The Stovebolt Site Forums The Parking Lot Big Bolts '70 c50 master cyl

hello all, i just brought home a 1970 c50 saturday, had to trailer it home because it doesn't have any brakes. I have not had a chance to do anything other than look at it a little. it has a vacuum booster on it under the cab and a single pot reservoir master cylinder. i may have to replce the master cyl, if i do, has anyone put a dual reservoir unit on their truck? this one has drum on both ends, and i don't plan to change that anytime soon. i just need to get it driveable first. so let me know what the rest of you have done,

jason

Hy macgyver, you may be able to find a dual mster cylinder with a large enough volume in each system, and you may be able to mount it to a booster on the firewall, but you will have a lot of plumbing to do to install that setup. Why don't you investigate what is actually wrong with the system and repair that, rather than re-invent the wheel. Lest We Forget.

my concern with a single reservoir master cylinder is the safety aspect of it. if the truck had a problem with one axle or wheel, it could potentially drain the whole system, a dual reservoir would at least give me 50% and i wouldn't be completely without. I will do my homework and i am sure that i will fix it as factory, but if there is a viable dual reservoir master cylinder, i would put it on.

When the factory went to dual system on the larger trucks they used 2 booster's. Later they went to hydroboost with electric pump backup. I don't think a car/light truck master cyl behind a vacuum power unit would have enough stopping power.

As Wrenchbender points out when the market changed to dual reservoir master cylinders on the larger medium duty trucks they used two hydro-vac units splitting the hydraulics between the front and rear brakes. As time marched on they later began using hydro-boost systems.
Either system could be found on a later truck of the same size and retro fitted on to your big bolt. Just be sure that the donor truck is drum and drum and that it has close to the same working area to its wheel cylinders as your truck. Some bracket fabrication and plumbing is all either will take. However with the use of the hydro-boost system if if you don't already have power steering you would also need to mount a power steering pump for the hydraulic power weather or not you add power steering too good luck.

Thanks all for the help. It sounds like there is not a quick go to master cylinder for this application. I am confident that unless it needs a new everything that I will get the factory system working properly. before I got it I noticed that someone has been into the rear system, there is new hardware on the backing plates, and some new lines ran, i might get lucky and there is just a leak somewhere. but, knowing trucks that have sat a long time, I doubt I'll be that lucky. I'll try to post any updates as they happen,

thanks again

macgyver37,

Do not trust ANY of the old brake system replace it all 100% front to back. The old DOT3 fluid would wick water and cause the inside of the steel lines to rust. They might look good on the outside and really be paper thin and ready to blow.

The rubber lines also go bad on the inside...

Be safe,

Mike B

mike, thanks for the reminder, i have ran into that before with both the steel lines rusting out and the rubber hoses failing. the worst was trying to figure out a buddies camaro that the front right would apply the disc but wouldn't release. we went through alot of things before we figured out the inside of the rubber line had partially broken apart and was acting like a check valve. i will definately go through all the parts before doing anymore than around the shop with it. some of the lines have been replaced already, so other than double checking the work, i plan to reuse them.

is this truck's brake system going to be in a motor manual or similar? it has 2 lines (i think) running to each rear wheel cylinder, i'm sure i will figure it out when i pull the rear apart, but i haven't messed with this style yet and it would be nice to have a manual.

jason

Jason,

For peace of mind I would redo ALL of the lines...the steel lines are the cheapest part of the entire job and they are easy to do if you use the old lines as patterens...

My 2 cents,

Mike B

well, due to weather and other commitments i hadn't had a chance to get back in to the truck, but last saturday it got up to 68 deg, so i figured i would dive into the system to see what was wrong.
I pulled off the master, took it apart fearing the worst, but it cleaned up nicely with no pitting in the bore and the cups looked good. I put it back on then decided to see if the system would even build pressure and got it to the point i got a bit of pedal and as i was checking the fluid level i heard a hissing noise. it was the front steel line going to the passenger wheel, it had a rust hole in it as Mike had suggested.

I was honestly pretty happy about it, because I would much rather replace cheap lines than the expensive master cylinder ($145) or whatever is on the booster (didn't even price that). So, i replaced all the steel lines on the truck and i got it to the point that it had a good pedal so i fired it up to see if the booster would work and also to move the truck to test them out.
By the time i got to this point it was very dark and have you ever tried to back up a box truck without a passenger side mirror? my neighbor had also parked alot of cars around it. i didn't really feel like running into anything that night, so a short 40 ft burst of speed and hitting the brakes had to suffice for my test, but as best i could tell the brakes work as they should... woo hoo!
Since then I haven't had the ambition to go out in the bitter cold and deep snow to play with it in the daylight.
so, now that i can move it and stop with more than the parking brake, i am eager to make a bunch of laps around the block and maybe drive it to work a few times. still need to do the mirror, check all lights, turn signals, and charging system, but now that i can drive it, it will get some use and attention.

jason

Jason,

To confirm the Hydrovac is working do the following test;

With the engine off (first start of the day). step down on the brake pedal (medium/hard pressure) and start the engine. As the truck builds vacuum you should feel the brake pedal fade away just a little bit. If it does then the hydrovac is working.

While replacing ALL of the steel lines did you replace the three rubber flex lines? They go bad from the inside out as well.

Be safe!

Mike B

no, i did not replace the rubber lines yet, i will have to wait a little while before i can get them. once it gets back up to freezing i may try to fire it up and do the hydovac test, thanks for the info