Is there a a difference between drum/drum master cylinders and drum/disc cylinder that wouldn't allow them to be interchanged? I took a dual diaphragm booster off of a '78ish F350 and a new master cylinder for the same application and put it on my drum/drum truck and it just doesn't work quite right. The booster seems to be working (after truck sits overnight step on the brake pedal and start it and the pedal falls a smidgen) but it still takes a lot of effort on the pedal. All of the rubber lines are new in the past two years as are all of the brake parts in the axles.

And no, it isn't just the drum brakes. It take much more effort than Dad's '75 highboy and even grandpa's '66 highboy (no booster).

 

I would imagine the bore diameter is different due to drums needing more or less fluid than discs.

They certainly carry different part numbers.

 

The problem is most likely the MC. If you used the MC from the F350, it has a huge 1-1/4" bore.

This will move a lot of fluid volume but, it will have less output pressure than a MC with a smaller bore. It can also make the pedal very stiff to push and difficult to get the truck to stop within a reasonable distance.

It's not a good idea to use a drum/drum MC on a disc/drum application. The reservoir of a disc/drum MC is larger than the reservoir for the drum brake circuit.

As the front pads wear down, the calipers move further outward. This requires more fluid from the larger reserve of the MC to take up the void. A drum/drum MC does not have this extra fluid reserve to compensate for this --if you run out of fluid, you run out of brakes.

A MC with a bore diameter of 1-1/16" to 1-1/8" would probably work much better. --it's also better to install a new MC, not rebuilt and especially not use a MC from a donor vehicle.

 

I went back to my notes. Booster is off of a '76 F350. Master cylinder is not for the same application - it is off of a '79 F350 and has an 1 1/16" bore.

I originally used the '76 master cylinder (1 1/4" bore) but it took a lot of pressure so I bought the new (not reman) m/c that I have now and nothing changed.

 

Ideas? Since my booster is a dual diaphragm could one diaphragm be blown out and only giving me "boost" from one diaphragm? Or are the master cylinders different enough to negatively affect braking? Is there a better way to isolate the problem so I don't have to just throw money at new parts hoping the problem goes away?

 

You may perform these checks to determine if you have a bad booster.

How to: Diagnosing a Hard Brake Pedal | Master Power Brakes

 

The disc brake front on the f250-F350 had dual piston calipers hence the added fluid .. The difference I think is in the proportioning valve ..
See here

 

OP said the booster was on his truck with all-wheel drums.
Proportioning isn't used on all-wheel drum brake systems. Proportioning would be used on a discs front/drums rear brake system.

From '67-up, all-wheel drum brake systems (from the factory) would have had a pressure differential valve.

In '73, front discs became standard on the 4x2 F100-F350 trucks --power brakes was optional. Also beginning in '73, the multiple functions of metering to the front discs, pressure differential sensing between the front and rear brake circuits and proportioning to the rear drums was integrated into one valve body assembly.

Proportioning, in a brake setup that requires proportioning, only limits the rate of pressure rise after a predetermined amount of pressure has been reached --used for the purposes of reducing the chances the rear wheels will lock up during a panic stop.

Before this point, brake pressures between the primary (front) and secondary (rear) brake circuits will be equal.

 

Forgot about that. Pressure differential valve is off of a '74 highboy (all drum).

Reading through the above link piqued my interest. How do I tell if the pressure diff. valve is centered?

Also, what if just one diaphragm was good and the other was bad? Would it test good?

 

I realize you're dealing with a Slick and as such, it wouldn't have had the brake warning light on the dash from the factory, like you would find on a '67-up model truck.

If you didn't also add the brake warning light, then obviously you're not going to get an indication from it that your pressure differential valve spool has shifted --if it's shifted.

Internal view of the pressure differential brake valve (top half of photo).



Testing brake valve switch if you don't have a brake warning light:

If the brake warning light circuit isn't hooked up, you can still determine whether it's centered or whether it's shifted, with an ohmeter.

The first year for the pressure differential valve was in 1967. This is the only year that the valve used a warning switch with a single pin conductor. From 1968-on, the switch had two pins.

On '68-up, the second conductor goes back to the ignition switch. When the key is turned to the "start" position, it temporarily illuminates the brake warning light (prove out circuit). It lets the driver know the warning bulb is still good.

When the key moves from "start" to the "run" position, the warning light goes out --if the pressure differential valve spool is centered. If it's not centered, the light will remain lit.

To test for whether the pressure differential valve spool is centered or, if it's shifted, place one meter lead to the outside of the valve body and the other lead to the pin on the switch.

If there are two pins (not just one) in the switch, it doesn't matter which pin you touch. If there are two pins, they are at the same potential, electrically, since they are both tied together by a small bar at the lower end of the pins.

View of the switch pins tied together by the bar between the pins.



This is what you want to see when you check the valve with a multimeter --no continuity through the switch. This means the spool is centered.



This is what you don't want to see --continuity, indicating the spool is shifted off-center.



If your meter has an audible feature like mine, no "beep" means the spool is centered. If it does "beep," it means the spool is shifted off-center.

 

Never thought of that. I'll go check it out. Thanks.