterfly is fully closed and manifold vacuum is at an all-time high.

However .. the whole system is limited, because vacuum can theoretically only reach an absolute maximum pressure of 14.7 psi (air pressure at sea level) .. and in practical terms, it's more like about 12 psi.
So, you can see, you don't have a lot of effort being applied to boosters .. thus boosters have to be huge.
Then .. add in, a braking system that operates at about 1/10 of the speed of air brakes, and you have delays in braking of up to a couple of seconds.

This was O.K., back in the days of average speeds of 60-70 kmh .. very little traffic .. and plenty of road space to swerve around something, that you were otherwise going to hit, because you couldn't stop in time ..
But .. in todays world, they are basically highly inadequate, and border on outright dangerous.

653Detroit - I can pretty well guarantee you, that driving a full-vacuum braked rig in todays traffic would turn all your hair grey, in one short drive .. :'(

Not only are vacuum trailer brakes dreadfully slow to actuate .. they just don't reach high enough pressure in application, to provide substantive braking effort.
In a single vehicle, such as a truck or car, vacuum-boosted brakes work O.K., because the lines are short, and the vacuum booster is a large diameter.
On the old semi-trailers, the vacuum lines are long and tortuous, the boosters are inadequate size, application speed is slow, and braking effort is just moderate, at best.
In addition, vacuum leaks were common, and a nightmare to try and find. At least with high pressure air, you can soon hear hissing.

Your best bet would be to install a compressor, and the necessary air lines and valves, to operate the current air trailer brakes. Repco-PBR should be able to help out with all that equipment.

Hey Onetrack, you wouldn't have the same diagram for air over hydraulic trucks would you?

How times have changed. You didnt see blackies leading up to stop lights in the old vac trailer brake days.
Could be wrong but I dont think there were as many jackknives in the wet back then either.
Oh and that feeling when you go for an emergency stop and the system takes a big gulp and stalls the donk.

Basil

600Dodge - Here ya go .. but, I've only got the full air system .. just add fluid .. (the insert at top left shows the air over hydraulic setup - the CA34 air/hyd master cylinder is No. 7)