Can someone explain or show a pic of the physical differences of the disk/drum verses drum/drum master cylinder? I may have installed the wrong one.

Thx, Jim

 

I have never paid much attention to it so I can't answer your question. I have always used a proportioning valve on the rear to control front to rear bias.

 

Sorry I don’t have a picture, but saw one just the other day either here or over on classicbroncos.
Until the very early 70s (70 or 71) the master cylinder had equal size reservoirs and an oval shaped cap.
The next generation used a larger reservoir for the front brakes (where the disks usually were) but kept the smaller one for the rear brakes. This has the more familiar shape with one round end and one flat end.

This was actually used even with four-wheel drum brakes too, until 76 when the rest of the trucks got discs. Likely just to cut down on extra parts when one could work for all.
Because the disc brakes were already in use on passenger cars by 66 and trucks a few years later, they had already started using the new design master cylinder, even if you had drum brakes still.

Very little, if any, internal differences to the pistons and reservoirs after that.
Just inside the output ports they continued to use residual pressure check valves like they traditionally had for drum brakes and for back when many master cylinder locations were below the actual brake height.
The typical ratings are 10 psi for drum brakes and 2 psi or four disc brakes, but I don’t know if they installed lower pressure check valves back then or simply left them out of the front brakes.
These days most drum brake masters don’t even come with a check valve installed. Don’t know about disc brake masters. And never checked one back in the 70s when they were still installing them regularly.

Just removing them was what we always did when installing disc bricks and using a drum brake master.
As has been mentioned before, the other reason for a check valve was to keep a minimal amount of pressure on the wheel cylinders in the drum brake.
This is a good thing but not absolutely necessary if your wheel cylinders have springs inside them, but it’s something to be aware of.

There may be other differences I’m not covering, and there are probably differences in modern master cylinders, but that’s what I know about the old ones.

 

Forgot to ask. What makes you think yours is wrong? And do you have a picture you can show us?

 

Im not sure about different size reservoirs but the drum/drum M/Cs have 1 inch diameter bore and drum/disc have a 1.125 inch bore.

 

Did a complete brake job including,... new pads, rebuilt calipers, rebuilt mbc, new power unit, rebuilt inline valves and successfully flushed the system.
Now I drive a few miles in traffic and my front brakes start dragging and eventually lock up. Let it sit for awhile and cool down, and it starts out fine,... for a few stop-n-go miles.
A friend advised me to check the adjustable pedal push rod for free play. I even shortened the rod for more play. Seemed like it was going to work but not for long. You can feel the free play start getting less as as the brakes are used until they eventually pump up and lock.
Thought I might have installed the wrong MBC.

 

Not sure why you opened a new thread:

https://www.ford-trucks.com/forums/1...ompletely.html

 

If you have a booster, the actuator rod between the booster and the master cylinder is too long and is not letting the master cylinder fully retract. The symptoms are just as you describe. While driving the brakes will essentially lock and not release but will eventually bleed down while sitting. Many boosters have an adjustable rod. You need to screw it in and try for .010" clearance. If you drive it and they lock up you can quickly loosen the two nuts holding the master cylinder and they will release. If you loosen the master and it rocks on the surface of the booster the rod is too long. Thread it in one turn at a time until it no longer rocks. You don't want to thread it in too far or you will have excessive pedal travel.

If you have manual brakes, many have a threaded actuator rod and if not properly adjusted the brake pedal may be keeping pressure on the master.

 

Wrong rod.
As mentioned, it’s the rod between the master and booster. NOT the rod from the pedal to booster.
that one can sometimes need adjustment to, of course, but your problem is more likely to be coming from the other rod.

One way to tell if it’s up at the master is to crack open the hard line between the front brakes and the master cylinder.
This is usually the rear or larger reservoir.
If the brakes release immediately, then the pressure is being held in the line from the master. If not, then the pressure might be further down the line, such as a collapsed hose or something.
You can crack open the fittings or bleeders on down the line to narrow down the location of the issue.
it can even be a sticky caliper, but more often than not. It’s what we’ve been talking about.

Once you crack open the line to the front brakes, you can remove it fully and check for a residual pressure check valve.
Take a small poker thingy that can fit through the cone seat fitting in the master and gently probe. If you feel somethings spongy or springy then there may be a check valve installed.
If you do not, and the poker just goes straight in to the hard surface, then you do not have one.

How is yours plumbed? Rear reservoir to the front brakes?

 

I was a little surprised when I read one application guide that had my F350 with the twin piston calipers using a 1 inch bore master.
Came is kind of a surprise, but I guess it shouldn’t have since that’s what For did with the Broncos as well when they went to disc brakes in 76.
They didn’t change the master bore from the original 1” size.

 

I am only going off two of my truck and there parts. My 1970 f250 dual piston disc/drum uses a 1.125" bore M/C. My 1972 f250 4x4 with drum/drum uses the 1" bore. I have a 1970 f350 as well but it has been a parts truck and I havent got into any of the brake parts.

 

Yeah, the 1.125 size actually makes sense, but the smaller one did seem to work super well too, so I'm sure there's more to it than just bigger-is-better in some cases.

paul

 

Don't forget we had single pot MC's forever.