The Stovebolt Site Forums The Parking Lot Big Bolts dual master cylinder conversion

We have a 1948 gmc fc303, 1 1/2 ton flatbed. We're trying to restore it mostly to original. But one exception we decided on is to go to dual master cylinder.

Now forgive me in advance for this question, because we haven't done more than glance at the hydrovac power unit underneath the bed or do any research. I'm just curious about how hard this might be to convert? I'm guessing there might not be a separate place to hook a second line into it?

Anyone tried this and gotten the process figured out? is there another power unit that needs to be installed?

The power booster and or dual master kits sold by the regular "restoration" parts suppliers for smaller trucks of the same years will fit, then all you need to do is find a dual master cylinder with a large enough bore from a drum/drum brake truck.

I did mine a little different and used a hydrobooster, works off the PS pump, I also have disc disc, so can't help you with a master cylinder. Unless you plan on having PS, then the Hydrobooster may not be as easy as a vacuum booster, the hydrobooster is most often a better system though.

There are two kinds of dual master and or booster kits, some mount with a bracket to the original pedal assembly where the old master was. The second kind mounts it with a bracket inside the frame rail behind the pedal assembly, then uses a longer push rod to reach it. I would use the second type if I were to do it over again.

Grigg

The original hydrovac does not support a dual master setup. You might be able to run dual hydrovacs though its probably not a practical approach.

Another option is to remove the hydrovac completely and simply use a dual master with no assist. The power assist was an option on the 1-1/2 ton trucks and some of them did not have it.

The later trucks and some of the 3 ton trucks with dual master setup did just what OldSub suggest using two hydovac units one for the front and one for the rear. I did what Grigg did with the hydro boost system pictured here with the bracket that mounts it on the inner frame rail using the stock pedal assembly. This unit came from ABS Power Brake. http://rides.webshots.com/photo/2701798080096773078qEsBli

On the big trucks there's room enough to mount a regular pick-up vacuum booster that normally mounts to the firewall. All you have to do it remove the old master and make a bracket to mount it to the frame. I used a 1970 pick-up master which had the drum/drum system and mounted it to my booster. I also put a pressure switch in line while replumbing the brake lines, I went 12 volts. Fab up an adjustable pedal rod and run a vacuum line down to the booster and you're done, after bleeding of course,
Scott

That's what my 1970 C50 has from the factory.It has a duel master cylinder going down to 2 hydrovacs under the cab. One for front, and one rear. Idon't know if this is really practical for a retrofit. I think they used this system due to the fact that these trucks were not running power steering, and the hydroboost was not available yet( to my knowledge ) I would think that you could adapt a hydro boost if you wanted to add a pump, or get a large vacum booster.

Hey 48bigtrucks I'm doing the same thing although I didnt add in a 12v pressure switch, can you explain why you did this thanks!!