Its also possible the line out of the hydrovac only goes to the rear wheels, and the fronts are unboosted. I could be completely wrong there, too.
Its also possible the line out of the hydrovac only goes to the rear wheels, and the fronts are unboosted. I could be completely wrong there, too.
Streetwiseguy wrote: ...I like the looks of the one you have pictured. I think I would retain the vacuum reservoir tank if possible. I can imagine there might be times out on the road where you are going up a grade, and haven't actually created any vacuum for quite some time.
I liked it too, so there is one on the way. If the 1" bore isn't enough, they make the exact same master cylinder with a 1 1/8 bore for about $50. If that doesn't work, I'll punt and start over.
Good thought on retaining the vacuum reservoir, I'll do that.
If anyone suddenly realizes that I'm about to kill myself, shout out.
Toyman01 wrote: As it sits now it's a single circuit system. It looks like fluid comes off the master cylinder and goes to the booster cylinder. This causes the booster to actuate and push fluid back to the master cylinder distribution block. This pushes the wheel cylinders as well as pushes back on the brake pedal.
That's how it should work, but I'm not seeing it working that way as installed unless the manifold on the end of the MC is independent and only one port on it (the gray line?) is actually connected to the MC. In any event I think you can put in a normal booster setup and size the MC the same as the current one.
One option for the front axle would be to just swap spindles from something newer with disk brakes onto your axle. There aren't a lot of king pin sizes when you get that big, and the bushings should tolerate something decently close when you ream them to fit.
In reply to Toyman01:
O was actually looking for pics of the brake drum set up, but that is a curious design. If I'm looking at those lines right, only part of the system is boosted?
I also did some digging of my own and found some diagrams that be helpful. These are probably very close to what you have.
Diagram of hydraulic and vacuum system
Dual leading shoe front brake system with the same type of wheel cylinders that you were talking about. 
In your earlier post it sounded like you were saying that each drum only had one single piston wheel cylinders, maybe I misunderstood. This arrangement makes more sense. These style brakes are sort of directional, in that they will grab a lot harder going forward than backwards. When set up correctly they are pretty strong .
oldopelguy wrote: One option for the front axle would be to just swap spindles from something newer with disk brakes onto your axle. There aren't a lot of king pin sizes when you get that big, and the bushings should tolerate something decently close when you ream them to fit.
I wouldn't be surprised if a whole disk brake Ford school bus front axle from a more recent bus could just bolt on.
If your pedal effort is high the hydrovac isn't working properly. Check to see if you have a vacuum leak at the booster. You can actually separate the booster to check the diaphragm visually without separating the fluid lines. I had that same system on a truck I had and when the rubber diaphragm split the pedal effort was reall high, but the brakes still worked fine.
In reply to HappyAndy:
It looks like just about every newer front axle with disks, is a foot too wide to put the tires under the fenders.
I did find those diagrams earlier, but the front setup is not what I have. I only have one wheel cylinder on the front. The sole parts book I found shows a single sided wheel cylinder like the ones in the diagram though. Most of the diagrams I can find are for the 60s and later busses. There is a complete absence of information on the web. I did just order a factory service manual though. Maybe that will help. If not, I'll have to pull the front end apart and see what is in there.
I do have a lathe and mill. Something will damn well fit and work. 
In reply to Toyman01:
Is it possible then that you could just take the spindles from a newer axel, kind of like what oldopelguy said?
Seeing if the current vacuum assist setup will hold vacuum might be worthwhile before you start spending money.
Hearing everyone say to call Wilwood made me LOL because I was thinking, "you know I can hear you, right?!?!"
Toyman, shoot me an email to my screen name at gmail dot com. I speak drum brake. But I won't get it til lunchtime Thursday.
90's gmc 3500hd axle(s) with discs all around? They came with 19.5" wheels. I can measure them for you, and you could pillage hydroboost from one too
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