Just bought a 1957 F600 2 ton grain truck with V8 and dump bed, will use it to haul rock and dirt locally, sat unused for 20 years. Going 5-10mph is plenty, the stock drum brakes should be fine, but terrain is hilly and brakes must be dependable. Brakes are currently shot with pitted cylinders and lines, will likely wind up pretty much replacing the entire system. Moving up from a single master cylinder to dual seems prudent. I guess that means keeping the same size slave cylinders in the wheels, and get a dual master cylinder and vacuum assist for a later model year but meant for my size of slave cylinders. I assume vacuum boost is pretty much required for a truck this size hauling 5 yards of rock, but would prefer the simplicity and reliability of no vacuum boost if that's a reasonable option for a guy with a strong right leg. Any and all advice is welcome, especially advice on exactly what parts to consider obtaining. Stock truck has vacuum assist bolted to the frame below, but there's plenty of room on the firewall and I'm not afraid to drill some holes.

 

You MUST have vacuum or other boost. Trust me. You will NEVER stop a loaded truck with leg power alone. (well if you are on a dry lake you might, but you won't be)

Look on the frame under the cab. THAT is your booster. They are available as rebuilt units. You just need to find out what you have.

You could do a firewall mounted dual pot with vacuum boost, but you would need to make sure you have enough fluid capacity and balance the system via trial and error.

 

Yes, I know where the current vacuum boost is, I've looked over all the brake lines and
took some of this stuff apart, concluded most of it should be replaced.
Yup, I had assumed I pretty much needed needed the vacuum boost, but figured it was worth the question.
I mostly just want reliable at 10mph, requirements might be different for somebody taking a load down the freeway.
This will not be on the freeways. Or byways for that matter.

Wish there was a better system for sizing the new dual master cylinder and vacuum boost beyond trial and error.
If that's what it takes I'd probably opt for just replicating the original equipment.

Thanks for the reply!

 

85e150six4mtod wrote:
> You could do a firewall mounted dual pot with vacuum boost, but you would need to make sure you have enough fluid capacity and balance the system via trial and error.

That sounds like a good solution.

But I'm not quite sure what "balance" means here.
Perhaps the dual master cylinder units have screws to adjust how hard to hit front vs rear?
Current system just tee's out of the vacuum boost and heads off to all four brakes.
These are all drum brakes, and will remain so.

As should be pretty clear by now, I have not messed around a lot with brakes and have a lot to learn.

Perhaps all I have to do is get a dual master cylinder with vacuum boost (all one unit)
that's big enough for an F600 size truck with drum brakes, and re-plumb
That should be easy enough.
The redundancy of separate front and rear would be reassuring,

Jerry

 

Front to rear balance via pressure regulating devices. A dual master for all drum brakes or all disc might be best. Disc/drum require different pressures. You don't want your fronts locking up early, even at 10mph.

F350 units might be a place to start.

 

Thanks for the advice.

Sounds like this pressure regulating device is a separate piece, not an adjustment to the dual master cylinder.
The fronts can contribute a lot of braking power, though I do want the front to continue facing forward.

I'm pretty sure the original equipment has equal hydraulic pressure to all four brakes.
This thread on a 1954 F600 shows those rear slave cylinders had a larger bore and thus press harder on the shoes.
https://www.ford-trucks.com/forums/1...s-sources.html
Will need to get me a 1-1/2" socket before I can take the wheels off and look.
Have a shop manual ordered.
Will likely punt and just replace the stock equipment as needed rather than guess at
how to redesign a brake system.

Jerry

 

Please go to a dual piston master. If you lose the fronts, the rears will still work & vice versa!! I was involved in a rear ender because one wheel cylinder went south & emptied my master, pedal to the floor!!!!!!!!!!!!!

 

Bruman,

Thanks for the reply!
That is exactly why I posted, though wasn't fully understanding some of the design issues.
So a bit hesitant to go with a dual master.

A few further thoughts, please correct as you see fit:

A dual master with the same bore will put out double the amount of fluid for a given amount of travel of the brake pedal.
Likely not too much an issue, except if the vacuum boost fails you have to press the pedal twice as hard.
So I may want to go with a smaller bore.
Would be interesting to pull the vacuum line on level ground, see how hard it is to stop.

The existing vacuum boost attached to the chassis is simple enough, I think I have it figured out.
Key point is that the volume of fluid exiting to the brakes is equal to the volume entering from the stock single master cylinder.

The fluid pressure to the slave cylinders at all four corners is equal on the stock truck, but cylinder bore is larger
on the rears than on the front. (Haven't yet pulled the wheels to check.) That seems appropriate, if hauling 5 ton
down the highway at 60mph the rears are not so likely to skid when you hit the brakes. Rears could skid if empty,
but in that case stopping distance will be considerably less and thus less brake pressure is required.
So designed for the worst case load.

This advice from 85e150six4mtod seems correct: "Front to rear balance via pressure regulating devices."
Otherwise could well have full on front brakes with zero at the rears. Or vice versa.
Not yet clear to me how that regulating device works, or how it does not compromise the desired redundancy.

Currently thinking I'll go to a dual master cylinder of smaller bore than the current single master cylinder.
Have a pressure regulating device in there such that brake lines to fronts remain at about the same pressure
as brake lines to the rears, just like the stock system. Keep wheel cylinder bores as they are on the stock truck.
Replace the brake lines, especially the rubber hoses, and the rusty pitted steel I see in places.

This is not a race car, no intention of ever taking it up to 20mph.
But I would prefer some assurance that at least two of the wheels have brakes.

Jerry

 

A dual master cylinder will but out the same amount fluid as the singe bore just split between 2 ports. The volume of brake fluid needed to apply the brakes won't change, if the bore size of the double master is the same bore as the single master cylinder the pedal effort will be the same and not change.
On Drum brakes there is no proportioning valve the brakes are proportioned by the size of the slave cylinders in the brakes. All you need to do to switch to a dual master cylinder is to get one of the same bore size as the single and make the necessary plumbing changes. On drum brakes either power or manual they will use the same bore size Master unlike disks As for pedal travel if you want more pedal travel go to a smaller bore dual the will decrease pedal effort and increase effective force applied to the brakes with the same pedal effort. There is some very erroneous and bad advice in is thread.

 

 

Could be right, I'm not yet convinced.
Here's a diagram of a dual master cylinder: https://auto.howstuffworks.com/auto-...wer-brake1.htm
Pressing the pedal will move the master piston and the slave piston forward by some distance,
forcing out double the volume of brake fluid we would get it it was only a single piston travelling that same distance.
Double the volume per unit distance of piston travel at the same slave cylinder pressures means twice as much force must be applied to the pedal when the vacuum fails.

I'm not sure either way if the front and rear hydraulic lines can be kept entirely separate.
If the front shoes get tight against the drums first, does that mean the rears will never be used unless there is some way to (slowly?) equalize pressure?
Or is that equalization inherent in how the ports into the reservoirs of the dual master cylinder are arranged,
allowing excess fluid to flow back up into the front reservoir when the pedal is released? (Still assuming it was the fronts that got tight first.)

I'm ok with rebuilding to the stock design.
If there's a clear path, I would consider a dual master cylinder.
Mostly, just want to understand how this stuff works.

 

Dual Master cylinders are self balancing. the same pressure is applied to each port. The Primary circuit (front brakes and nearest the fire wall) piston has one seal the Secondary circuit (Rear) piston has 2 seals The First seal on the secondary piston seals the back side of the primary circuit. Both circuits are tied to each hydraulically but the fluid is separated.

So lets take your example So lets say the front shoes are tight against the drums and the rears are loose. You apply the brakes the pressure in the primary chamber rises applies the brakes they stop but the rears are not applied. So you keep pushing on the pedal the primary and secondary piston will still move as the secondary piston is sealing the back side of the primary chamber the primary piston will continue to move forcing the secondary one until the pressure is equalized in both circuits.

You do the same amount of work with X amount of force whether the Master is dual or a single. When they moved to Dual circuit masters in 67 the bore size of the masters never changed on manual drum brakes nor did the wheel cylinders. The just had 2 ports. Think about it this way say you cut a second port in a single master cylinder and instead of joining the front rear brakes at the Y union you plumbed to the master. Is pedal effort going to change ? Of course not. Now let's say you but check valve in each circuit that would shut if pressure was lost in a circuit is pedal effort going to change ? Of course not.
It is the same in a dual master cylinder as both circuits are hydraulically linked to each other just like in a single master cylinder with 2 ports. Only difference is the fluid in each circuit is separated.

You will need a master cylinder for Drum Drum brakes as they will have the correct residual valves If you use a Disk Drum master cylinder it may have no residual valves or Disk residual valves in it and the brake shoes will fully retract leading to full stroke or more brake application. .

 

Many thanks for the concise explanation!
I'll be looking over the various drawings of dual master cylinders out there more closely.
But I think you nailed it, you have convinced me the bore should remain the same size.

I had been thinking there was a mechanical linkage from primary to secondary piston. If there is no direct mechanical linkage between the two pistons, then pressing the pedal is only doing the work of moving primary piston, pressurizing the fluid going to the front brakes. That fluid pressure is also moving the secondary piston to actuate the rear brakes, giving them the same hydraulic pressure. If the front brake lines blow, that fluid pressure in the primary master cylinder drops to zero, the primary piston moves forward till it is in mechanical contact with the secondary piston, and thus the rear brakes can still be actuated.

 

Yup that is exactly how it works think you explained it better than I did lol. I think your truck has a 1.5" bore MC it could be a trick to find a 1.5" Bore dual MC but there is likely one out there that has the same 4 bolt pattern as it is a common pattern on medium trucks and with a bore of that size you will be looking at medium truck masters.

 

Was told by a local mechanic (and 85e150six4mtod in this thread as well) that a dual MC from an F350 and up
would likely do fine, though he didn't want to discuss it in all the annoying detail we've gone through here.
Have the old MC pulled apart, pretty sure it's stock, bore measures 1.25".
Local NAPA store is pretty good, had no trouble finding wheel bearings for my nearly 60 yr old MF35 tractor.
Lots of old 2 ton grain trucks still being used here in far NE Oregon.

So long as I'm buying a new MC, I'm happy to get a dual MC with integrated vacuum boost.
Simpler to install than stock with the separate vacuum boost, and should be much more robust.

Might be a month or two before I get time.
Once up, perhaps I'll post a photo of the truck getting loaded with pit run
from a quarry at the top of the canyon wall here.
It's about 1000 feet of 100% grade from there to the creek at the bottom.
Brakes are a requirement.

Will be nice to have something other than our thick clay mud to turn around on come next spring.