I looked at a 1976 f600 with the 391 engine and a bad brake system.
My question is has anyone else worked on one of these trucks?
The power brake booster is under the truck behind the cab with a long vacuum line running from the engine to the back of the truck .
The trucks brake pedel goes right to the floor .
Basically all I am looking for is how this power assist system even works .
Any information would be great!

 

You have a problem with the brakes themselves. If it was only the booster the brakes would just be hard to push.

Try your local library and see if they have an old Motor's truck repair manual. They cover things like the brake systems on HD trucks.

If your local library has book sales sometimes you can pick up old Motor's manuals.

 

I will have to check that out . thanks
It is a sweet truck (wrecker) - 4 speed and a split rear

 

Has the previous owner ever taken it to a heavy truck shop? I'd go to a local HD repair shop and ask a few questions, see if they're familiar w/ the setup and where you can get parts from locally...plus, I know where there's remnants a 70's F700 I have access to. I'll look and see if it has the setup you are looking at. good luck!

 

Those brake boosters are pretty tough and dont go bad all that often tho its a possibility. I'd check the master cyl first. check fluid level and make sure the piston isnt stuck. Old brake fluid will have alot of moisture in it and it reaks havock on the system. You can get rebuild kits for both the master cyl and the brake boost unit at most NAPA stores. Plan on going thru the whole system. Once you get the master cyl and booster up to speed you may find some leaking wheel cylinders and old lines that leak.

 

I have replaced a few of those. Some times air will get into it some how their should be a bleader screw on the back of the booster. If the truck has drum brakes they may be way out of adjustment. But I would also check the master cylinder and basic things first.

 

What is the name of the "thing" on the back of the power assist booster?
This truck has drums on every wheel AND it has 2 wheel cylinders on each wheel.

 

They call that thing a frame mounted vacuum brake booster single or duel diaphragm. The thing on the back is only called a hydraulic cylinder, and the thing facing down down is called the valve body. Does the backing on the backside have 2 hex headed bolts at 9 and 3 O clock with springs under them. If so they are your adjustments. To adjust lift the wheels off the ground spin the wheels and turn those hex bolts until you hear the shoes rubbing the drum. If that is kind you have.

 

Thanks for the advice I will check it out!

 

The unit under the floor is a hydrovac. The one on my truck, come to find out is larger than the factory unit, and came off of larger truck. Once you get the master cylinder replaced/working and push the pedal with the engine running, be sure you don't hear a continuous sucking sound in the cab behind the seat. The hyrdrovac needs access to atmospheric pressure and gets this via a line up into the cab to get clean/debris free air. If you hear a continuous sucking sound while holding the brake pedal, it means the diaphram is ruptured. You can use a larger unit. I replaced mine with one identical to the one I removed even though Ford never used a unit that size on these trucks. All it means is I have less pedal effort. Good Luck. I'm still working on my brakes as well.

 

I have a 20000# GVW version with a 5 speed and split axle. Similar brake system. It has been a good truck for occasional use. The power assist system is similar in principal to the power brakes in your car with one important difference, instead of the push rod from the brake pedal directly operating the hydrovac (brake booster) it operates a master cylinder which operates a piston in the hydrovac which controls the operation of the booster. You fill the system from the master cylinder. The hydrovac simply allows atmospheric pressure to increase the pressure in the truck's brake lines by applying a vacuum to the back side of a large diaphragm. Thus there is a vacuum hose to the engine's intake manifold and a vent behind the seat. The vent behind the seat is the source of the power to the brakes by allowing atmospheric pressure to enter one side of the booster. It will make a sucking/breathing sound when the brakes are operated. If the hydrovac has a problem in the vacuum system it can also cause a vacuum leak to the intake manifold which will cause the engine to run lean/rough. Since a hydrovac gets it's power from atmospheric pressure they tend to be less powerful at higher elevations (mountains.) The braking system itself consists of a master cylinder on the firewall, a single line from the master cylinder to the hydrovac, a single line from the hydrovac to a tee and from the tee a line to each wheel. There are plenty of tees and connections in this system. At each wheel there is a jumper line from the one cylinder to the other. The cylinder at the end of the line has the bleeder. This is a single circuit system so a failure anywhere in the system will cause complete brake failure. There is a bleeder on the hydrovac, and a bleeder on each wheel. There is a flexible brake line (rubber) for each front wheel and for the rear axle. Most of the brake system is pretty straight forward. Problem areas include: 1. Brake adjusters rusting tight preventing removal of drums or inability to adjust. 2. Worn drums preventing removal when adjusters cannot be backed off. 3. Dirt in the system preventing the hydrovac from working properly including brakes that won't release, low braking force/high pedal force. 4. Aged rubber in the wheel cylinders allowing air to be drawn into the system. 5. Corrosion in the wheel cylinders causing fluid leakage or brake failure. 6. Bolts mounting master cylinder becoming so rusted they break when removing said cylinder. 7. Seals in full floating rear axle leaking differential oil onto the rear brake linings. The truck also has a drive shaft mechanical brake. The best I can say about it is that if needed when the truck is moving it will suggest to the empty truck it should slow down but it is not too sure about it.* The drive shaft brake will be more effective if the split axle is in low range. It is mainly meant for parking and even then chocks should be used if there is any kind of incline or when loading at a dock. *From my testing it will bring an empty truck (9,800LBS tare) to a stop from 30mph on level road in about 150ft.