RE: [A/S] Vacuum Disc Brakes

["Craig Eldridge" <craig_e@xxxxxxxxxxx>]

I have it on my '77 Excella 500 31' trailer and disc brakes are so much 
better than drums.  The discs are huge and heavy.  They were an option on 
the '77 model year but I am not sure what other years they were available.  
Many do not understand the complexity of the system and end up converting to 
electric drums which is a step backwards.  Part of the problem comes from 
providing a vacuum source for the booster tank located on the tongue of the 
trailer and parts are not always available just anywhere.  Inland RV in CA. 
has most of what you need but if you had electric brakes most parts would be 
available across the country at RV as well as horse trailer repair shops.  
For the vacuum source, many have used fulltime manifold vacuum on gas 
engines and electric vacuum motors for diesel engines.  One guy used a 
vacuum motor out of the Buick Grand National.  I run off a manifold vacuum 
source from the back of the manifold of my 8.1 engine then through an 
airbrake hose back to the rear bumper where I use a 1/2" female vacuum 
coupler.  A back flow preventer (check valve) is inline with the hose just 
like you would find in the water system of your Airstream.  The system works 
like this:
     When the vehicle is operated without application of brakes, supply 
vacuum passes through the check valve and open synchronizing valve to 
provide equal pressure to both sides of a power diaphragm in the power brake 
booster located on the tongue of the trailer.  It does not exert hydraulic 
output to the disc brakes at this time.
     The electric brake controller sends a signal to the unit on the tongue 
and the synchronizing valve shuts off the vacuum supply line and admits 
atmospheric air directly to one side of the booster power diaphragm.  The 
diaphragm exerts force on the booster hydraulic cylinder generating 
hydraulic  pressure to operate the disc caliphers.  The brake away trailer 
vacuum supply line will cause automatic application of the disc brakes 
similiar to an electric brake away switch on drum brakes.

I was told that the Hayes-Lemmerz Energize III was the proper brake 
controller for this setup and it has worked fine for me.  I would think most 
other brake controllers would be fine also.