I am needing to upgrade my bus to a dual circuit brake system and am not able to find a master cylinder/booster combination that will fit. It has occurred to me that I may need to piece together components to work. How do I figure out the bore and stroke requirements for the otherwise stock brake system? Or better yet, can someone tell me what specific booster/master cylinder combination I can bolt right in?

 

Your bus should have a hydro-vac set up as OEM.

Like so:

Buy Cardone Hydro-Vac Power Brake Booster - Remanufactured 51-8030 at Advance Auto Parts

This is the same principle as the on-the-firewall set ups like a car or pickup, but a bit more powerful.

There is a primary master that feeds pressure to this unit, which in turn feeds more pressure to the wheel cylinders.

Is that the way your vehicle is set up?

There is some discussion here, indicating that some late '70s and '80s mediums had dual circuit hydro-vac set ups.

There are also later hydro-boost set ups with electrical backup.

Hydroboosted drum type brakes [Archive] - Cummins 4BT & Diesel Conversions Forums

If you are just trying to do away with the hydro-vac and just have a vacuum boosted master on the firewall, that might be difficult to do give the fluid requirements. Do you have dual cylinders in the rear wheels?

Sorry if this isn't what you are looking for. I think you need to stick with medium truck parts and not try to adapt pickup or car parts.

 

My bus had a single circuit vacuum boosted master cylinder OEM. It does have dual rear wheel cylinders and single piston front wheel cylinders.

http://www.autoatlanta.com/centric/s...p?pn=130.65068

 

So you had the vacuum can on the firewall behind it like a car or pickup?

Wow--I'm sure those brakes worked, but it seems like a pretty "weak" system compared to what they put on F500/600 trucks in the day, with the frame mounted hydro-vac.

In looking for a formula for the master, I found:

Reference / FAQ's: Master Cylinders

Power brake with dual mc conversion kits....cars and F100 and Chev 3100s...

Booster / Master Conversion Kits

They might have something that will work. You need a drum brake MC, and if I'm reading the whole brake thing correctly, the fluid capacity with a drum set up is less than disc. Of course, with the way these old vehicles poop out wheel cylinders, maybe a 10 gallon fluid reserve would be a good idea.....

Another outfit, if they will talk to you about your set up:

Results for HYDROVACS:medium duty truck

Brake talk, maybe a late '70s C60 for a firewall mounted dual circuit MC. You'd have to adapt it for mounting--childs play, right?