I've got a '74 F250 4X4 - stock D60 rear and D44 front with drums on both. When I got it, it had been sitting for quite some time so I decided to go through the axles and brakes - new shoes, wheel cylinders and hardware on all four corners. The booster and master cylinder were missing so I picked up a new master and booster for a '78 F350 (dual diaphragm booster). I bench bled the master and then bled the whole system, starting from the farthest wheel and working to the closest. I adjusted the brakes and thought I was good to go.

Meanwhile I got the engine running and tuned up so I buttoned everything up and took it for a spin. It runs great, but the brakes are poor.

For light braking, everything works perfect. But if I want more braking, the pedal is rock solid but it takes a LOT of force to get any more braking power.

With the engine off, if I pump the brake pedal to use up the vacuum in the booster and than hold my foot on the pedal as I start it, the pedal drops a little when I start the engine. This means the booster is good, right?

The pedal is always firm - it's never squishy and never goes the floor and has about the right amount of travel. I would think this means that the master is good...

I have tried bleeding it twice more and have made sure the brakes are adjusted correctly to no avail.

I am at a loss as to why the brakes suck (and yes, I have been around drum brakes all my life and know what to expect from vacuum and drums).

Any suggestions?

 

What about the rubber lines? If she's been settin' for years chances are good the rubbers are shot from within - collapsing during braking. They may look good from the outside but junk/weak on the inside.

You have an open system while bleeding them ( relatively low pressure) but a closed system when braking (higher pressure). I think that may make a difference....

 

Forgot to mention that I also have new lines to the front wheels. Everything is new except the steel lines and differential valve down on the frame rail.

 

Could the booster you bought be for a drum/drum combination? I had a similar problem and when I changed to a drum/disc booster (externally they look the same) that solved my issue.
Rich

 

Does the brake pedal rod have enough travel ?

 

Even with perfect brakes the drums I removed never stoped as well as disc brakes do. Mine worked exactly like yours. Fine for regular stoping but if panic stoping it felt like braking with a 2x4 for shoes.

 

I replaced the entire brake system in my '64 last year. Well not the pedal. Everything else though.

One thing I noticed is they (shoes) seem to take a couple hundred miles to really break-in to the drums. Years ago they used a brake machine to arc the new shoe linings to the size of the drums. They sold oversize shoes to fit drums that had been turned etc. This made for good braking right away. Understandably due to (then) asbestos health hazards shoe arc machines today are scarce.

They also need bedding in. This is true of any type of brake material, regardless of brake design. If the linings become glazed or improperly bedded braking action will be poor, even with discs. They will seem like they don't "grip", because they aren't. This has to be done right away though.

Basically to do this need to perform a (safely) series of a dozen or so repeated moderate brake applications from say 60 mph down to 10 or 20 mph ... until you can smell 'em. They will smoke a little bit. Then keep driving till cool down. Nobody hardly does this. Some linings are much softer than others and will wear out quicker, but provide better braking.

Manual brakes do need a lot more leg power, but without regard to that, you should still be able to stop on a dime even with drums all the way around. Make sure the short lining "primary" shoe is installed towards the front.

I can lock 'em up on all 4 corners in the 64 if necessary. Unless you're towing a lot in the mountains or something like that seems to me drum brakes work fine. They sort of suck to work on, with all the springs and adjusters and the chinese puzzle angle, but properly setup I think they work excellent.

 

I would make sure u have the correct master cylinder, a disc/drum master will not properly operate a drum/drum setup.

Also in a former life I was a mechanic. On a new brake job I always take the vehicle out, quickly accelerate and brake hard from about 50 three times, nearly stopping the first two times and stopping completely the last time. This helps seat the brakes. After the three stops I try to cruise a bit and not touch the brakes so they can cool off.

 

OP said the booster application: 1978 F350. All 1973/79 F350's have dual piston caliper front disc brakes.

The only trucks (Dentsides) that have front & rear drum brakes are 1973/75 F100/250 4WD's.

 

So, this could be the problem then, huh?

What is different about a drum/drum and drum/disc cylinder? I know the proportioning valves are different but I never realized the master cylinders were different...

 

For one thing, a disc rotor brake system needs much higher brake pressure than drum. Bendix drum brakes are "servo" or self energizing. That's why the rear master cylinder on a dual (typically used for the front axle though) is much larger, than for the rear axle.

I'd think you should have trouble with the front wheels locking up when using a disc/drum reservoir and drums on all four corners, but I dunno.

 

A MC (drum/drum, disc/drum or disc/disc) of a given bore diameter will produce the same amount of output pressure for a given amount of force applied on the brake pedal.

The smaller the MC bore the more pressure it will produce but, it will displace a lesser volume. The larger the bore, a lesser amount of output pressure will be produced but at a greater volume.

A smaller bore MC will have more pedal travel. A larger bore MC will have less pedal travel.

The pressure being applied on the front brakes is the same pressure being applied to the rear brakes, if the tandem MC has a consistent bore diameter on both the primary and secondary sides. The only thing that changes the force acting on the brake rotors/drums are the diameters of the front wheel cylinders vs. the rear or, on a disc/drum setup, the size of the front caliper piston(s) vs the rear and the reduction in the rate of pressure rise on the rear brakes by the proportioning valve.

 

So, the master cylinder isn't causing my problem?

 

Does the input rod from the booster connect directly to the brake pedal or, does it go to a bellcrank (cantilever) assembly on the firewall and then to the brake pedal?

Also, do you know what bore diameter your MC is?

 

I didn't see mention of getting the drums surfaced or measured. Uneven wear or worn out drums might be part of the problem. I also didn't see mention of break in time like Tedster9 suggested. Digging into the MC or booster might not be necessary yet.